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Title: The Geologic Setting of the John Day Country - Grant County, Oregon
Author: Thayer, Thomas Prence
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.

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                        GEOLOGIC SETTING OF THE
                           JOHN DAY COUNTRY,
                          GRANT COUNTY, OREGON


      UNITED STATES DEPARTMENT OF THE INTERIOR/ GEOLOGICAL SURVEY

[Illustration: The town of John Day, the John Day River valley, and the
Strawberry Range, looking south and southeast. The tailings piles left
by gold dredges have been levelled for building sites since the
photograph was taken in 1946.]

One of the Pacific Northwest’s most notable outdoor recreation areas,
the “John Day Country” in northeastern Oregon, is named after a native
Virginian who was a member of the Astor expedition to the mouth of the
Columbia River in 1812.

There is little factual information about John Day except that he was
born in Culpeper County, Va. about 1770. It is known also that in 1810
this tall pioneer “with an elastic step as if he trod on springs” joined
John Jacob Astor’s overland expedition under Wilson Price Hunt to
establish a vast fur-gathering network in the western states based on a
major trading post at the mouth of the Columbia River.

The expedition arrived in the vicinity of the Grand Tetons, in what is
now Wyoming, in September 1811, and with the onset of winter met
disaster along the Snake River. When they ran out of food near the
present site of Twin Falls, Idaho, Hunt divided the expedition into four
parties to seek food and a feasible route through the canyons. As
described in Washington Irving’s _Astoria_, the party that included John
Day became widely separated from the others; experienced terrible
hardships while wintering with Indians near Huntington; and was
eventually reduced to just John Day and Ramsey Crooks. By mid-April of
the following year, Day and Crooks reached the junction of the Columbia
and Mah-hah Rivers, where a band of Indians took everything they had,
including their clothes. Because of this incident, the Mah-hah River was
renamed the John Day. Returning up the Columbia to seek help from
friendly Indians, they were rescued by a party of trappers in canoes,
and finally reached Astoria on the 11th of May 1812.

Although the first discovery of gold in Oregon reportedly was made in
1845 on one of the upper branches of the John Day River by a member of
an immigrant train, the settlement of the John Day Country really began
in 1862, when gold was discovered in Canyon Creek just above Canyon
City. Since then, possibly $30,000,000 worth of gold has been mined,
mostly from gravels in and along Canyon Creek and along a 10-mile
stretch of the John Day River. Lumbering and ranching are now the
principal industries of the region.

The growth of tourism in Oregon and Grant County and the accompanying
increase of interest in geology have stimulated the preparation of this
leaflet. The Grant County Planning Commission and State Department of
Geology and Mineral Industries have cooperated most cordially in the
program to better inform interested visitors about the geology of the
country they are seeing.



                           _GEOLOGIC HISTORY_


The John Day Country of today covers an area of 4,000-5,000 square miles
in the southwestern part of the Blue Mountain region of Oregon, and is
in the borderland between two major geologic provinces. One, to the
north, is the Columbia Plateau which consists of flat or gently tilted
flows of basalt covering about 100,000 square miles. The other, to the
south, is the Basin and Range Province which extends into Mexico, and is
characterized by a wide variety of complexly folded and faulted rocks.
The Strawberry-Aldrich Mountain Range, rising from 7,000 to 9,000 feet
in altitude along the south side of the John Day River valley (see
Figure 13 on page 21), is part of a 150-mile long, east-trending
mountain chain that locally separates the two provinces.

Many features that record events in the geologic history of northeastern
Oregon may be seen in the rocks along the paved highways within the John
Day Country. A road log describing some of these features is included as
a part of this booklet. The locations of principal points of interest,
keyed in the road log to state highway mile posts, are shown on the map
accompanying the road log.

The known part of the geologic history of the John Day Country began
with lava flows and deposition of volcanic ash, sandstone, shale, and
small lenses of limestone in a late Paleozoic sea more than 250 million
years ago. Sometime between 200 and 250 million years ago, peridotite
and gabbro (dark, magnesium-rich varieties of igneous rock) rose from
great depths and, as molten material (_magma_), invaded the preexisting
marine deposits. These igneous rocks now form the core of Canyon
Mountain and can be seen along the precipitous walls of Canyon Creek.
Masses of chromium ore (_chromite_) were carried upward with the molten
material. After erosion had exposed the peridotite and gabbro, the area
was submerged again and, during Late Triassic and Early Jurassic time
(about 180 million years ago), the Aldrich Mountain area was part of a
seaway into which lavas flowed. Thousands of feet of volcanic ash from
active volcanoes accumulated in the sea, and between eruptions great
thicknesses of mudstone and shale were deposited. The cuts along U. S.
Highway 395 between Canyon Creek and Bear Valley are in these rocks.
During part of this volcanic activity, Canyon Mountain stood as a high
landmass, but finally it too was deeply buried. Again the region emerged
and probably was dry land during the last half of Jurassic time (135 to
150 million years ago).

