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Title: The Geological History of Fossil Butte National Monument and Fossil Basin
Author: McGrew, Paul Orman, Casilliano, Michael
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

*** Start of this Doctrine Publishing Corporation Digital Book "The Geological History of Fossil Butte National Monument and Fossil Basin" ***

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As the Nation’s principal conservation agency, the Department of the
Interior has basic responsibilities for water, fish, wildlife, mineral,
land, park, and recreational resources. Indian and Territorial affairs
are other major concerns of America’s “Department of Natural Resources.”
The Department works to assure the wisest choice in managing all our
resources so each will make its full contribution to a better United
States—now and in the future.

This publication is one in a series of research studies devoted to
special topics which have been explored in connection with the various
areas in the National Park System. It is printed at the Government
Printing Office and may be purchased from the Superintendent of
Documents, Government Printing Office, Washington, D.C. 20402.

Library of Congress Cataloging in Publication Data

  McGrew, Paul Orman, 1909-
    The geological history of Fossil Butte National Monument and Fossil
      (National Park Service occasional paper; no. 3)
      Bibliography: p.
      Includes index.
      1. Geology—Wyoming—Fossil Butte National Monument.
      2. Geology, Stratigraphic—Eocene.
      3. Paleontology—Eocene.
      4. Fossil Butte National Monument, Wyo.
      I. Casilliano, Michael, joint author.
      II. Title.
      III. Series: United States. National Park Service. National Park
          Service occasional paper; no. 3.
      QE182.F67M3    557.87'82    75-17511

The Geological History of Fossil Butte National Monument and Fossil Basin

                                                          Paul O. McGrew
                                                      Michael Casilliano
                          _Department of Geology, University of Wyoming_


    [Illustration: Excavations in progress]


  INTRODUCTION                                                          1
  LOCATION                                                              1
  HISTORICAL BACKGROUND OF RESEARCH                                     1
  STRATIGRAPHY                                                          5
      Thaynes Limestone                                                 7
      Evanston Formation                                                7
      Wasatch Formation                                                 7
      Basal Conglomerate Member                                         9
      Lower Member                                                      9
      Main Body                                                        11
      Sandstone Tongue                                                 11
      Mudstone Tongue                                                  12
      Bullpen Member                                                   12
      Tunp Member                                                      12
      Age of the Wasatch Formation                                     12
      Depositional Environment                                         13
      Green River Formation                                            13
      Fossil Butte Member                                              13
      Angelo Member                                                    14
      Age of the Green River Formation                                 14
      Depositional Environment                                         14
      Fowkes Formation                                                 15
      Sillem Member                                                    15
      Bulldog Hollow Member                                            15
      Gooseberry Member                                                15
      Age of the Fowkes Formation                                      15
      Depositional Environment                                         15
  QUATERNARY                                                           15
  THE GEOLOGIC STRUCTURE OF FOSSIL BASIN                               15
  PALEONTOLOGY                                                         19
      Flora                                                            19
      Invertebrates                                                    20
      Vertebrates                                                      20
      Fish                                                             20
      Amphibians                                                       24
      Reptiles                                                         24
      Birds                                                            24
      Mammals                                                          25
  PALEOECOLOGY AND TAPHONOMY                                           30
  GLOSSARY                                                             34
  REFERENCES                                                           35
  BIBLIOGRAPHY                                                         37
  ACKNOWLEDGMENTS                                                      37


Fifty million years ago the southwestern corner of what is now Wyoming
was part of a system of three freshwater lakes that covered that area
plus adjacent parts of Colorado and Utah.

These lakes began with Lake Flagstaff and later became Lake Gosiute,
Lake Uinta, and Fossil Lake (Fig. 1). These lakes developed in
intermontane basins that were created as a result of the geologic events
that formed the Rocky Mountains. The three lakes, collectively known as
the Green River Lake System, had a long, complex history of expansion
and contraction. Varying climatic and geologic conditions were
responsible for the changes in lake size and distribution.

The shores surrounding the lakes were blanketed by a lush, green canopy
of palm, cinnamon, maple, oak, and other familiar trees. Hazel and lilac
covered the forest floor. Rushes and other aquatic plants lined the lake

The air was humid and warm. Streams flowing down from the hills and
mountains built up flood plains and fed sediment into the lake, where it
was deposited in shallow water near the shore. Deposits formed by
chemical processes settled to the bottom further from shore in deep,
quiet water. From these processes were formed the rocks from which the
past history of Green River Lake System is read.

In the forest and undergrowth lived the ancestors of modern mammal
groups. Ancestral rodents and tiny insectivores lived a furtive
existence in the brush and mold that carpeted the forest floor, or else
they sought a livelihood among the branches of trees. Large, bizarre
animals with strange names lumbered through the reed-covered streamsides
eating soft, succulent plants. The earliest members of the horse family
browsed on the soft vegetation. The trees overhead were alive with the
chattering and antics of early primates. Carnivorous mammals preyed on
their plant-eating neighbors and so maintained a balanced community.
Crocodiles and turtles basked in the sun on the beach. Flamingos
concentrated in large nesting grounds. Snakes and lizards crawled about
in the undergrowth. Insects, many exceedingly similar to modern types,
flew about in the warm air or crept about on the plants.

The waters of the lakes teemed with many types of fish. Relatives of the
perch, herring, and sting ray swam in the warm lake water. Occasionally,
large-scale mortalities of the fish occurred. As the fish died, they
sank to the bottom of the lake, and were preserved in the lake

Today, the lakes are gone and where once there were lush tropical
forests there is now a semi-desert covered with sagebrush and
greasewood. The history of these lakes, forests, and animal life can be
read in the thick sediments deposited so long ago.

This report concerns itself with one of these lakes, the smallest of the
three, Fossil Lake. Famous for its fossil fish beds, part of this
ancient lake is now Fossil Butte National Monument. It is the purpose of
this report to relate the history of Fossil Lake and its now fossilized
inhabitants, thereby, hopefully presenting the order of events in a
landscape obscured by 50 million years of time.


Fossil Butte National Monument is near the geographical center of Fossil
Basin. The basin itself is located in the extreme southwestern part of
Wyoming in Uinta and Lincoln counties, near the Utah-Idaho border (Fig.

The monument is situated about 10 miles west of Kemmerer, Lincoln
County, Wyoming (Fig. 3). U.S. Highway 30N runs just south of the
monument as does the Union Pacific Railroad line to Oregon. The
abandoned town of Fossil is also immediately south of the monument.

The Kemmerer and Sage quadrangles of the United States Geological Survey
cover the entire monument and the surrounding area.


The principal rocks involved at Fossil Butte are the Green River and
Wasatch formations. The first published notice of these two rock units
was by Hayden (1869). Although short, Hayden’s descriptions were the
basis for later refinement of the stratigraphy of the Green River and
Fossil basins. Hayden also mentioned the quantities of fossil fish from
the Green River Formation.

The work done by Hayden was conducted under the auspices of the U.S.
Department of the Interior, Geological and Geographical Survey of the
Territories. The survey lasted from 1867 to 1878, during which time
Hayden and his associates published annual reports of their findings.

In the 1870 report, Hayden (1871) mentioned the discovery of the
Petrified Fish Cut. This cut is located on the main line of the Union
Pacific Railroad about 2 miles west of Green River, Wyoming. A. W.
Hilliard and L. E. Rickseeker, employees of the Union Pacific Railroad,
discovered the cut and obtained many fossil fish which they turned over
to Hayden.

Previous to Hayden, rocks now known as the Green River and Wasatch
formations were occasionally mentioned in the various diaries, journals,
and reports of early missionaries like those of S. A. Parker or those of
explorers like Fremont (Knight 1955).

    [Illustration: Fig. 1. Paleocene-Eocene lake system (after Schaeffer
    and Mangun 1965).]

    [Illustration: Fig. 2. Geographic features of southwest Wyoming and
    adjacent areas (from Oriel and Tracey 1970).]

    [Illustration: Fig. 3. Physiography of the Fossil Butte National
    Monument region (from U.S. Department of the Interior 1964).]

Hayden’s report (1871:425-437) included a description of the fish
fossils from Petrified Fish Cut by E. D. Cope, a vertebrate
paleontologist. Several years earlier Leidy (1856), another
paleontologist, described a fish from an unknown locality in the Green
River Formation.

Hayden divided this survey area into several districts. The Green River
district was put in charge of A. C. Peale. Peale’s (1879:535) report
contained the first geologic description of Fossil Butte itself, as well
as a short discussion of the fish fossils obtained there. The
description of the butte is quoted here:

  ... In the lower part of the bluff from which these specimens are
  taken, the bright coloured beds of the Wasatch are seen outcropping,
  although the entire section cannot be seen, as their softness causes
  them to weather so that the debris conceals the strata. The fossils
  are found at several horizons in the shales. Near the top of the bluff
  is a band of hard, bituminous, or oily shale, which burns rather
  freely with a strong bituminous odor.... It is brownish-black in color
  and on the weathered surfaces a bluish white....

The next major publication dealing with Fossil Basin was that by Veatch
(1907). He mapped the rocks in certain areas of the basin in an attempt
to bring a semblance of order to Hayden’s (1869) broad stratigraphic
descriptions. His work resulted in the designation of most of the rock
units in Fossil Basin from Jurassic to Recent.

Schultz (1914), an associate of Veatch, published a paper dealing with
the structures to the north and east of Fossil Basin. This helped to
delineate the features surrounding and forming the basin.

Subsequent work on the geology of southwest Wyoming mainly has been
concentrated on the Green River Basin just east of Fossil Basin.
Significant work on Fossil Basin has been done by Rubey et al. (1968a,
b) who mapped the northern part of the basin, and by Oriel and Tracey
(1970) who have published results of the latest study on the
stratigraphy and age of the rock units in Fossil Basin. This latest work
is the culmination of many years of study and incorporates and refines
data presented in shorter, earlier papers.


The complete geologic history of the Fossil Basin involves many
stratigraphic units. Most of these are not exposed within the monument
itself but are well known from outcrops outside the boundaries of the
monument and from deep wells (Fig. 4).

At the base of the stratigraphic section are rocks of Precambrian age.
These have not been penetrated by wells within the Fossil Basin but
should occur as much as 25,000 ft beneath the surface. They probably
consist of metamorphic and intrusive rocks. Beginning about 600 million
years ago, at the beginning of Cambrian time, sedimentary deposits
accumulated. These are both marine and continental in origin and range
in age from early Cambrian (600 million years old) to early Tertiary (50
million years old). Some 34 geologic formations are recognized in this
thick section, a list of which follows:

  Eocene                Fowkes Formation
                        Wasatch-Green River Formations
  Paleocene             Evanston Formation (upper part)
  Upper Cretaceous      Evanston Formation (lower part)
                        Adaville Formation
                        Lazeart Sandstone
                        Hilliard Shale
                        Frontier Formation
  Lower Cretaceous      Aspen Shale
                        Bear River Formation
                        Gannett Group
  Jurassic              Stump Sandstone
                        Preuss Sandstone
                        Twin Creek Limestone
                        Nugget Sandstone (upper part)
  Triassic              Nugget Sandstone (lower part)
                        Thaynes Limestone
                        Woodside Formation
                        Dinwoody Formation
  Permian               Phosphoria Formation
  Pennsylvanian         Wells Formation
  Mississippian         Brazer Limestone
                        Madison Limestone (upper part)
  Devonian              Madison Limestone (lower part)
                        Darby Formation
  Silurian              Not Present
  Ordovician            Leigh Dolomite
                        Bighorn Dolomite
  Cambrian              Gallatin Limestone
                        Gros Ventre Formation
  Precambrian           Complex of metamorphic and intrusive rocks

Within the monument itself the oldest rocks exposed are those of the
Thaynes Limestone near Prow Point. As will be seen in the discussion of
the structure of the Fossil Basin, all of the rocks beneath the Evanston
Formation are folded and faulted and within the monument are buried by
deposits of Tertiary age. For this reason detailed discussion of
stratigraphic units will be confined to those rocks that can be seen
within the boundaries of the monument or closely adjacent to it (Fig. 5
). The distribution of the various formations within the monument is
illustrated in Figs. 7 and 8.

    [Illustration: Fig. 4. Geologic time scale.]

  Era/Period/Epoch             Age  Major Biological & Geologic Events
                        of years)
    Quaternary                      Ice Age begins and evolution of
                                    modern mammals; man reaches N. A.;
                                    general uplift of continent.
      Holocene (Recent)        .01
      Pleistocene                3
    Tertiary                        Evolution and Development of major
                                    mammal groups; Cascadian Orogeny in
                                    northwestern United States; in latest
                                    Cenozoic, Eocene lake system.
      Pliocene                 5.5
      Miocene                 25.5
      Oligocene                 36
      Eocene                    54
      Paleocene                 65
    Cretaceous                130   Extinction of dinosaurs; flowering
                                    plants flourish; height of
                                    Cordilleran Orogeny.
    Jurassic                  185   First birds, dinosaurs and mammals
                                    continue to evolve; beginning of
                                    Cordilleran Orogeny which formed the
                                    Rocky Mountains.
    Triassic                  230   Conifers evolved; first dinosaurs;
                                    first mammals; Appalachian Orogeny;
                                    some orogeny in Cordillera.
    Permian                   265   Archaic reptiles and mammal-like
                                    reptiles evolve. Appalachian Orogeny.
    Pennsylvanian             310   First reptiles, coal swamps formed;
                                    beginning of the Appalachian Orogeny.
    Mississippian             355   Expansion of amphibians; much
                                    carbonate deposition; Antler Orogeny
                                    in Cordilleran Mobile Belt (Rocky
                                    Mountain area).
    Devonian                  413   First land plants; first jawed
                                    fishes, sharks and bony fish;
                                    amphibians appear; Acadian Orogeny in
                                    the Appalachian belt.
    Silurian                  425   Jawless fishes evolved; much reef
                                    development; evaporite mineral
    Ordovician                475   First vertebrate remains known from
                                    this time; Taconic Orogeny in
                                    Appalachian belt.
    Cambrian                  600   First shelled invertebrates; major
                                    flooding of North America.