In Early Cretaceous time, molten material was intruded to form the
granitic rocks in the Aldrich Mountains and near Dixie Butte, northeast
of Prairie City. The gold veins in Canyon Mountain and in most of the
Blue Mountain region probably were formed at that time by solutions from
the granitic magma. Scattered patches of fossiliferous sandstone and
conglomerate like that in Goose Rock (see Figure 5 on page 10) show that
the sea encroached on the Blue Mountain region briefly during Cretaceous
time after erosion had exposed the granites. The shoreline was not far
east of the John Day area.

For the past 60 million years, eastern Oregon has been a land of
volcanoes, mountain building, and erosion. After the retreat of the
Cretaceous sea and an undefined period of erosion, volcanic eruptions
from widely scattered centers during the Eocene Epoch buried the region
under several thousand feet of volcanic rocks which now form the Clarno
Formation; locally these rocks consist mostly of andesitic lava flows
and coarse mud-flow breccias. The Clarno Formation was extensively
folded and faulted and deeply eroded before another series of volcanoes
in and somewhat east of the Cascade Mountains erupted rhyolitic ash that
was blown eastward and deposited as the John Day Formation during the
Oligocene Epoch. The John Day Formation appears to have been restricted
to a lowland area which geologists today call the John Day basin,
located between the present site of the Cascade Mountains and the
ancestral Blue Mountains. The Clarno and John Day beds in this basin are
world famous, having yielded thousands of bones, leaves, and pieces of
petrified wood that were buried and preserved by the volcanic ash in a
manner similar to the burial of Pompeii by the eruption of Mt. Vesuvius
in 79 A. D. Archaic large mammals such as the titanotheres, and small
ancestors of modern mammals such as the 4-toed forest horse _Eohippus_,
which was the size of a fox terrier, roamed subtropical forests during
the Eocene Epoch. Many important links in the evolutionary chain of
mammals have been found in the John Day fossil beds; for example, the
3-toed horse _Mesohippus_, which was a forest dweller the size of a
sheep, had teeth for browsing instead of grazing.


The modern day landscape began to take form in Middle Miocene time, when
the basalt flows which cover most of the area and are exposed in the
walls of Picture Gorge (hence their name, Picture Gorge Basalt) buried
the landscape formed on the John Day Formation and older rocks. The
basalt erupted from long cracks or fissures in the Earth’s crust and
formed floods of very fluid lava which flowed for distances up to one
hundred miles. These basalts are recognized by geologists to be of a
distinct type, commonly called plateau or flood basalt. Basalt, frozen
in the fissures, forms dikes that commonly weather out above the
adjoining softer rocks, as shown in the picture of Kimberly Dike on page
16.

While the Picture Gorge Basalt was flooding most of the John Day basin,
several volcanoes in the vicinity of Strawberry Mountain erupted
andesitic to rhyolitic lava and ash, and built up cones similar to Mt.
Hood and the other high peaks of the Oregon Cascades. Geologists have
named the remnants of these cones the Strawberry Volcanics. Because
these volcanoes rose several thousand feet above the top of the Picture
Gorge Basalt, ash and erosional debris from them washed out over the
basalt; these materials constitute the Mascall Formation. Although the
mountains were timbered, the lowlands probably were open and grassy.
Bones and teeth of a pony-size three-toed horse, _Merychippus_, have
been dug from the Mascall Formation. When the eruptions ceased in Early
Pliocene time (about 10 million years ago), the entire Blue Mountain
region probably resembled the eastern part of the Cascade Mountains in
Oregon today, particularly the area between Mt. Hood and Crater Lake.