    Reformatted table to combine ‘Era’, ‘Period’, and ‘Epoch’ columns
          into one column in outline form.
    Beneath heading ‘Epoch’: “(Cenozoic only)”; beside other periods,
          “Epochs for these periods are not definitely established”
    As the last line, “Precambrian time began 5 billion years ago.”

                           Thaynes Limestone

At the very northeastern corner of Fossil Butte National Monument can be
seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit
deposited some 200 million years ago. This formation is noted for its
rich and varied assemblage of marine fossils, mostly forms related to
oysters and clams. The Thaynes Limestone consists predominantly of
sandy, gray limestone and green-gray limey siltstones. The formation
weathers to a dark brown in the lower part and a grayish-yellow in the
upper. The fact that the Thaynes Limestone is exposed in the monument at
all is because of the faulting that took place just prior to the
deposition of the Evanston Formation of latest Cretaceous and earliest
Tertiary age (refer to section on structure) and the erosion of the
Tertiary rocks that once covered it.

                           Evanston Formation

Although not seen within the boundaries of the Fossil Butte National
Monument, the Evanston Formation is exposed just south of Highway 30N,
1.5 miles southeast of the southeast corner of the monument. The
Evanston was not involved in the complex folding and faulting but it is
somewhat disturbed and rests under the Wasatch Formation with angular
uncomformity. The Evanston Formation bridges the time boundary between
the Cretaceous and Tertiary periods. In the lower part of the unit are
found many fossil leaves, pollen, and spores and a jaw of the horned
dinosaur _Triceratops_ that prove its Cretaceous age, and in the upper
part are found fossil mammals of Paleocene age.

The Evanston Formation has been studied in detail by Oriel and Tracey
(1970). These authors divide the formation into three members. The
lowest, which they called the Lower Member, is predominantly “gray to
very dark gray mudstone, siltstone, claystone and gray carbonaceous
sandstone.” The Lower Member reaches a thickness of 500 ft in some
places. Above and in part interfingering with the Lower Member is a 1000
ft thick unit that was named the Hams Fork Conglomerate Member. This
unit consists of beds of boulder conglomerate interstratified with thick
beds of coarse, partly conglomeratic brown sandstone and gray mudstone.
Where the Lower Member of the Evanston is missing, the Hams Fork
Conglomerate forms the base of the formation.

The Upper Member of the Evanston Formation is termed by Oriel and Tracey
(1970) the Main Body. It is more than a thousand feet thick and the
lower part intertongues with the Hams Fork Conglomerate. The Main Body
is “light to dark gray carbonaceous sandy to clayey siltstone
interbedded with gray, tan, yellow and brown sandstone and conglomerate
and carbonaceous to lignitic claystone.” It is this Main Body that can
be seen along the highway just southeast of the monument.

The types of sediments and fossils found in the Evanston indicate that
the formation was deposited by streams on flood plains and in marshes
and ponds. A subtropical climate is indicated and the area was heavily

                           Wasatch Formation

The term Wasatch was first used by Hayden (1869:91) as follows:

  Immediately west of Fort Bridger commences one of the most remarkable
  and extensive groups of Tertiary beds seen in the West. They are
  wonderfully variegated, some shade of red predominating. This group,
  to which I have given the name of Wasatch group, is composed of
  variegated sands and clays. Very little calcareous matter is found in
  these beds.

  In Echo and Weber Canyons are wonderful displays of conglomerates,
  fifteen hundred to two thousand feet in thickness. Although this group
  occupies a vast area, and attains a thickness of three to five
  thousand feet, yet I have never known any remains of animals to be
  found in it. I regard it, however, as of middle Tertiary age.

The Wasatch is well exposed in Fossil Basin. There the unit was regarded
by Veatch (1907) as a group and divided by him into three formations:
the Almy, Fowkes, and Knight. He wrote (1907:88):

  In the Wasatch group as thus defined by Hayden the field work of the
  season 1905 showed three divisions: 1) a basal member composed of
  reddish-yellow sandy clays, in many places containing pronounced
  conglomerate beds, which has been named the Almy Formation; 2) a great
  thickness of light-colored rhyolitic ash beds containing intercalated
  lenses of white limestones with fresh-water shells and leaves—the
  Fowkes Formation; and 3) a group of reddish-yellow sandy clays with
  irregular sandstone beds (the Knight Formation) closely resembling 1)
  lithologically and separated from 1) & 2) by a pronounced period of
  folding and erosion.

Veatch, however, erred in his field work and did not realize that the
Fowkes Formation had been downfaulted into the position in which he saw
it (Tracey and Oriel 1959; Eardley 1959). The Fowkes is actually the
youngest of the three formations of Veatch and is considerably later in
age than the true Wasatch.

The Almy and Knight formations are not separable (Oriel and Tracey
1970:16), and can be seen to grade into each other at the basin edges.

    [Illustration: Fig. 5. Intertonguing relationship of latest
    Cretaceous and Tertiary stratigraphic units of Fossil Basin.]

  Recent, Pleistocene
    Pleistocene and Recent gravel and alluvium
    Gooseberry Member of Fowkes Formation
  Oligocene to Pliocene
      Fowkes Fm.
    Bulldog Hollow Member
    Sillem Member
    Wasatchian: Wasatch Formation, Tunp Member
      Bullpen Member
    Green River Formation
    Angelo Member
    Mudstone Tongue
    Fossil Butte Member
    Sandstone Tongue
    Main Body
    Lower Member
      Basal Conglomerate Member
    Possible hiatus or unconformity
    Evanston Formation
      Main Body
      Hams Fork Conglomerate Member
  Latest Cretaceous
      Lower Member
  Late Cretaceous
    Adaville and older formations

    [Illustration: Fig. 6. Subdivisions of Paleocene and Eocene time.]

    Late Eocene
    Mid Eocene
    Early Eocene
    Late Paleocene
    Mid Paleocene
    Early Paleocene

The Almy and Knight, as defined by Veatch, are probably different facies
of Wasatch, the Almy being a more peripheral facies and the Knight a
more basinal facies (Oriel and Tracey 1970). These authors proposed that
the terms Almy and Knight be dropped and that the name Wasatch be
applied to all these rocks to avoid confusion.

The latest subdivision of the Wasatch Formation in the Fossil Basin is
that of Oriel and Tracey. The members of the Wasatch Formation they
propose are described in order from oldest to youngest.

                       BASAL CONGLOMERATE MEMBER.

This member has only local development in Fossil Basin. Where found, it
is a lenticular conglomerate with pebbles and cobbles of buff and tan
sandstone from the Nugget Sandstone and limestone fragments from the
Thaynes and Twin Creek formations.

The basal conglomerate is essentially a channel fill in ancient stream
beds cut into Mesozoic rocks.

                             LOWER MEMBER.

This is an irregular sequence of flood-plain and stream-channel
deposits. It is exposed along the southern part of the Tunp Range and
extends into the far western section of Fossil Butte National Monument
just below Prow Point.

Mudstone is the main rock type. It can be tan, brown, pink, red, or gray
in color. Black, carbonaceous siltstones are also present. Gray
sandstone that weathers yellow or brown and coarse-grained, cross-bedded
conglomerate and sandstone are also prominent. Limestone occurs as thin
lenses and is often brown, platy, and carbonaceous.

It is interesting to note that the Lower Member is intermediate not only
in stratigraphic position but also in color and composition between the
underlying Evanston Formation and overlying Main Body of the Wasatch
Formation. The Lower Member thus appears to represent a gradual change
in either climatic and/or sedimentary conditions in Fossil Basin (Oriel
and Tracey 1970).

    [Illustration: Fig. 7. Geologic map of Fossil Butte National
    Monument (after Rubey et al. 1968).]

    Quaternary—Qg-Qaf—Gravel & stream deposits
      Green River Fm.
    Tga—Angelo Member
    Tgf—Fossil Butte Member
      Wasatch Fm.
    Twb—Bullpen Member
    Twm—Mudstone Tongue
    Tw—Main Body
    Twl—Lower Member
    Triassic TR†—Thaynes Limestone

    [Illustration: Fig. 8. NW-SE section across Fossil Butte National
    Monument, Sage and Kemmerer quadrangles (mapped by Rubey et al.

    Twb—Bullpen Member of Wasatch Fm.
    Tga—Angelo Member of Green River Fm.
    Twm—Mudstone tongue of Wasatch Fm.
    Tgf—Fossil Butte Member of Green River Fm.
    Tw—Main body of Wasatch Fm.

                               MAIN BODY.

This unit of the Wasatch Formation is that part of the formation which
produces the spectacular red-colored badlands in Fossil Butte National
Monument. Particularly typical exposures can be seen in the south-facing
scarp of Fossil Butte where the Main Body makes up the lower portion of
the butte.

The most remarkable feature of the Main Body is its color. On the lower
slopes of Fossil Butte are bands of bright to dull red, pink, purple,
yellow, and gray color arranged in various patterns. This unit is best
observed at a distance, especially after a rain. The bright hues of the
Wasatch contrast markedly with the whites and tans of the overlying
Green River Formation.

Individual bands of color range from 1 to 10 ft thick (Oriel and Tracey
1970:78). The colors are brightest in the upper part of the member and
drabber in the coarser-grained lower part.

The predominant rock types in the Main Body are banded, variegated
mudstone with interlayered sandstone, conglomerate, marlstone,
siltstone, and claystone.

The upper part of the Main Body is mainly mudstone composed of fine silt
and very fine, bedded sand with a clay binder. Conglomerates occur as
channel fills and contain calcium carbonate as cement as do a number of
sandstone and siltstone layers in the upper Main Body.

Conglomerates and sandstone are more common in the lower part of the
Main Body. Some are part of Veatch’s old Almy Formation. They are best
developed along the edge of Fossil Basin. The Main Body overlaps the
Evanston Formation in some places and may rest directly on Mesozoic or
Paleozoic rocks.

                           SANDSTONE TONGUE.

This is a tongue of cross-bedded sandstone. It is brown in color and is
composed mostly of quartz with some black chert grains. This unit is
limited in distribution to the south of Fossil Butte National Monument.
The Sandstone Tongue thins and pinches out to the north and is not
present within the monument. The area of pinchout represents the
shoreline at one stage of Fossil Lake. The Sandstone Tongue wedges into
the Fossil Butte Member of the Green River Formation. The sediment
sources were probably the Uinta Mountains at the south edge of Fossil
Basin. Tectonic events caused an uplift and erosion of Mesozoic and
Paleozoic rocks in the Uintas and debris was deposited in Fossil Lake as
an encroaching delta of sand. When deposition of the sand halted, the
lake expanded and covered the sand, encasing it within the shales of the
Green River Formation.

The Sandstone Tongue can be traced into the Main Body of the Wasatch,
hence its assignment to the Wasatch Formation.

                            MUDSTONE TONGUE.

This tongue of the Wasatch Formation can be seen in the northwestern
part of Fossil Butte National Monument and over most of the northern
part of Fossil Basin. In the area of its distribution, the Mudstone
Tongue separates the underlying Fossil Butte Member of the Green River
Formation from the overlying Angelo Member of the Green River Formation.
The tongue thins and disappears to the south, that is, basinward. As
with the Sandstone Tongue, the edges of the Mudstone Tongue where it
pinches out represent an ancient shoreline of Fossil Lake. To the north
and west the Mudstone Tongue merges with the Tunp Member of the Wasatch

The Mudstone Tongue is a composite of dark-red mudstone which becomes
lighter in color basinward, changing to light red, pink, or
greenish-gray claystone. The unit is a mixture of silt and clay derived
from the north and west, and was deposited as a large delta in Fossil

An interesting aspect of the Mudstone Tongue is the presence of algal
logs. These are cylinders of limestone that apparently formed as an
encrustation of calcium carbonate around logs and branches that fell
into the edge of the lake. The calcium carbonate resulted from the
action of algae which grew around the log. Successive growths of algae
resulted in successive layers of calcium carbonate being deposited.

                            BULLPEN MEMBER.

This uppermost member of the Wasatch Formation is found mostly to the
west and south of the monument. A few, small, isolated caps on the top
of the Green River Formation east of Prow Point occur within the
monument. These low hills, mere bumps, are remnants of a once more
extensive distribution of the Bullpen Member.