The present Strawberry-Aldrich Mountain range and the ridges and valleys
north of it were formed when the Earth’s crust buckled and broke under
strong compressive forces from the north and south. Partly by bending or
folding, and partly by breaking along the John Day and other faults, the
Strawberry-Aldrich Mountains were gradually raised a mile and a half to
two miles above the valley to the north. The floor of the main John Day
River valley was filled with gravels eroded from the rising mountains
and, as the folding and faulting and erosion slowed, an extensive,
broad, gently sloping surface was formed on top of the valley fill.
While these gravels (called the Rattlesnake Formation) were
accumulating, however, a great volcanic eruption spread an ash flow more
than 100 feet thick over the entire length of the John Day valley floor;
remnants of this flow form prominent rim-rocks between the town of John
Day and Picture Gorge. Bones of _Hipparion_, a horse the size of a pony
which had feet and teeth like modern horses, have been found in the
gravels under the ash flow.



                        _THE PRESENT LANDSCAPE_


Differences in the rates of erosion of various rocks were as important
as folding and faulting in forming the topographic features in the John
Day Country. The chief agents of erosion are chemical weathering
processes that eventually break down or decompose all rocks, and running
water which carries away the weathered or partly weathered and broken
material. Picture Gorge is narrow because the basalts in the walls are
very resistant to weathering and break along vertical joints into large
blocks that do not move easily. In contrast, the John Day River has cut
a broad, flat-floored valley in the Mascall Formation at the south end
of Picture Gorge because the ashy beds weather to fine clay which is
easily washed away.

Landslides have marked the face of the John Day Country. Large rock
masses commonly slide where steep cliffs form in soft rocks that are
capped by hard resistant rocks. When the cliff face becomes too high and
steep, the soft beds give way and large blocks or masses slide downward,
usually tilting backward as they move, as shown in the diagram of
Cathedral Rock on page 15. Cathedral Rock and two small landslides north
of the river, about two miles east of John Day, show this classic form.
Many landslides, however, are just jumbled, hummocky masses of slumped
material. Large-scale landsliding in the John Day Formation under the
Picture Gorge Basalt is colorfully displayed along the river for 8 miles
north of Picture Gorge. Some landslides occur suddenly, but many move
forward only as fast as the toe or lower end is eroded away, as at
Sunken Mountain.

Glaciers have sculptured the principal valleys that lie above an
altitude of about 5,000 feet. Moving ice, hundreds of feet thick,
plucked out semi-circular amphitheaters called _cirques_ at the heads of
the valleys. The 2,000-foot cliffs above Little Strawberry Lake were
formed this way. Rocks held in the ice, like the teeth of a giant rasp,
ground off irregularities and widened the valley bottoms to a broad U
shape as the glaciers moved down the valleys. The effectiveness of ice
scour can be seen by comparing glaciated Strawberry Creek above
Strawberry Lake (as shown in figure 15 on page 22) with unglaciated
Picture Gorge or with Canyon Creek just above Canyon City.



                    ROAD LOG OF THE JOHN DAY “LOOP”


NOTE: Mileages at lettered stops on the map (Fig. 1) refer to nearby
mile posts. These are not in numerical order because the route covers
parts of four different numbered highways. By taking the tour clockwise,
the traveler will be at higher altitudes later in the day.


                                A. 155.4

_Holliday Rest Area._ The main John Day fault, which here is buried
under river gravel, is believed to be just south of the rest area. A
parallel step fault (Fig. 2) cuts from left to right across the southern
slope of Mt. Vernon Butte about where the juniper trees thin out. Flows
of Picture Gorge Basalt in the upper part of the butte slope down to
valley level on your left. Rocks in the foreground and to the right are
rudely bedded volcanic breccias of the Clarno Formation. The Clarno
Formation has been raised as much as 250 feet by movement along the
fault to position it against the basalt as indicated in the diagram. The
fault can be seen best in afternoon light in the ravine to the right,
just south of some brick-red layers in the basalt.

Movement on the main John Day fault to the south appears to have raised
the rocks at least 1,000 feet, so the two faults have stairstepped the
rock layers. The main John Day fault has been traced about 80 miles.

[Illustration: Fig. 2.—View of Mount Vernon Butte and diagram of
faulting along its south slope.]

[Illustration: ]

[Illustration: Fig. 3.—View of the John Day fault in Mascall Formation
at the mouth of Fields Creek, and its relation to the structure of the
John Day River valley.]


                                B. 144.0

_Fields Creek Road._ In the road cuts just south of the highway, the
John Day fault (Fig. 3) passes through the Mascall Formation where the
slope or dip of the beds changes abruptly from gently southward to
steeply northward. The fault continues westward under the floor of the
valley. Fossil leaves and snails can be found in the beds south of the
fault, and 1,000 feet farther south vertical Picture Gorge Basalt flows
are exposed.