Veatch (1907:99) originally considered what is now called the Bullpen
Member as being the Bridger Formation, this was based mostly on the
Bullpen’s position above the Green River Formation rather than on any
lithologic resemblance to the Bridger. The Bullpen is much redder in
color than the Bridger.

In early work Tracey and Oriel (1959:729) called these beds an upper
tongue of the Wasatch and have since named them the Bullpen Member.

The Bullpen Member is lithologically very like the Main Body of the
Wasatch and can be traced into the peripheral units of the Wasatch

The rocks included within the Bullpen Member are layered sequences of
red, pink, gray, and green claystone and mudstone. Bentonite is present
in some claystone beds and causes sloughing because of its property of
expanding when wet. Some sandstones are present in the northern areas of
the member’s distribution. Limestones are also present. They are thin
and slabby and have a brown, white, or gray color. Some contain varying
amounts of clay. A conglomerate is present in the upper part of the
Bullpen toward the basin periphery and merges with the Tunp Member of
the Wasatch Formation.

The Bullpen Member is conformable with the underlying Angelo Member of
the Green River Formation. The contact is a transitional one reflecting
a gradual change from the lake environment of the Green River Formation
through a swamp environment to that of a flood plain. The light-colored,
fine-grained shales, marlstones, and limestones of the Green River
Formation grade upward into drabber, coarser grained clay and mudstones
of the Bullpen Member as a reflection of this change of environments.

                              TUNP MEMBER.

This peripheral unit of the Wasatch Formation was first described by
Oriel and Tracey in 1970. It is not exposed in the monument, but forms a
belt of outcrops around the edge of Fossil Basin and in channels cut at
right angles to the basin edge.

The Tunp Member is seen to intertongue with nearly all of the Wasatch
and Green River formations. The member grades laterally basinward from
coarse, angular conglomerates to fine mudstone. Two limestone tongues of
the Green River Formation are interbedded with the Tunp, indicating that
at least twice there was major expansion of the lake.

Lithologically, the Tunp Member is a diamictite. This is a sedimentary
rock with a wide range of particle sizes. The Tunp is best described as
a red, conglomeratic, sandy mudstone with angular, poorly rounded to
smooth, well-rounded clasts with a size range from pebble to boulder.
There is no bedding or orientation of clasts.

The Tunp Member probably originated from mudflows and gravity sliding
(Tracey et al. 1961). Environmental studies indicate that the area had a
warm, humid climate. This would cause deep weathering of the surrounding
slopes. This weathered material would then be a prime source to be acted
upon by rain, gravity, and possibly earthquakes. These agents of
deposition would then cause the material to flow and be deposited with
little chance for sorting and rounding of the rock particles. The result
was a belt of coarse, unsorted detritus on the basin edges now called
the Tunp Member.

                     AGE OF THE WASATCH FORMATION.

The exact age of the units within the Wasatch Formation can be
determined only if fossils are present. None has been found in the basal
conglomerates but it is believed to be earliest Eocene. The Lower Member
is also not dated with fossils but is believed to be very early Eocene.
A number of fossil mammals are known from the Main Body of the Wasatch
Formation. The lower part is early, early Eocene as demonstrated by the
presence of a very primitive ungulate, _Haplomylus speirianus_. In the
upper part of the Main Body another primitive ungulate, _Hyopsodus
browni_, is found which indicates a mid to early Eocene age. The
Mudstone and Sandstone tongues are not dated by means of fossils but the
stratigraphic relationships indicate an early Eocene age. Fossil
mollusks in the Bullpen Member are not completely diagnostic but suggest
either a late early Eocene (Lostcabinian) (Fig. 6) or mid-Eocene age.
The Tunp Member interfingering as it does with the entire Wasatch
section represents all of early Eocene age.

                       DEPOSITIONAL ENVIRONMENT.

The sediments that make up the Wasatch Formation in the Fossil Basin
were deposited mainly by streams flowing into the basin from the
surrounding uplands. Rock types are variable and for the most part
individual types cannot be traced over large areas, a condition typical
of fluvial environments. The mudstones and siltstones probably were
deposited along flood plains, while the lenticular sands and
conglomerates were laid down in stream channels. Where streams entered
the lake tongues of deltaic deposits, sands or mudstones wedge into the
Green River Lake sediments.

The reason for the red color of the Wasatch Formation is not fully
known. According to Van Houten (1948), the red is most likely the result
of tropical, red lateritic soils forming in the uplands and being
deposited on flood plains. The oxidation of organic material in a humid,
subtropical environment causes the red color. The bands of purple, gray,
orange, and other colors are due to various stages in the reduction of
the iron oxides in the rocks.

The presence of palms in the lake sediments and of bones of primates and
crocodiles in both the lacustrine and fluvial sediments indicate a
heavily forested, tropical environment.

                         Green River Formation

The Green River Formation is easily recognized by its light color and
continuous bedding, in strong contrast to the red, discontinuous,
variegated Wasatch Formation below. The Green River Formation can be
thought of as a gigantic lens of lacustrine sediments enclosed in the
fluvial Wasatch Formation. The name Green River Formation is applied to
all of the roughly contemporaneous deposits laid down in lakes of Utah,
Colorado, and Wyoming. It is probable that at one time or another all of
these lakes were connected.

Oriel and Tracey (1970) have divided the Green River Formation of the
Fossil Basin into two members: the Fossil Butte Member and the Angelo

                          FOSSIL BUTTE MEMBER.

The type section for this member is near the southeastern end of Fossil
Butte within the monument boundaries. In the type area the Fossil Butte
Member can be seen to consist of four lithologic units.

The lowermost unit is predominantly mudstone. It occurs in a sequence
about 45 ft thick and contains light gray, fine-grained, calcareous
mudstone and siltstone.

The next overlying unit is about 75 ft thick and is mainly a limestone
unit. It consists of tan to gray limestone, shaly limestone, siltstone,
and paper shale which weathers into thin, curled flakes. A yellow-brown
mudstone tops this unit.

The third unit in the sequence is 45 ft thick and mainly composed of
shales. These shales weather a buff color and are calcareous. Oil shale,
organic rich paper shale, and marlstone comprise the actual layers in
the unit. A few, thin ash beds are also present. This unit is the most
significant one for Fossil Butte National Monument. About 10 ft below
the top of this unit (about 155 ft above the Wasatch) is a bed of varved
shales, one foot thick, that contains the fossil fish for which the
monument was established.

The uppermost unit contains a number of beds of oil shale that are brown
on a fresh surface but weather a grayish-white. An
orange-yellow-weathering limestone caps the 40-ft thick upper unit. Ash
beds are common and traceable over a wide area of Fossil Basin.

South of the monument the Sandstone Tongue of the Wasatch Formation
wedges in between the lowermost unit and the overlying limestone unit of
the Fossil Butte Member.

In the Fossil Butte Member an interesting sequence of facies changes can
be seen which reflect lateral changes in the environment of Fossil Lake.
In those areas where the member represents a deep-water environment,
organically formed limestones and shales are predominant. Shoreward,
these rocks grade into ostracodal limestone, gastrapodal limestone, and,
closest to shore, algal limestone. Then a muddy, sandy beach facies is
encountered where the Green River and Wasatch formations intergrade.

The contact with the underlying Wasatch is conformable and sharp. It is
often marked by a bench or by slump blocks of Green River Formation.

The Fossil Butte Member may in some cases overlap the Wasatch Formation
and be deposited on Paleozoic and Mesozoic rocks. This is a reflection
of topographic relief during the Eocene.

The thickness of the Fossil Butte Member, as reported by Oriel and
Tracey (1970:37), is from 208 to 269 ft.

                             ANGELO MEMBER.

In the northern part of Fossil Basin, the Mudstone Tongue of the Wasatch
Formation separates the Angelo Member of the Green River Formation from
the Fossil Butte Member. Further south, the two members rest directly on
one another. The southern extent of the Angelo Member has not been
determined as yet, but Oriel and Tracey (1970:32) believe it extends up
to, and intertongues with, Wasatch conglomerates near the Uinta
Mountains. Toward other edges of Fossil Basin, the Angelo Member thins
and pinches out into the Wasatch Formation. The Bullpen Member of the
Wasatch rests on top of the Angelo Member.

The Angelo Member consists of white to blue-white weathering limestone,
marlstone, and mudstone. Some sandstone lenses, claystone, oil shale,
and siliceous limestone are present as well. In general, Oriel and
Tracey (1970) have found that buff limestones prevail to the north and
white, siliceous limestone, to the south. Like the Fossil Butte Member,
the Angelo Member shows a facies change from deep-water to shore.

This member forms the very uppermost, rounded slopes of Fossil Butte in
contrast to the more vertical cliffs formed by the Fossil Butte Member.


The age of the Green River Formation is dated on its intertonguing
relationships with the Wasatch Formation because the latter has datable
mammal fossils. The well-preserved fossils of fish, leaves, and insects
from the Green River are, unfortunately, of little value in dating the
Green River Formation.

Although no Lostcabinian mammals are known from Fossil Basin, the Green
River Formation is believed to be of that age (Gazin 1959; Schaeffer and
Mangus 1965).

                       DEPOSITIONAL ENVIRONMENT.

The Green River Formation, on the basis of its lithology and fossil
content, is a fresh-water, lacustrine deposit. Clay and silt were dumped
in Fossil Lake by streams. Most of this fine debris was deposited near
the shore. Chemical and organic processes formed limestones and
marlstones in the deeper central part of the lake.

Examination of the edge of the Green River Formation indicates that
Fossil Lake expanded and contracted several times.

Fossil Lake was eventually filled in with chemical precipitates and
deltaic deposits. The end of the lake was gradual as seen in the
transitional and gradational Green River-Bullpen contact.

Although oil shale is not extensively developed in the Green River
Formation of Fossil Basin, it does occur in small quantities.

  Oil shale is a fine-grained sedimentary rock containing organic matter
  which was derived chiefly from aquatic organisms, waxy spores and
  pollen grains ... and of which a large portion is distillable into a
  liquid similar to petroleum. Despite the name, most rich beds of oil
  shale in the Green River Formation cannot be regarded strictly as
  shale. Instead, they are dolomitic marlstones rich in organic matter.
  Nevertheless, a few are shaly (Bradley 1964b:19).

Oil shale has a structureless ground mass that is yellowish-orange to
reddish-orange in color. Pyrite crystals are found indicating a
partially anaerobic or reducing environment. Pollen, waxy spores,
filaments of algae, and other plant parts are preserved along with
insects and larvae. The preservation is akin to mummification. Crystals
of calcite, dolomite, or authigenic feldspar are also found in the oil
shale (Bradley 1966).

The exact mode of origin of oil shale is not positively known because of
a lack of a modern analogue for comparison. Oil shale probably
originated as an organic ooze on the bottom of the Fossil Lake. This
ooze was composed of the remains of phytoplankton, blue-green algae,
zooplankton, bacteria, and some pollen and spores. The ooze was dense
and uncompacted. Little clastic debris is found, either because the ooze
accumulated in deep water or plants near the shore filtered out the

Decay was reduced effectively in the ooze because of either an
antibiotic in the ooze which inhibited bacteria of decay or the ooze
accumulated in waters where anaerobic conditions prevented decay.

With time and the weight of overlying sediments, the ooze was compacted
and most of its water driven off. Continuing pressure from compaction
and heat generated by burial and compaction caused a variety of complex
chemical reactions which converted the ooze into a petroleum product
called kerogen. Kerogen is distillable and is the important constituent
of oil shale.

An alternate hypothesis (Eugster and Surdam 1973), would have some oil
shale forming in a desert-playa environment. This is based on
geochemical evidence found in Gosiute Lake sediments to the east of
Fossil Lake. There, certain minerals are found in association with some
oil shale that could only have been deposited during periods of extreme
evaporation and in a shallow lake. Much study is now being directed
toward a solution to these problems.

The combustible quality of oil shale has been known for a long time.
Many of the pioneers used it as a fuel for cooking and heat. Hayden
(1871:142) wrote of how workmen on the Union Pacific accidently ignited
the oil shale in a cut they were excavating. The burning shale provided
enough light for night work.

Many of the shales of the Green River Formation appear to be varved. A
varve consists of two layers, one of calcium or magnesium carbonate and
one of organic material. The limnological conditions that led to the
formation of varves will be discussed in their proper place in the
section on Paleoecology.

                            Fowkes Formation

This is the middle formation of Veatch’s (1907) tripartite division of
the Wasatch Group. It is now found to be the youngest formation in
Fossil Basin. It is not exposed in Fossil Butte National Monument.

Oriel and Tracey (1970) have formally divided and named three members of
the Fowkes Formation: a lower Sillem Member, a middle Bulldog Hollow
Member, and an upper Gooseberry Member.

                             SILLEM MEMBER.

Like most of the Fowkes Formation, this sequence of rocks has been
eroded extensively and is preserved as erosional remnants, where
protected by faulting, in the western part of Fossil Basin.

The Sillem Member consists of a lower conglomeratic sequence with some
sandstone and mudstone. The conglomerate contains well-rounded clasts of
gray quartzite, chert, and Paleozoic limestone. The sandstone is light
gray, calcareous to muddy, and coarse to medium-grained. The mudstone is
pink, gray, or tan in color.

The upper part of the Sillem Member is a mudstone and claystone unit. It
ranges in color from pink and yellow to gray and green. Some volcanic
debris is found. There are also interbedded layers of marlstone and
limestone. Some sandstone is present.

The Sillem Member is between 100 and 400 ft thick and most probably
rests unconformably on the Bullpen Member of the Wasatch.

                         BULLDOG HOLLOW MEMBER.

This middle member of the Fowkes Formation has the thickest and most
extensive outcrops. The Bulldog Hollow Member is exposed along the west
side of the basin.

Included rocks are green, white, and blue-green mudstone with ash beds,
green and buff claystone, and tuffaceous, limy sandstone. A high
percentage of the iron mineral, magnetite, occurs in the sandstone.
Conglomerate occurs as lenses.

The Bulldog Hollow Member has a gradational contact with the underlying
Sillem Member. The amount of volcanic material increases upward from the
Sillem, indicating an increase in volcanic activity during the
deposition of the Bulldog Hollow Member.

                           GOOSEBERRY MEMBER.

Oriel and Tracey (1970:55) place this uppermost member provisionally
within the Fowkes Formation. Most of the Gooseberry Member is a
puddingstone, a lithology with well-rounded, spherical pebbles in a
marlstone, sandstone, or sandy limestone matrix. The pebbles are too
rounded for the rock to be a diamictite, and too separated from each
other to be called a conglomerate.

The nature of the Gooseberry-Bulldog Hollow contact is not completely
known. It appears to be gradational in some areas and to be an angular
unconformity in others.

                        AGE OF FOWKES FORMATION.

Fossils date the Sillem and Bulldog Hollow members as middle Eocene in
age. These fossils consist of ostracodes, gastropods, leaves, and
vertebrates from the Bulldog Hollow Member (Nelson 1973). The Gooseberry
Member has yielded a few vertebrate remains and is late Miocene or early
Pliocene in age (Oriel and Tracey 1970).

                       DEPOSITIONAL ENVIRONMENT.

The Fowkes Formation is an alluvial deposit, much like the Wasatch
Formation. The chemical and climatic conditions of deposition were
different from those of the Wasatch, and the extensive red-beds are not

Small lakes were present in which limestone and marlstone accumulated.
The puddingstone may be a mudflow. Volcanic activity left its record in
the ash found in the Fowkes Formation.

In the past, the Fowkes Formation had a greater distribution.
Postdepositional faulting down dropped parts of the Fowkes protecting
them from subsequent erosion.


Rubey et al. (1968a, b) have mapped several forms of Quaternary deposits
in Fossil Basin. These include stream alluvium, rock and landslide
debris, river terraces, and gravels, all derived from local formations.
These deposits are the work of water, wind, and ice acting in relatively
Recent time.


The Fossil Basin is a small, linear and structurally controlled basin in
the southeastern part of the Wyoming overthrust belt. This “overthrust
belt” is represented by a number of small mountain ranges and high
ridges formed by the “thrusting” of sedimentary rocks over other
sedimentary rocks. Topographically, the Fossil Basin is bounded by the
Crawford Mountains and Tunp Range on the west, by Oyster Ridge on the
east, and by the Uinta Mountains on the south. The Crawford Mountains,
Tunp Range, and Oyster Ridge (Fig. 2) are areas of high relief developed
upon southerly extended salient ridges of deformed Paleozoic and
Mesozoic strata. In the center of the Fossil Basin, these earlier rocks
are covered by a veneer of early Tertiary sediments. Superficially, the
Fossil Basin appears to be a broad syncline with tilted beds dipping
sharply or gently basinward from the basin margins. The Tertiary
sedimentary cover, however, partially obscures what is a more complex
structural history.

Following deposition of the Late Cretaceous Adaville Formation, the
Fossil Basin was included in a period of intense structural deformation.
This deformation was the result of compressional forces acting along a
more or less east-west alignment. The strain, or the resolution of these
forces, was developed along a north-south alignment or perpendicular to
the compressional forces. As the stress level became too great, the
rocks were first folded and then faulted. Initial faulting of the rocks
relieved some of the stress; however, with continued application of
compressive forces many stages of folding and faulting were generated.
Because the alignment of the compressive forces remained about the same
throughout deformation (i.e., east-west) and because the strongest
relative compressive forces were from the west, successively younger
folds and faults were generated in an eastward direction.

The structural evolution of the Fossil Butte area may be interpreted as
follows from the schematic diagrams:

  1. The Rock Creek-Needles Anticline is developed at the western edge
  of the Fossil Basin. The present topographic highs developed on this
  structure are the Crawford Mountains and Tunp Range (Fig. 9-B).

  2. With continued application of compressional forces, Paleozoic and
  Mesozoic rocks to the east of the Rock Creek-Needles Anticline are
  folded (Fig. 9-C).

  3. The fold becomes sharply asymmetrical and when the rocks can no
  longer accommodate the compressional forces by further folding, a
  low-angle fault is developed (Absaroka thrust fault). This fault
  probably began as a fracture parallel to bedding which rose to
  topographic surface at a low angle at the point of greatest strain in
  the fold. Most likely, this fracture was an eastward extension of the
  fault underlying the Rock Creek-Needles Anticline, as depicted in Fig.

  4. The rocks to the west of the newly developed fault overrode those
  to the east. This was accompanied by further downwarping in the
  syncline to the east of the fault and the erosion of uplifted
  sedimentary rocks on the west side of the fault (Fig. 9-E).

  5. Another cycle of folding and faulting was initiated to the east of
  the Absaroka fault. This post-Absaroka deformation instigated further
  downwarping of the Fossil Basin area between the Tunp-Crawford
  Mountains and Oyster Ridge. Tertiary sediments began to accumulate in
  the Fossil Basin (Fig. 9-F).

    [Illustration: Fig. 9. Structural development of the Fossil Butte

    K₂, K₁—Cretaceous
    X—Position of Fossil Butte
    ←, →—Relative movement along faults

    [Illustration: A. Pre-deformation: Paleozoic & Mesozoic Formations]

    [Illustration: B. Pre-Absaroka Deformation]

    [Illustration: C. Development of the Absaroka thrust fault: folding]

    [Illustration: D. Development of the Absaroka thrust fault:

    [Illustration: E. Development of the Absaroka thrust fault:

    [Illustration: F. Post-Absaroka Thrust Deformation]

    [Illustration: G. Simplified structure section through present-day
    Fossil Basin
    (Oyster Ridge is off the diagram to the East)
    Structural interpretation by Dr. D.L. Blackstone]

In early Eocene times, the effects of continued downwarping allowed
Fossil Lake to form in the Fossil Basin. By the late Eocene, however,
basinal subsidence could no longer keep pace with deposition and the
dominantly lacustrine (lake) sedimentation was replaced by a fluviatile
(riverine) sedimentary regime. Deposition of fluviatile sediments
probably continued into the later Tertiary when, in the late Pliocene,
regional uplift of the Rocky Mountain interior reversed the sedimentary
cycle from depositional to erosional. The last 3 or 4 million years of
geologic history have witnessed the excavation of much of the Tertiary
sedimentary fill from the Fossil Basin. Fossil Butte and the Ham’s Fork
Plateau to the northeast of the butte are high erosional remnants of
this early Tertiary basin fill. The traces of the Absaroka and other
thrust faults are buried beneath these remaining Tertiary deposits in
the Fossil Basin.

While most of this complex structural history took place before the
Tertiary sediments were deposited, some deformation continued into the
early Eocene. Near Prow Point, for example, is a fault that developed
during early Eocene time. This can be noted in the very northwest corner
of the monument, just to the west of Prow Point (Fig. 6). Here, the
Lower Member of the Wasatch Formation on the west is faulted against the
Main Body on the east (USDI 1964). This fault also is responsible for
elevating the Thaynes Limestone so it is exposed (Fig. 10).

    [Illustration: Fig. 10. Structural relation of older rocks to
    Wasatch and Green River formations near Prow Point in the northwest
    corner of the monument (from U.S. Department of Interior 1964).]


Fossils of plants, invertebrates, and vertebrates are abundant in rocks
exposed in the Fossil Basin. For example, Oyster Ridge received its name
because of the well-preserved and abundant fossil oysters found there.
Of course, Fossil Butte and Fossil Butte National Monument were so named
because of the beautifully preserved fossil fish. A brief review of the
main groups of fossils to be found in the basin follows.


Many of the sediments in the Fossil Basin contain an abundance of fossil
plants. These consist of pollen, spores, wood, and leaf impressions.
Those of the Green River Formation are best preserved and have received
the most study.

Brown (1929, 1934) and Lesquereux (1883) have presented the best
information on the flora of the Green River Formation. Much of the
present knowledge is based on their work.

Brown (1929, 1934) concluded that the Green River flora is a mixed
forest type. It contains some plants common to warm, wet lowlands and
others adapted to cooler, drier uplands. Pollen studies support Brown’s
original concept that several plant communities, including mountain
communities, existed around Fossil Lake.

The flora was situated in an inland, mountain basin. Fossil Lake was in
the middle of the basin. Swamps and flood plains bordered the lake,
while the flanking ridges and mountains provided altitudinal variations
in the plant assemblage.

Spruce, fir, and pine are indicated by fossil pollen. The presence of
these conifers suggest that at least some of the nearby mountains rose
to heights of 6000-8000 ft.

Leaf imprints indicate that deciduous trees occurred on the lower
slopes. Oak, elm, maple, and beech were common forms.

Closer to the lake where it was wetter and warmer there were willows,
laurel, and fig trees. Palms grew in the sandier soils (Fig. 11).
Cypress grew in quiet embayments of Fossil Lake. The shoreline plant
assemblage had a rather subtropical appearance.

    [Illustration: Fig. 11. Impression of a palm frond from the fish
    beds of Fossil Basin. Width of original, 160 mm. _Collection of
    University of Wyoming._]

Ground cover was provided by a mold of dead leaves, holly, liverworts,
mosses, and ferns (Fig. 12). Prairie-type grasses are not present and
large savanna areas probably were not developed.

    [Illustration: Fig. 12. A fern leaf from the Green River Shales of
    the Green River Basin. Width of original, 4 cm. _Collection of
    University of Wyoming._]

In the lake itself small, one-celled algae floated about in the waves
and currents. Bacteria grew on the lake bottom and in the water and
helped to contribute to the organic ooze that built up on the bottom.
Fungi, wonderfully preserved in oil shales, are further indicators that
Fossil Lake was a fresh-water lake and deep enough to have depths below
the zone of sunlight penetration (Bradley 1964a). Reeds, rushes, and
similar lake-shore plants grew in shallow, near-shore waters.

The large amount of organic material in the rocks is evidence of the
prolific microscopic plant growth in the lake.

Occasional dry periods in the Fossil Basin are indicated by fossils of a
plant called _Ephedra_. This plant had leaves with a thick, waxy cuticle
to prevent desiccation during drought. The absence of saline minerals in
the Green River Formation in Fossil Basin suggests that these arid
periods were never of the intensity that occurred in the Green River
Basin to the east.

The flora of Fossil Basin, especially that from the Green River
Formation, is usually compared to that of the present-day Gulf Coast.
Seaward from the Appalachians, the composition of the flora is very
similar to that of the Green River. Spruce, pine, and fir grow in the
Appalachians at elevations of 6000 or more feet, supporting the idea
that similar elevations existed near Fossil Lake and that a general Gulf
Coast climate prevailed in Fossil Basin.


Molluscs, ostracodes, and insects comprise the invertebrate fauna of
Fossil Basin.

A large number of marine forms are found in the earlier marine deposits,
while fresh-water and terrestrial forms are common in the Tertiary
units. It is these later forms which are considered here.

The fresh-water and terrestrial molluscs from the Wasatch and Green
River formations include bivalves (clams) and gastropods (snails).

Aquatic clams from the Green River Formation include _Unio_ and
_Plesiellipta_. _Pisidium_ and _Sphaerium_ are common in the Wasatch

Land snails from the Wasatch include such genera as _Grangerella_,
_Discus_, _Oreoconus_, and _Glypterpres_. _Oreoconus_ is also known from
the Green River Formation.

Fresh-water snails are very abundant from both the Wasatch and Green
River formations. _Physa_, _Planorbis_, _Elimia_, _Bioniphalaria_,
_Virpains_, and _Omalodiscus_ are from the Wasatch. _Physa_, _Elimia_,
_Diomphalaria_, and _Goniobasis_ are recorded from the lacustrine Green
River sediments. In some areas of the Green River Formation, these snail
fossils are so abundant that they form a major constituent of the
limestones. The presence of land snails in the Green River and
fresh-water molluscs in the Wasatch is evidence that the two formations
are not totally representative of a single environment, but represent
several related ones.

Most of these fresh-water snails were herbivorous and frequented the
shallow, well-lighted areas of Fossil Lake which had a good growth of
plants and algae. This naturally occurred close to shore, hence the
gastropodal limestones reported by Bradley (1926) and Oriel and Tracey
(1970) are interpreted as being near-shore facies.