To the north across the valley at the White Hills, beds of the Mascall
Formation have been dropped down against the Picture Gorge Basalt along
the Belshaw fault. The White Hills are a well-known locality for
collecting fossil leaves.


                           C. 138.5 to 135.0

_Vertical Ribs._ The prominent vertical ribs, visible south of the river
in steep slopes below the high bench (pediment), are flows of Picture
Gorge Basalt tilted vertically in the north limb of the Aldrich Mountain
anticline. The John Day fault follows the base of the steep front in
which the ribs are exposed.


                                D. 132.6

_Volcanic Ash Flow._ The rimrock north of the John Day River between
here and Dayville is a volcanic ash flow that erupted about five million
years ago as red hot pumice highly charged with gas. A rapidly moving
incandescent cloud probably filled the ancient John Day River valley and
deposited ash to a depth of more than 100 feet over a distance of 60 to
70 miles. As shown in Figure 4, the ash flow blankets about 200 feet of
gravels that had been deposited in the valley. The rimrock and the
gravels above and below it constitute the Rattlesnake Formation.

[Illustration: Fig. 4.—View northwest across the John Day River valley
toward Picture Gorge.]


                                E. 126.0

_Picture Gorge._ Visible to the left of Picture Gorge, from north around
to west, in order of their deposition and geologic age from oldest to
youngest, are the Picture Gorge Basalt, Mascall Formation, and
Rattlesnake Formation (Fig. 5). The Picture Gorge Basalt flows and ashy
beds of the Mascall Formation were tilted southward together and eroded
before the Rattlesnake Formation was laid down horizontally across them.
Picture Gorge and the present valley were then cut by the John Day River
after the Rattlesnake Formation had been tilted in its turn. The five
benches or terraces on the basalt just east of Picture Gorge mark
temporary halts in downcutting of the John Day River.


                                F. 122.0

_Thomas Condon Viewpoint, John Day Fossil Beds State Park._ From Picture
Gorge to the cliffs opposite, the Picture Gorge Basalts and varicolored
ash beds of the John Day Formation rise together nearly 2000 feet. The
basalt that caps Sheep Rock is an erosional remnant. The lower beds in
the John Day Formation are colored red by clay eroded from thick soil on
the Clarno Formation, which is exposed in the farthest red hill. The
soil was formed by tropical weathering some 30-35 million years ago,
before the John Day Formation was laid down. Faulting on a small scale
is illustrated in Sheep Rock, where the thick olive-drab ash flow in the
middle of the John Day Formation is offset 75-100 feet (Fig. 6). The
fault slopes about 45° eastward. About two miles downstream, large-scale
movement on two faults has dropped the basalt flows in Middle Mountain
2000-2500 feet. One of the faults follows along the upstream base of
Middle Mountain (Fig. 6).

[Illustration: Fig. 5—North-south section along the John Day River
through Picture Gorge and Middle Mountain.]

[Illustration: Fig. 1.—Geologic map of the John Day Country showing log
route.]

                         [Higher-Resolution Map]


                                G. 118.7

_Munro Area, John Day Fossil Beds State Park._ The valley of the John
Day River has been widened to nearly five miles by erosion in the John
Day Formation. Large tilted slide blocks of the John Day Formation and
basalts jumbled together show how important landsliding of soft beds
under hard rocks can be in widening valleys.

[Illustration: Fig. 6.—Sheep Rock from Thomas Condon viewpoint.]

Here one can appreciate the regularity and extent of basalt flows of the
flood or plateau type, which form the Columbia Plateau. Individual flows
have been traced 100 miles. Travelers will see few other rocks between
here and The Dalles, Wenatchee, Pendleton, or Spokane as they cross
parts of the Columbia Plateau.


                                H. 116.2

_Cathedral Rock._ The bluff called Cathedral Rock is the front face of a
large block of the John Day Formation that has slid from the west (Fig.
7). Inside the next horseshoe bend downstream a large mass of basalt is
tilted down against Cathedral Rock. From the highway 1.1 miles farther
north one can see the side of the tilted block along the river, and the
same two prominent red and olive-drab ash layers in the high bluff from
which the block slid. The horseshoe bend was formed as the river was
pushed eastward by the nose of the landslide.

[Illustration: Fig. 7.—View of Cathedral Rock and diagram of
landsliding.]