Ostracodes are extremely small arthropods that produce a two-piece,
hinged shell within which they live. _Pseudocypus_ is the most common
form. Like the snails, the ostracodes were so abundant in areas that
they, too, formed a large part of some Green River Formation limestones.
And like the snails they lived in a near-shore environment.

Within the laminated shales of the Green River Formation is preserved an
abundant and diverse assemblage of fossil insects (Fig. 13).

These insects are important because they demonstrate that many modern
families and even genera were in existence during the Eocene. In
contrast to this, knowledge of insect evolution prior to the Eocene is
rather poorly known. Scudder (1890) and Cockerell (1920) have described
most of the Green River insects.

Beetles are the most common forms, followed by dragonflies. Maggots and
larvae of flies are commonly preserved.

Except for these insects preserved as whole "mummies" in the oil shales,
the majority of the insect fossils are preserved as distilled outlines.
This distillation process resulted from the weight and heat of overlying
sediments of driving off the volatile substances from the buried
insects, leaving a hydrocarbon outline. The process is so precise that
the fine hairs and wing veins and even body-color markings are

    [Illustration: Fig. 13. An impression of a horse fly from the Fossil
    Lake deposits. Length of original, 12 mm. _Collection of University
    of Wyoming._]



The single factor that has made Fossil Butte world famous is the
occurrence of literally millions of beautifully preserved fossil fish.
In one layer approximately 14 inches thick, the fish are nearly black
and preserved in a nearly white shale. Most of the fish are perfectly
preserved, retaining every detail of the skeleton and even undisturbed
scales. Fish from Fossil Butte have been collected and sold since the
1870s and may be found on sales counters, in museums, and on living room
walls throughout the world.

The fossil fish from the Green River Formation have a history of
discovery stretching back to 1856. At that time Leidy (1856:256)
described a fish given to him by a Dr. John Evans. Leidy called the fish
_Clupea humilis_, a type of herring. Exactly where Dr. Evans obtained
the specimen is not known. He passed through the area of the Green River
Formation several times and could have collected it during any one of
those trips (Knight 1955:12).

During the construction of the Union Pacific Railroad in the 1860s,
workmen blasting a cut through Green River shales, about 2 miles west of
Green River, Wyoming, came upon remains of well-preserved fish. Hayden
(1871:742) first mentions this cut as "Petrified Fish Cut." Many of the
fish from this cut were given to Cope who described them in Hayden’s
(1871) report. Insects, plants, and a bird feather were also obtained
from "Petrified Fish Cut" at this time.

Sometime in the 1870s the first fish were obtained from Fossil Butte
itself. Cope (1877, 1878) described fish from the Green River Formation
which may be from Fossil Butte. His locality is "nearer the mainline of
Wasatch Mountains" (Cope 1877:807). This may be Fossil Butte. His 1878
locality information is just as poor. He does mention a specimen of
_Priscacara peali_ given to him by A. C. Peale. Since it was Peale who
first wrote about Fossil Butte, the specimen may have come from the

Peale (1879) first mentioned the quarrying of fish at Fossil Butte, but
he had no specific date as to the discovery of the butte or the
beginning of quarrying of fish.

Cope (1884) published two large volumes about the Tertiary vertebrates
of the West. Here he mentioned three fossil localities for his Green
River Fish: "Petrified Fish Cut," "The Mouth of Labarge Creek," and
"Twin Creek." Twin Creek is now known as the Fossil Butte site.

Since that time, the quarries at Fossil Butte have been extensively
worked, mostly by commercial collectors.

The fish from Fossil Butte form an unusual array of genera (Table 1)
unlike any now found living together. Included in the assemblage are
forms usually found in marine waters and tropical, fresh-water fish
(Schaeffer and Mangus 1965).

    TABLE 1. _Simplified classification of fish from Fossil Basin._

      _Xiphotrygon_ (sting ray)
      _Polyodon_ (paddle fish)
      _Lepisosteus_ (gar)
      _Amia_ (bowfin)
      _Notogoneus_ (sand fish)
      _Diplomystus_ (herrings, shad)
      _Ameiurus_ (catfish)
    Aphredoderidae (pirate perches)
      _Mioplosus_ (perches)

More than 40 species of fish have been described from the Green River
shales, but authorities do not agree on how many of these are valid.
Because of this, only super specific groups will be considered in the
following discussion.

    [Illustration: Fig. 14. _Xiphotrygon_, a fossil ray. These forms are
    present but rather rare in the fish deposits of Fossil Butte. Length
    of original, 39 cm. _Collection of University of Wyoming._]

One of the strangest forms to come from Fossil Butte is that of
_Xiphotrygon_ (Fig. 14). This is not a true fish, but is a sting ray,
related to sharks. Closely related forms still survive, with little
change, in coastal waters in many parts of the world. Like sharks, sting
rays have a skeleton composed of cartilage. Normally, cartilage is not
preserved as a fossil as it disintegrates readily. The excellent
skeletons of _Xiphotrygon_ are good evidence of how well the shales of
the Green River Formation preserve the fossils. _Xiphotrygon_ is
characterized by being shortened from back to front and flattened from
top to bottom. It possesses a long, whip-like tail and bears enormous,
flat pectoral (anterior) fins. The mouth is on the underside of the body
indicating that _Xiphotrygon_, like its modern relatives, fed along the
bottom of the lake. The strong flattened teeth form something like a
pavement in the mouth, suggesting that it fed on clams and other
hard-shelled invertebrates. Close relatives of _Xiphotrygon_ live mostly
in marine waters, but occasionally enter fresh water.

The paddlefish, _Polyodon_, is rare but present at Fossil Butte. Similar
forms still survive in China and the United States. The prominent
features of _Polyodon_ include a virtually scaleless body, a long,
depressed snout, and long gill rakers. The skeleton, unlike that of most
modern fish, is composed of cartilage, hence skeletons of _Polyodon_ are
rare. The difference between the modern and Eocene paddlefish are so
slight that the two probably filled similar or the same ecologic niches.
This consists of feeding on plankton and other small organisms which can
be obtained from the water. The long gill rakers would provide a large
surface area on which food could be trapped as the water passed out
through the gill slits. The large mouth would also provide a large
surface area for catching and trapping food.

Quite common to streams and rivers of North America from the Cretaceous
to the Recent is the garfish, _Lepisosteus_ (Fig. 15). The most striking
feature of the gar is its diamond-shaped scales, which are extremely
hard and shiny. The body is long, essentially of the same depth
throughout, and ending in a broad-based tail that is slightly
asymmetrical. The mouth is armed with many small, sharp teeth for
catching prey. One specimen over 5 ft long has been found in the Fossil
Butte area.

    [Illustration: Fig. 15. The fossil gar _Lepisosteus_. Only a few gar
    have been found at Fossil Butte itself. Length of original, 70 cm.
    Across the highway from the butte, one specimen over 5 ft long was
    found by Carl Ulrich. _Collection of University of Wyoming._]

Fossils of the bowfin _Amia_ (Fig. 16) are recorded from the Green River
shales at Fossil Butte and in stream deposits from Cretaceous to Recent.
This fish is still another “living fossil.” _Amia_ is a nocturnal
predator. Presumably, the Eocene _Amia_ had similar habits. The body in
_Amia_ has become elongate, as has the dorsal fin. The thickness of the
scales has been reduced, but they are still rather heavy and cover the
body. Specimens are rather large and deep-bodied. The tail fin is nearly
symmetrical above and below. _Amia_ is rare in the Fossil Butte fish

By far the most abundant fish from Fossil Butte is the genus _Knightia_
(Fig. 17, 18). This fish is a member of the Clupeidae, a family that
includes modern herrings, shad, and sardines. The tail fin is small
relative to the rest of the body. A row of modified scales extends on
the back from the skull to the dorsal fin. Possibly they aided in
streamlining the fish. _Knightia_ apparently fed on the large amounts of
plankton, especially algae, which lived in the waters of Fossil Lake.
Fossils of _Knightia_ are also known from Eocene lake sediments in South

_Knightia_ appears to have been susceptible to mass mortalities. Some
layers of shale at Fossil Butte contain literally hundreds of thousands
of these fish that must have suffered catastrophic mass mortality.
Possible reasons for the obvious sudden death of so many fish will be
discussed in the section on paleoenvironment and taphonomy.

Another Eocene relative of the herring found at Fossil Butte is
_Diplomystus_ (Fig. 19, 20). Like _Knightia_, _Diplomystus_ bears a row
of modified scales on its back. _Diplomystus_ is the second most
abundant fish at Fossil Butte. The jaw in _Diplomystus_ has a rather
pronounced oblique angle to it. The deepest portion of the body is
directly behind the gill region, with a continuous narrowing of the body
toward the tail region. _Diplomystus_ is known from Cretaceous rocks of
Brazil and Syria and from Tertiary sediments in Brazil and West Africa
(Schaeffer and Mangus 1965). Close relatives of _Diplomystus_ now
inhabit the waters off the coasts of Peru and eastern Australia.

    [Illustration: Fig. 16. _Amia_, the bowfin. This too is rare but
    present at Fossil Butte. Length of original, 67 cm. _Collection of
    American Museum of Natural History._]

    [Illustration: Fig. 17. The fossil herring, _Knightia_. This fish is
    by far the most abundant in the fish beds at Fossil Butte and at
    least in one layer was involved in a catastrophic mass mortality.
    Length of each fish, 13 cm. _Collection of University of Wyoming._]

    [Illustration: Fig. 18. The herring, _Knightia_.]

    [Illustration: Fig. 19. _Diplomystus_, another type of herring, is
    the second most abundant fish at Fossil Butte. Length of original,
    12 cm. _Collection of University of Wyoming._]

    [Illustration: Fig. 20. A large _Diplomystus_ with four small bass,
    _Priscacara_. Length of _Diplomystus_, 45 cm. _Collection of
    University of Wyoming._]

_Phareodus_ (Fig. 21) belongs to a family of fish, the Osteoglossidae,
which began as marine forms and has since become a fresh-water family.
At present, they are restricted to tropical rivers and lakes of South
America, Africa, and Australia. In the past they had a much greater
distribution, as its occurrence at Fossil Butte indicates, including
North America. It is a deep-bodied fish with a large head. The anal and
dorsal fins are close to the tail fins. The many sharp teeth in the
mouth attest to the carnivorous habits of _Phareodus_.

The family Gonorhynchidae, the living sandfish of the Indo-Pacific area,
is represented at Fossil Butte by _Notogoneus_ (Fig. 22). _Notogoneus_
is very long-bodied and quite slender. The body width does not vary much
from head to tail, although narrowing does occur in the tail region. The
tail fin is symmetrical, forming a good rudder. The anal, dorsal,
pectoral, and pelvic fins are all small. The body is covered with small,
elongate scales. _Notogoneus_ was apparently a bottom feeder, living on
small organisms picked off or out of the bottom of Fossil Lake.

The Recent catfish, _Ameuirus_, has also been found at Fossil Butte in
the Green River shales. The form of the Eocene catfish was very much
like the Recent one. The habits of the Eocene catfish were probably like
those of the modern _Ameuirus_, i.e., adaptations to a scavenger-type

Rather common in the sediments of Lake Gosiute, but not yet known from
the Fossil Basin, is _Erismatopterus_, a member of the family
Aphredederidae or pirate perches. This small, elongate fish has few
distinguishing characters, the rounded front edge of the skull being
most distinguishable.

_Asineops_ is an Eocene relative of _Erismatopterus_ and also one of the
pirate perches. _Asineops_ is, however, placed in a different family,
the Asineopidae. _Asineops_, like _Erismatopterus_, is a rather
plain-looking fish. The dorsal fin is long relative to the rest of the
body. The body is slightly deeper behind the gills than elsewhere,
giving _Asineops_ a rather common appearance.

The Percidae, or perches, are represented at Fossil Butte by _Mioplosus_
(Fig. 23). _Mioplosus_, as indicated by its well-developed teeth, was a
carnivore that probably preyed on its piscine relatives in Fossil Lake.
The large head blends in well with a strongly built, long body. The anal
and dorsal fins are subequal in size and positioned opposite each other.
The tail fin is large and fan-shaped. It is easily identified by the
presence of two dorsal fins.

    [Illustration: Fig. 21. A large _Phareodus_, fairly common in the
    Fossil Butte fish beds. Length of original, 54 cm. _Collection of
    University of Wyoming._]

    [Illustration: Fig. 22. _Notogoneus_, a long, slender bottom-feeder,
    occasionally found at Fossil Butte. Length of original, 56 cm.
    _Collection of University of Wyoming._]

    [Illustration: Fig. 23. _Mioplosus_, a fairly common perch from
    Fossil Butte. Length of original, 30 cm. _Collection of American
    Museum of Natural History._]

    [Illustration: Fig. 24. _Priscacara_, a small bass that is common in
    the Fossil Butte fish beds. Length of original, 13 cm. _Collection
    of University of Wyoming._]

The one genus for which no close living relatives can be found is
_Priscacara_ (Fig. 24). The family Priscacaridae is thought to be
related to the bass. The family is known only from the Eocene. The size
of _Priscacara_ is extremely variable. The body is deep and nearly round
like that of the common sunfish, the head blending in well with the body
and its contours. The most distinguishing feature of _Priscacara_ is the
series of strong, stout spines supporting the anal and dorsal fins.
These spines may have acted as cut-waters for better swimming and/or
they may have protected _Priscacara_ from its more voracious relatives.


To date, no amphibian fossils are known from Fossil Butte. They must
have been present, as this class of animals is recorded from Devonian
rocks more than 350 million years old and from rocks younger than those
at Fossil Butte. Living amphibians are typified by frogs, toads, and
salamanders. The lack of amphibian remains at Fossil Butte is more
likely due to nondiscovery than nonexistence.


Reptile fossils are very abundant in the Green River and Wasatch
formations. Also a jaw of the three-horned _Triceratops_ is known from
the Cretaceous part of the Evanston Formation.

Small lizards are normally represented only by jaw fragments and
vertebrae. Their remains are most often found by a process of washing
and screening of the sediments containing the fossils. Snakes are also
represented in the Green River Formation. Remains of these animals are
also often restricted to jaws and vertebrae. A complete skeleton of a
fossil boa, _Boavus idelmani_, has been recovered from the Green River
Formation and is the most complete fossil snake from North America
(Schaeffer and Mangus 1965).

Turtles are among the most abundant reptile fossils. They are usually
found as isolated shell plates, although many complete shells are known.
The turtles _Trionyx_ (Fig. 25), _Emys_, _Baena_, and _Notomorpha_ are
known from the Fossil Basin.

The largest Eocene reptiles were the crocodiles (Fig. 26). The presence
of these predaceous animals is further evidence for a prevailing warm
and humid climate during the Eocene times since their tolerance ranges
include only comparable conditions at the present time. A few incomplete
specimens have been recovered from near the monument.


Birds, which first appeared in Jurassic time, are rare as fossils in
rocks of any age because their skeletal remains are exceedingly fragile.
Their fossilization requires special conditions because many of their
bones are hollow as a weight-reducing adaptation for flight. These
hollow bones are especially susceptible to breakage and poor
preservation. Scattered remains of birds, mostly limb bones, are found
in the Green River sediments. Feather imprints have also been recovered.
A complete skeleton of _Gallinuloides_, a bird related to the South
American fowl-like chachalaca, is one of the most significant finds from
the Green River Formation. Recently, Brodkorb (1970) described the wings
of puff birds similar to those now living in South America from near
Fossil Butte.

    [Illustration: Fig. 25. An aquatic turtle, _Trionyx_, from the
    shore-line deposits of Lake Gosiute. Several turtles have been found
    in the fish beds near Fossil Butte.]

    [Illustration: Fig. 26. The skull of a large crocodile from near the
    shoreline of Lake Gosiute. Several crocodile fragments have been
    found at Fossil Butte beds. Length of original, 75 cm. _Collection
    of University of Wyoming._]

In the adjacent Green River Basin to the east, paleontologists from the
University of Wyoming have found nesting grounds of Eocene flamingos in
slightly younger sediments of the Green River Formation. These
“rookeries” have yielded many hundreds of fossil bones of this curious
bird. Since these bird fossils were found proximal to Fossil Butte, they
demonstrate the near subtropical nature of the environment in the Fossil
Butte area in Green River times.


The rocks of the Evanston, Wasatch, and Fowkes formations contain
various fossilized remains of mammals which once inhabited the Fossil

Mammal fossils are highly significant since they are used extensively
for correlation and dating of the Tertiary rocks of the West. Fossils
which are readily recognizable, limited in time, and widely distributed
geographically are called “index fossils.” These fossils are restricted
to a particular time horizon in the rock. In this way, rocks of unknown
age which are found to contain index fossils can be dated relatively and
hence correlated with other fossil-bearing rocks. The small condylarth
_Haplomylus_ (Fig. 27) is a very early Eocene index fossil. Its presence
in certain sediments in the Fossil Basin shows conclusively that these
sediments were laid down in earliest Eocene times.

    [Illustration: Fig. 27. A jaw of the primitive mammal _Haplomylus_.
    Length of original, 1.3 cm. _Collection of University of Wyoming._]

The mammal fossils found in Fossil Basin are also important in
documenting the temporal changes in evolution and environment of the
biotic community. The picture of Eocene Wyoming drawn through
interpretation of these fossils is vastly different than that of the
present day.

Mammals have a long evolutionary history that began over 200 million
years ago in the Triassic period. At that time mammals had only just
evolved from their reptilian ancestors. These earliest mammals were
small, furtive creatures. However, with the extinction of the dinosaurs
some 65 million years ago at the end of the Cretaceous, the mammals were
able to diversify rapidly and fill the empty ecologic niches that the
dinosaurs once occupied.

This filling of ecologic niches voided by the dinosaurs coupled with the
spreading of mammals into hitherto unoccupied niches resulted in the
development of a large variety of mammals. Some became extinct, others
were the ancestors of modern mammals. Fossils of mammals found in
sediments in the Fossil Basin are proof of the extent of this Late
Cretaceous-early Tertiary mammal radiation.

Early mammals bore little resemblance to their later descendants.
General trends in mammalian evolution were increase in size and the
tendency to become more “modern”-looking through adaptation to changing
Tertiary environments.

Most mammal fossils are extremely fragmentary. Teeth are most often
preserved as they are the hardest part of the skeleton and therefore
most resistant to wear and breakage. Thus, it often happens that the
knowledge of a particular fossil mammal is derived entirely from its
preserved dentition. This has obvious limitations for the completeness
of our understanding about mammals in question.

The Paleocene portion of the Evanston Formation has yielded mammals of
Torrejonian and Tiffanian age (Fig. 6).

The Torrejonian assemblage (middle Paleocene) (Gazin 1969) is small and
poorly preserved. Its main significance is that the fauna records a
definite time interval for the Evanston Formation. Gazin’s work has
demonstrated this fauna to be intermediate in composition between
similarly aged faunas recovered from sites to the north and south.

The major faunal elements are insectivores, primates, condylarths
(primitive ungulates), and multituberculates (a type of extinct
rodent-like mammal, with no known descendants).

The poorly preserved condition of these fossils makes discussion
impossible except to note that they extend the known time range of
mammalian habitation in the Fossil Basin.

The younger Tiffanian fauna (late Paleocene) (Gazin 1956) is also known
from largely fragmentary remains. The multituberculate mammal
_Ptilodus_, also present in Torrejonian deposits, is the most
“primitive” mammal in the fauna. The multituberculates first appeared in
the Jurassic (Fig. 4), and are therefore the oldest lineage of mammals
that survived into Tertiary times. The molar teeth of these animals are
characterized by multiple cusps arranged in parallel rows. _Ptilodus_,
like most other multituberculates, was specialized in that its fourth
lower premolars were expanded into shearing blades, the function of
which is not known. _Ptilodus_ was in many respects similar to rodents
in the development of procumbant incisors and in general build and
appearance. Multituberculates are, however, not related to rodents
except that both are mammals. The differentiation of rodents in the late
Paleocene and their diversification during the Eocene probably presented
strong competition for the multituberculates who were unable to compete
successfully and so became extinct.

The primates were represented by _Plesiadapsis_. This animal was about
the size of a squirrel and had chisel-like incisors. Flesh
reconstructions of _Plesiadapsis_, based on skeletal remains, show this
primate to look much like a rodent with a long-snouted skull, clawed
feet, long body, and tail. There is conceivable relationship between the
rodents and _Plesiadapsis_-like primates.

The dominant Tiffanian herbivorous mammals were members of the Order
Condylarthra. These primitive ungulates were diverse in both size and
appearance. It is probable that later modern ungulates (Artiodactyla,
Perissodactyla) evolved from the Condylarthra; however, there are no
surviving members of this group. The Paleocene condylarths were mostly
small animals and include such forms as _Haplaletes_, _Litomylus_, and
_Gidleyina_. These mammals had somewhat insectivore-like teeth, and may
have bridged the gap between insectivores and archaic hoofed mammals.
One of the larger Tiffanian condylarths was _Phenacodus_. This mammal
had a long, massive skull and a long, probably flexible body. The limbs
were stout and short. The body ended in a long tail. Small hooves were
present on all digits, which numbered five per foot. The length of the
largest _Phenacodus_ was about 6 ft.

Carnivorous and omnivorous condylarthra were also common. _Thryptacodon_
and _Claenodon_ are two types known from the Evanston Formation in the
Fossil Basin.

The true carnivores (Order Carnivora) were also present. The Paleocene
carnivores were small and possibly arboreal in habit. The stem carnivore
stock was represented by the family Miacidae. In the Fossil Basin the
miacid _Didymictis_ was common. In _Didymictis_, the mouth was armed
with small, sharp teeth. Most important, the typical carnivoran
carnassial (shearing) teeth had developed. Among carnivores, from
Paleocene to Recent, the shearing blades developed on the last upper
premolar and the first lower molars. Various kinds of shearing teeth
have evolved in other mammal lineages.

Mammals from the Wasatch Formation in Fossil Basin represent the
Greybullian and Lysitean provincial ages (see Fig. 6). These fossils are
of earliest Eocene and mid to early Eocene age. Fossil localities in the
Wasatch Formation of southwestern Wyoming are Knight Station (partly
Lysitean), at Elk Mountain (Greybullian), and at Fossil Butte itself.
The age of the Wasatch Formation at Fossil Butte is Lysitean.

The insectivores are represented by _Diacodon_, a small mammal known
only from fragmentary material.

Primates also continued to expand and diversify in Wasatchian time.
_Microsyops_ was a very unusual type that belongs to the extinct family
Microsyopidae. The teeth were sharp and adapted for eating fruit and/or
insects. _Microsyops_ was about the size of a rat or slightly larger.
The microsyopid primates were limited to the Paleocene and Eocene and
left no later known descendants.

Another group of primates is represented by _Pelycodus_. This was a
small lemur-like animal that may possibly have been an ancestor of
higher primates (monkeys, apes, and men). _Pelycodus_, like
_Microsyops_, was an arboreal (tree-dwelling) animal which inhabited the
forests of the Fossil Basin during the early Eocene.

The Order Taeniodonta is a strange and little known group of mammals. In
the sediments at Fossil Butte the taeniodonts are represented by
_Ectoganus_. This was a moderately large animal. The skeletal
adaptations were similar to those of moles, but there was no actual
relationship between the two groups. The front feet and legs were
robust, and bore large claws. _Ectoganus_ probably used these structures
to grub for food, possibly roots. The incisor teeth were rootless and
persistently growing. The tooth enamel was restricted to two bands on
either side of the teeth. The single pair of upper and lower incisors
was greatly enlarged. The taeniodonts became extinct in the Eocene and
left no descendants.

Equally as strange a group of animals as the taeniodonts are mammals in
the Order Tillodonta. These were herbivorous animals, some of which
became quite large. They have many rodent-like characters, but were not
related to rodents. In many respects, the morphology of rodents seems to
have lent itself to convergence. _Esthonyx_ was the most common
tillodont from the Fossil Basin. Prominent, rootless incisors were a
characteristic of the later members of the group. These incisors were
chisel-like, as in rodents. The molars were unusual and bore certain
resemblances to primitive carnivoran and insectivoran teeth. The
tillodonts, however, bore uncertain relationships to other mammals and
left no descendants beyond the middle Eocene.

In comparison with recent faunas, Paleocene rodents were rare faunal
elements. Two early rodents were found in the Fossil Basin Wasatch

_Paramys_ was relatively unspecialized but was diverse in the number of
species. One species of _Paramys_ was about 2 ft long and possessed a
long tail. The body was long and slender. The skull was small and rather
squirrel-like. The cheek teeth were quadrate and had low, blunt cusps.
The typical, single pair of chisel incisors was present both on lower
and upper jaws. _Paramys_ was probably an arboreal form that did not
look too unlike a modern squirrel and may have had a similar mode of
life. Another rodent, _Reithroparamys_, was somewhat similar to
_Paramys_ in size and appearance. The differences between the two
animals were mainly in the teeth and hind limbs. The latter showed
certain very minor structural modifications that suggested a saltatorial
(jumping) mode of locomotion for _Reithroparamys_.

Bats (Order Chiroptera) are extremely rare as fossils because their
volant and cave-dwelling habitat as well as a fragile skeleton did not
lend their remains to preservation. From the Green River Shales, from
near Fossil Butte, a complete articulated skeleton of a bat has been
found (Jepsen 1966). This animal, named _Caronycteris index_, was small
and generalized in form. Superficially, it looked much like a typical
brown bat. This fossil is important because it demonstrates that the
bats had already become good fliers by early Eocene time.

The condylarths were prominent in the early Eocene but diminished in
importance following this time. _Phenacodus_ was still extant; however,
it was replaced in importance by the smaller _Hyopsodus_.

_Hyopsodus_ is perhaps the most commonly recovered Eocene fossil mammal.
This animal was small and long-bodied and retained a more-or-less
generalized structural pattern. In some respects _Hyopsodus_ was similar
to both insectivores and primates and was at one time or another
regarded as belonging to either of these two orders.

Some of the later condylarths paralleled the more advanced ungulates.
Indeed, the Perissodactyla (horses, tapirs, rhinos) and Artiodactyla
(bovids, deer, pigs, sheep, etc.) were derived from early Tertiary
condylarths. _Meniscotherium_ was one such advanced condylarth. The
cusps on its molars, instead of being blunt points, had developed into
crescentic patterns, somewhat like those in deer or camels. The
relationship of _Meniscotherium_ to modern ungulates is only one of
parallel dental development. _Meniscotherium_ was a medium-sized animal,
but about the same build and size as that of a cocker spaniel.
_Meniscotherium_, however, had hoofs on its toes and was a forest-living

The condylarth _Haplomylus_ is found at the Elk Mountain Wasatchian
locality. This small animal is typical of the Greybullian level of
faunal organization (earliest Eocene). A supposed carnivorous condylarth
was _Pachyaena_. _Pachyaena_ was a fairly large animal for its time. The
strong, robust jaws and teeth and heavy build suggest that _Pachyaena_
was a predatory animal suited for preying on larger animals such as the
amphibious pantodont _Coryphodon_. The affinities of _Pachyaena_ to the
condylarths are uncertain and are based primarily on foot structure.

The miacid carnivores were abundant and varied. _Didymictis_ persisted
and both _Vassacyon_ and _Vulpavus_ continued the arboreal,
forest-dwelling habits of their Paleocene precursors.

The main carnivorous animals of the Eocene, however, were the creodonts
(Order Creodonta) which were essentially early experimenters in
carnivorous habits. As such they were diversely specialized.
_Proviverra_ (=_Sinopa_) was one of the smaller, predaceous creodonts
that inhabited Fossil Basin during Eocene times.

One of the most characteristic fossil mammals from the early Eocene of
Fossil Basin was _Coryphodon_. A specimen of _Coryphodon_ was also the
first mammal fossil to be found in the Fossil Basin. The order of
mammals to which _Coryphodon_ belongs, the Pantodonta, was a group of
large and heavily built, herbivorous mammals. _Coryphodon_ was about the
size of a small rhinoceros. The skull was large and heavily built. The
brain, however, was small. The canine teeth were somewhat enlarged and
strong. Stout limbs supported the bulky body. In habits, _Coryphodon_
was possibly semi-aquatic.

The modern ungulates, Order Perissodactyla (odd-toed ungulates) and
Order Artiodactyla (even-toed ungulates), first appeared in the early
Eocene. The earliest members of both orders were small and relatively

The earliest perissodactyls were the horses and tapirs. The earliest
horses were represented by _Hyracotherium_. This small, slender, and
lightly built horse was about the size of a fox terrier. The front feet
had four toes, and the hind feet three toes. The teeth suggest that this
animal was probably a browser, living in the forests. In the Fossil
Basin, tapirs were known from deposits of Lysitean (mid to early Eocene)
age of which _Heptodon_ was representative. The tapirs now live in the
tropics of Malaysia and Latin America. Their present habitat suggests a
similar environment may have been favored by the Eocene tapirs of the
Fossil Basin.

The first Artiodactyls were pig-like forms, although they are only
distantly related to true suids. _Protodichobune_ was small and may have
looked much like some of the early Perissodactyls. There are, however, a
complex of structural features which serve to distinguish these
mammalian orders from each other.

The mammal fauna from the Fowkes Formation places a late Bridgerian age
on the Sillem and Bulldog Hollow members (Nelson 1973). Small mammals
were most abundant in the Fowkes, especially primates and rodents. The
marsupials were characterized by _Peratherium_ which was a small,
probably arboreal, opossum-like animal. _Peratherium_ was but one stage
in a slow and relatively conservative evolution of opossums that began
in the Cretaceous.

The condylarths decreased in importance during Fowkes time and were
gradually replaced by more advanced ungulates, the artiodactyls and
perissodactyls. Only the small _Hyopsodus_ survived into the Bridgerian
of the Fossil Basin. The horses continued to differentiate as evidenced
by the appearance of _Orohippus_, an animal similar to _Hyracotherium_,
but with minor dental differences. The primitive tapirs were represented
by _Hyrachyus_, a medium-sized herbivore which was nearly as closely
related to the rhinoceroses as it was to the tapirs.

The primates were also diverse in Fowkes time; however, this group began
to decline in importance following Bridgerian times. As a group, the
primates were restricted mainly to warm and forested environments. The
lemur-like _Notharctus_ and the tarsier-like _Omomys_ and _Hemiacodon_
were common in the Fowkes. _Hemiacodon_ was typical of the late
Bridgerian (an index fossil).

The true carnivores, represented by _Miacis_, remained small in size,
perhaps to avoid competition with the larger surviving creodonts.

Insectivores from the Fowkes Formation are abundant and varied.
Unfortunately, they are known almost exclusively from teeth. Fowkes
insectivores include the hedge-hog-like _Nyctitherium_, _Talpavus_, and
_Scenopagus_. _Apatemys_ is an insectivore of uncertain affinities. The
feeding habits of these early insectivores were probably similar to
those of contemporary insectivore species (moles and shrews); however,
there is some evidence to suggest many of these small creatures may have
been semi-arboreal.

One of the most unusual of middle Eocene creatures is the giant
_Uintatherium_. This was a large browsing animal, about the size of a
rhinoceros. What was most unusual about _Uintatherium_ was its skull
which was large and strongly built. The large upper canines were
apparently formidable defensive weapons. The skulls of the males bore
six prominent bony protuberances which grew from the frontal region of
the skull. The function of these structures is unknown. They may have
been of use for defense.

During Bridgerian times, the rodents underwent an explosive adaptive
radiation which ultimately may have led to the near extinction of the
less well-adapted multituberculates. The diversity of Bridgerian rodents
from Fossil Basin is impressive. Species of _Leptotomus_, _Paramys_,
_Thisbemys_, _Microparamys_, _Sciuravus_, _Mysops_, and _Pauromys_
demonstrate that a wide range of adaptations of environments was
exploited by the rodents. This diversity in life habits has been a mark
of rodent evolution from the middle Eocene to the present day.

The above descriptions of the known mammal fossils from the Fossil Butte
area are intended to demonstrate the varied life forms that once existed
in southwestern Wyoming. Further collecting will almost certainly expand
the number of fossil mammal species from Fossil Basin and increase our
knowledge of the succession of ancient environments which prevailed at
different times in the Fossil Basin area.

               TABLE 2. _Mammals known from Fossil Basin_

  Faunal lists—Middle Paleocene: Torrejonian
    Class Mammalia
  Late Paleocene: Tiffanian
    Genus indeterminant.
  Early Eocene: Graybullian
  Early Eocene: Lysitean
  Early Eocene: Wasatchian (general)
  Late Middle Eocene: Late Bridgerian

                       PALEOECOLOGY AND TAPHONOMY

Paleoecology is the study of ancient biotic communities and their
relationship to the abiotic environment. The conclusions of paleoecology
are reached by studying in detail the fossils and sediments and whatever
relationships exist between them. Its ultimate aim is to build a picture
of the climate, flora, fauna, and topographic setting of an area. For
this reason it borrows heavily from paleontology, paleobotany,
sedimentology, and climatology.

From studying modern environments, it is observed that each has its own
type of sediment. When certain sediments are found in the rock record,
it is generally assumed that they represent environments similar to
modern ones that produce similar deposits. For example, limestone is
forming today in warm, shallow, well-lighted, well-aerated water. When
limestone is found in a rock sequence, it is usually assumed that the
same or similar environmental conditions occurred in that area in the
past. Fossils can further refine the interpretation.

As can be interpreted from foregoing discussions, the sediments
deposited in the Fossil Basin vary from stream and flood-plain fluvial
to lacustrine. At times, material eroded from surrounding uplands was
carried by streams throughout the basin. These rocks are exemplified by
the Evanston and parts of the Wasatch and Fowkes formations. When the
lake appeared, lacustrine sediments, marlstone, and shale were deposited
in the lake, while around the periphery of the basin, fluvial sediments
continued to accumulate.

Plants found in the lake sediments of Fossil Basin tell us much about
the climatic conditions that prevailed during the Eocene. The flora is
quite similar to that now existing in the southeastern United States,
reflecting a warm and humid climate. One of the striking examples is
that of huge palm fronds that occasionally are found in the fish
quarries at Fossil Butte. These seem to be fronds that blew from the
trees into the lake. Soon they became water-logged and settled to the
bottom to be preserved.

The land animals indicate that a rather wide range of ecologic niches
existed over the basin before the lake came into being, around the lake
during its presence, and again all over the basin after the lake
disappeared. Many of the smaller mammals, some of the rodents, and most
of the primates were almost certainly arboreal. The large mammals such
as uintatheres, pantodonts, and tapiroids may have been stream-side or
marsh dwellers. Probably inhabiting the forest floor and feeding on low
bushes and undergrowth were such forms as the condylarths, horses,
artiodactyls, and some of the rodents. Feeding on the rodents and other
smaller mammals were the creodonts and miacids. Tiny shrew-like forms
scampered about the undergrowth and fed upon worms and insects.

Bradley (1963) has estimated possible temperature and precipitation
levels in southwestern Wyoming during the Eocene. Bradley’s modern
analogues were the Gulf Coast and Great Lakes regions. From a series of
calculations, an average annual temperature of 65°F is postulated. This
could have fluctuated greatly in the inland setting of Fossil Basin.
Precipitation amounts were possibly on the order of 30-43 inches
annually. The amount of annual evaporation was also possibly in the
range of 30-43 inches.

It is believed that Fossil Lake was thermally stratified—that is, with
colder, denser water at depth (hypolimnion) and warmer, less dense water
(epilimnion) nearer the surface. The deeper waters probably would have
been devoid of oxygen, hence essentially uninhabited other than by
anaerobic bacteria that survive without oxygen. If the bottom had been
oxygenated, many types of life would have burrowed into the sediment
thus destroying the delicate varves (Fig. 28). The lake was probably
deep enough that wind and wave action did not roil the bottom sediments.

    [Illustration: Fig. 28. A section of shales from the fish beds
    showing thin laminae that are interpreted as varves. Enlarged six

There have been many attempts to interpret the taphonomy (see glossary)
of concentrations of fish at Fossil Butte. Bradley (1948) interpreted
the cause of death and the reason for their preservation as follows:

  In this basin (Fossil, Wyo.) hundreds of thousands of beautifully
  preserved fish are entombed in the varved sediments. Even the delicate
  fin and tail rays and other bones originally held in place only by
  tissue are virtually undisturbed, and even the scales are in place
  almost completely undisturbed. It seems to me that the picture of this
  lake as a thermally stratified water body provides nearly all the
  necessary information to account for the excellent preservation of
  these fish. Only in the stagnant hypolimnion could they have escaped
  being torn to pieces by scavengers or distorted by bottom feeders. It
  is significant that all the well preserved fish are in varved
  sediments. Those in non-varved sediments are a disordered mass of
  broken and chewed up bones.

  The only part of the story lacking now is how the fish died and got
  into the hypolimnion. Limnology offers two possible explanations.
  Sometimes when the surface of a lake gets excessively warm, fish will
  plunge into deep water and might thus penetrate the hypolimnion, be
  overcome by hydrogen sulphide, and also have the gas in their swim
  bladders chilled so that they sank at once to the bottom. Once there,
  only anaerobic bacteria would attack them. The other hypothesis is
  that the thermally stratified lake was suddenly chilled so that it
  overturned more rapidly than the hydrogen sulphide could be oxydized
  and so killed off large numbers of fish. This seems a little more
  probable as the fossil fish are of all ages and sizes.

The fish in these quarries have been collected since the 1870s but
mainly by commercial collectors. Most museum collections were purchased
from these commercial collectors, hence they consist almost entirely of
perfectly preserved fish, as poorly preserved specimens would be
discarded by the collectors as of no monetary value. The result has been
that most people, including Bradley, were misled into believing all of
the fish in the varved sediments of the quarry were perfectly preserved.
As nearly as can be determined, the first attempt to systematically
collect and study this concentration of fish was that by paleontologists
from the University of Wyoming (McGrew 1974). This work threw much new
light on the occurrence and made available much new data that permit new

Shales of the Green River Formation in general have been described by
Bradley (1931) and he specifically mentioned those of the fish layer in
the Fossil Basin as follows: “Plate 1 shows a thin section of the varved
marlstone in the small, unnamed Green River Lake west of Gosiute Lake,
where the varves are better developed. Each varve or annual deposit,
consists of a layer of microgranular carbonates and a thinner, dark
layer of organic matter” (Bradley 1948:645). The X-ray diffraction
analyses performed by John Ward Smith of the Laramie Energy Research
Center showed that the shales of the “fish layer” consist predominantly
of calcite (roughly 60%), aragonite and dolomite (approximately 20%),
and quartz (up to 10%).

Although fish are numerous throughout the thickness of this “fish
layer,” there are three laminae that contain so many fish that it is
almost certain that they represent catastrophic mass mortalities. Two
are made up primarily of _Priscacara_ and one consists almost
exclusively of _Knightia_.

X-ray photos show that a rather high percentage of the fish in the
shales are not perfectly preserved but have undergone varying amounts of
disarticulation. There appears to be an orderly sequence of stages of
decomposition—from essentially perfect articulation to total
disarticulation. Disarticulation first appears in the most anterior
vertebrae. From there it rapidly proceeds anteriorly into the head
region and appears to do so posteriorly at a slower rate. In many
specimens the head and anterior half of the body are completely
disarticulated, while the posterior part of the body shows no
disarticulation whatever.

It is assumed that after the dead fish settled to the bottom of the
lake, external anaerobic bacteria found access to the “innards” of the
fish via the opercular opening. This would account for the first sign of
decomposition and disarticulation being just back of the head.

In the blocks of shale covered by X-ray there were 385 fish. For
convenience, these were classified into six groups, group I showing no
discernable disarticulation and group VI (Fig. 29) showing total
disarticulation. The number and percentage of fish in each group are as

  Group I          223 fish        58%
  Group II          38 fish        10%
  Group III         14 fish         4%
  Group IV          27 fish         7%
  Group V           24 fish         6%
  Group VI          59 fish        15%

    [Illustration: Fig. 29. A partially disarticulated skeleton of a
    large _Priscacara_, Group IV.]

Because of the predominance of completely articulated fish throughout
the quarry and the fact that the fish involved in the mass mortalities
show no disarticulation, it seems probable that some connection might
exist between the death of the fish and conditions of the lake bottom
that would cause their perfect preservation. It would seem that rapid
burial might be the most obvious reason for excellent preservation. Thus
any factor or combination of factors that would cause rapid
precipitation of carbonates and also would cause mortality of fish would
satisfy our requirements. One obvious factor that could, at least
theoretically, fulfill these requirements would be an annual bloom of
blue-green algae that are known to be toxic to fish (Prescott 1948).
Such blooms usually occur during the warmest part of the summer when
CaCO₃ is least soluble. By extracting CO₂ from the water, these algae
are known to cause precipitation of CaCO₃. Thus we have a possible cause
for some annual fish kill and perhaps an occasional superbloom that
would bring about a catastrophic mass mortality. The highest mortality
of fish then might have occurred during late summer algal blooms and at
this time also would occur the most rapid precipitation of CaCO₃.

It is not known how much deposition of CaCO₃ would be required to
protect a fish from disarticulation. It may be that a very thin layer,
especially if mixed with organic ooze, might provide an effective seal
to slow decomposition and prevent disarticulation. If sufficient CaCO₃
was precipitated and deposited to cover the fish that died during this
period, one might expect perfect preservation. Such fish would fit into
group I. Those fish that died just after this period might lie exposed
on the lake bottom for most of a year and be subject to disarticulation.
If little or no deposition took place during the rest of the year, fish
that died just after the period of deposition should be the most
completely disarticulated and fit into group VI. During the fall,
winter, and spring, fish that died of attritional mortality would be
disarticulated according to the length of time they lay on the bottom
prior to the next period of deposition.

If the foregoing is true, one should be able to determine the
approximate time of year fish died by the degree of disarticulation. One
might assume that blooms of blue-green algae and hence precipitation of
CaCO₃ would take place sometime during August and/or September. Thus we
should expect the most fish and those most perfectly preserved to have
died during this time period (group I). Those fish that died in October
and/or November should be the most completely disarticulated (group VI),
and those that died in June or July should show only a slight degree of
disarticulation (group II). The distribution of the stages of
disarticulation seems to fit almost exactly the pattern that one would
expect if this interpretation is correct.

Gunter (1947) has shown that annual periods of excess salinity in Texas
lagoons cause an annual increase in the death of fish and occasionally a
catastrophic mass mortality. Because Lake Gosiute is known to have been
saline, it might be assumed that somewhat similar chemical conditions
prevailed in the Fossil Lake. It is not known definitely that the two
lakes were ever connected but if they were, it was most probably a
narrow connection near the southern end of Fossil Lake and probably a
rather temporary connection.

That a rather long period of aridity occurred in the general region is
demonstrated by various depositional features, primary structures, and
salt deposition in Gosiute Lake in a part of the Green River Formation.
These deposits appear to have settled down during Lostcabinian (late to
early Eocene) times. Because the Wasatch Formation immediately
underlying the Fossil Butte Member of the Green River Formation in the
Fossil Syncline Basin is Lysitean (mid to early Eocene), it is probable
that the fish deposits are of Lostcabinian age. Thus it may well be that
the long period of aridity occurred during the deposition of the fish
beds. While the Fossil Lake probably never reached the high degree of
salinity present in Lake Gosiute, it was probably sufficiently saline
that periods of excessive evaporation could increase the salinity enough
to contribute to the mortality of fish and occasionally cause a
catastrophic mass mortality such as those described by Gunter. The
presence of aragonite and dolomite in the shales suggests that at least
some of the carbonate deposition might well have been because of
excessive evaporation and high concentrations of carbonates (Smith 1974,
pers. comm.).

At the University of Wyoming, an attempt is being made to interpret
scales of fish from the quarries. In a number of specimens annuli can be
observed and circuli counted. Although removal of scales for study is
extremely difficult, a number has been removed and photographed with a
scanning electron microscope. In Lake George, Florida, black crappie
develop annuli during January, February, March, and April (Huish 1954).
Climatic conditions in north central Florida may be similar to those
that existed in western Wyoming during the early Eocene. Thus annuli may
have developed at the same time of year. By counting the number of
circuli between the last annulus and the edge of the scale, it should be
possible to determine the approximate time of death of a fossil fish. If
this should correlate with the degree of disarticulation, a check on
this interpretation should be possible. Sufficient data are not yet
available, however, for the results to be conclusive.

It is not intended to suggest that the conditions outlined above could
account for all of the fish concentrations in the Green River Shales. In
certain shales in the Green River Basin, for example, the concentrations
of _Knightia_ in nonvarved shales appear to have been deposited in quite
shallow water. These fish are extremely well preserved but were
obviously laid down under conditions quite different from those at
Fossil Butte. Much study of these occurrences will be necessary before
interpretations are possible.

The story, as told here, should make it clear that the earth is
ever-changing. What was once a beautiful, deep lake in an area of lush
tropical forests is now a dry, sagebrush desert. What was once an area
in which sediments were accumulating is now an area of erosion,
occasioned by a broad uplift of the region near the end of Tertiary

It is hoped that this brief story of the geologic history of Fossil
Butte National Monument will give the reader some appreciation of the
geologic complexity not only of the monument itself but of the
surrounding area as well. Certainly such knowledge will add to the
enjoyment of a visit to the monument.

It may seem that geologists have all of the answers. This, however, is
not so. Interpretations are made on the basis of available evidence.
Each time a geologist studies an area more geologic data and more
evidence become available and our interpretations become a little more
accurate. It will be many years before all details of this fascinating
history will be known.


  ANAEROBIC. Usually in reference to organisms that can live without
  ANGULAR UNCONFORMITY. Two rock layers which are not parallel; the
          underlying older layer dips at a different angle (usually
          steeper) than the younger top strata.
  ANNULI. Marks on fish scales produced by periods (usually winter) of
  ANTICLINE. A fold in stratified rock with the strata sloping downward
          in opposite directions from the fold crest.
  ARAGONITE. A carbonate mineral with specific characteristics.
  AUTHIGENIC. A mineral (such as quartz or feldspar) which is formed
          after the deposition of a sedimentary layer in which it
  BENTONITE. A light-colored, soft, porous rock formed from the minute
          clay crystals of eroded volcanic ash. It has the
          characteristic of swelling when wet (water absorption) and
          contracting when dry.
  BRACKISH. A condition in a body of water in which the salinity (salt
          level) is below that of sea water, but higher than that of
          fresh water.
  CARBONACEOUS. Rock or sediment which contains carbon or altered
          organic material such as coal.
  CHERTY. Containing chert: a dull-colored, flint-like quartz often
          found in limestone.
  CIRCULI. Ridges on fish scales produced during growth of the scales.
  CLAST. Rock fragments which are the result of weathering of a larger
          rock mass.
  CLASTIC. Rocks that consist of particles derived from pre-existing
          rocks or minerals.
  CLAYSTONE. An indurated clay without the lamination or fissility of
  CONGLOMERATE. A coarse-grained sedimentary rock composed of fragments
          larger than 2 mm in diameter in a fine-grained matrix.
  CROSS-BEDDING. An internal structure in sedimentary rock in which the
          upper sedimentary layer runs across the grain of the main bed;
          it is caused by changing currents depositing sediment across
          the grain of the original deposits.
  DIAMICTITE. A sedimentary rock containing a wide range of particle
  DIP. The downward inclination of a rock layer; the vertical angle is
          determined by its relationship to a horizontal plane.
  DOLOMITIC. Containing a measurable amount of the mineral dolomite; a
          mineral consisting mainly of magnesium carbonate and calcium
  FACIES. Lateral variations in the appearance or composition of a rock
          layer. The variations can be lithologic or paleontologic.
  FAULT. A fracture in the earth’s crust along which displacement
          (movement) has occurred.
  FLUVIAL. Pertaining to a river or rivers. Fluvial sediments are those
          transported and deposited by stream action.
  FORMATION. A rock layer that is mappable; has a distinctive lithology
          or series of lithologies. A mappable sequence of uniform or
          uniformly varying rocks.
  GASTROPODAL. A rock containing an abundance of gastropods.
  HOGBACK. A long, narrow, sharp-crested ridge formed by the outcropping
          edges of steeply inclined resistant rocks.
  IGNEOUS ROCKS. A rock or mineral that has solidified from molten or
          partly molten material.
  INTRUSIVE. Igneous rock formed by the forcing of molten material into
          a pre-existing rock.
  IRONSTONE. A rock composed of various iron minerals that accumulated
          during or shortly after deposition of the enclosing sediments.
  LACUSTRINE. Pertaining to, produced by, or formed in a lake or lakes.
  LATERITIC. Containing laterite: a red, porous material usually
          developed in a tropical to temperate climate. It is a residual
          or end-product of weathering.
  LIGNITE. A brownish-black coal that is intermediate in coalification
          between peat and subbituminous coal.
  LITHOLOGY. The scientific study of rocks: composition, texture, color,
          origin, etc.
  MAGNETITE. A black, opaque mineral that is strongly magnetic.
  MARLSTONE. An impure limestone.
  METAMORPHIC. Rocks whose structure has been changed by pressure, heat,
          chemical reaction, etc., such as limestone into marble.
  MUDSTONE. An indurated mud without the lamination or fissility of
  OPERCULAR OPENING. The gill opening of fish.
  OSTRACODAL. A rock, usually a limestone, that contains an abundance of
          the small crustacean, ostracods.
  OVERTHRUST. A low-angle thrust fault of large scale, usually measured
          in miles.
  PALEOLIMNOLOGY. The study of ancient lakes.
  PAPER SHALE. A form of finely laminated shale that weathers into
          extremely thin, curled flakes.
  PHOSPHATIC. Containing phosphates.
  PHYTOPLANKTON. Floating microscopic plant life that occurs abundantly
          in lakes and oceans.
  PLATY. Rocks (sandstone or limestone) which separate into small slabs.
  PORCELLANITE. A dense cherty rock resembling porcelain.
  PUDDINGSTONE. A conglomerate consisting of well-rounded pebbles and
          cobbles sparsely packed in a fine-grained matrix.
  SILTSTONE. An indurated silt without the lamination or fissility of
  STRATA. Rock layers of distinct composition and origin.
  SYNCLINE. A fold in stratified rock in which the strata slope up from
          the axis of the fold forming a v opposed to anticline.
  TAPHONOMY. The branch of paleoecology which deals with the change from
          living animals to fossils.
  TECTONIC. The forces which result in structural changes in the earth’s
  THRUST FAULT. A fault in which an upper segment of rock (hanging wall)
          moves upward at a low angle (less than 45°) relative to a
          lower segment (footwall).
  TONGUE. A rock unit that wedges into, but disappears within, another
          rock unit.
  TUFFACEOUS. Sediment that contains up to 50% volcanic ash or dust.
  UNCONFORMITY. A substantial break or gap in the geologic or
          stratigraphic record.
  UNGULATES. Hooved mammals.
  VARVE. A set of rock laminae in which different types of sediment were
          deposited in the winter and in the summer. Thus a couplet of
          each sediment type would represent the deposition of one year.
  WELL SORTED. A rock in which nearly all of the sediment particles are
          of one grain size.
  ZOOPLANKTON. Floating microscopic animal life that occurs abundantly
          in lakes and oceans.


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This report, which was prepared under National Park Service Contract CX
6000-3-0076, leans heavily on the publications of those many people who
have studied various geologic aspects of the Fossil Basin. We would like
to cite particularly Doctors Steven Oriel and Joshua Tracy of the United
States Geological Survey, whose studies have clarified the stratigraphic
relationships of the sediments involved. Dr. D. L. Blackstone, Jr., of
the University of Wyoming supplied information from his yet unpublished
interpretations of the complicated structure of the Idaho-Wyoming thrust
belt. Mr. Tom Bown helped with the structural part of the report and
critically reviewed other parts. The drafting, except for Figs. 3 and 9,
was done by Mr. Evan Groutage. Mrs. Elaine Hertzfeldt, secretary for the
Department of Geology, aided greatly in preparing this manuscript.

                                                          Paul O. McGrew
                                                      Michael Casilliano


                          Transcriber’s Notes

—Silently corrected a few typos, including inconsistent spelling of

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by

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