[Illustration: ]


                                J. 107.0

_Kimberly Dike._ The low bluff across the river is formed by a vertical
dike of basalt which is about 60 feet wide. This dike cuts through the
John Day Formation and, farther north, the Picture Gorge Basalt (Fig. 8
). It crosses the river valley diagonally and can be traced nearly four
miles. The dike is formed of once-molten rock that “froze” in a fissure
which was a channelway for lava that fed a flow on the earth’s surface.
Many similar dikes are visible along the road east of Kimberly. The
basaltic magma is believed to have originated at depths of 40 miles or
more within the part of the earth called the mantle.


(At 105.3 turn east on State Highway 402 along the North Fork of the
John Day River)

[Illustration: Fig. 8.—Basalt dike two miles south of Kimberly.]


                                 K. 5.0

_Basalt Dike._ A basalt dike 15 feet wide cuts basalt flows in bluffs
north of the road and forms a wall 20 feet high in places; it is visible
also across the river.


                                L. 11.2

_Parallel Dikes._ Two prominent parallel dikes, among others, form
crests of hogbacks south of the river. A small tapering dike cuts pink
and white beds of the John Day Formation in the river bluff. This dike
is the southwest end of an irregular intrusive mass of basalt in which
the river has cut a steep-walled gorge.


                                M. 13.2

_Basalt Intrusion._ The road is cut through 300 feet of basalt intruded
into the John Day Formation. Both contacts are well exposed. The
basalt—when molten—baked and reddened 3 to 6 inches of the adjacent
beds.

(At 13.9 the highway crosses the North Fork of the John Day River and
follows the Cottonwood Creek Valley.)


                                N. 18.0

_Irregular Dikes._ Above the highway several small irregular basalt
dikes cut the white beds of the John Day Formation. Parts of the largest
dike are 10 to 15 feet high.


                                O. 20.1

_Cottonwood Creek Valley._ The view northwestward down Cottonwood Creek
toward Monument (Fig. 9) exemplifies the development of broad valleys by
erosion in soft beds of the John Day Formation under the gently warped
Picture Gorge Basalt. To the south, the valley ends against massive
rocks of the Clarno Formation which were raised as a block by movement
along the Hamilton fault. The red beds are in the lower part of the John
Day Formation. Note the contrast between the irregular massive intrusion
in the valley bottom west of Monument and the thin regular basalt flows.

[Illustration: Fig. 9.—Northwest view down Cottonwood Creek toward
Monument.]

On your right, the irregular contact between the John Day Formation and
Picture Gorge Basalt reveals an ancient landscape buried under lava
flows (Fig. 10). Some of the flows wedge out against former hillsides
and one, marked by the dry falls, fills an old valley.

For the next two miles, to the Sunken Mountain viewpoint, the road winds
through landslides in the John Day Formation and Picture Gorge Basalt.

[Illustration: Fig. 10.—Ancient landscape on John Day Formation buried
under Picture Gorge Basalt.]


                                P. 22.0

_Sunken Mountain._ A small landslide in the lower part of the John Day
Formation is called Sunken Mountain. When the valley wall was
over-steepened by normal stream erosion, the jumbled material in the
lower part broke away and slid down from the steep bare slopes above.
Absence of tilted trees indicates that the slide is not very active now.
The bare “badland” slopes are being eroded by rain wash. The cliffs of
the John Day Formation, which the road climbs half a mile farther east,
are the result of rapid but normal headward erosion by the creek.
Eventually, because of its lower elevation and steeper gradient, this
branch of Cottonwood Creek will intercept and behead Deer Creek just
east of Hamilton. A photogenic perspective view of the future stream
piracy can be seen from the road on the ridge just south of Sunken
Mountain, about a mile and a half from the highway.


                                Q. 26.4

_Long Creek Mountain._ An uplifted block of Picture Gorge Basalt
1400-1500 feet thick forms Long Creek Mountain; Round Basin is eroded in
the John Day Formation on which the basalt rests (Fig. 11). The base of
the basalt can be seen in road cuts on either side of Basin Creek. The
Hamilton fault follows the gulch to the left just below the parking
area, goes up the tree-filled gulch across Basin Creek, along the low
ridge at the northeast edge of Round Basin, and then along the northern
foot of the mountain. The small slab of basalt south of the fault in
Basin Creek has been tilted about 10° north by downward drag along the
fault. The Hamilton fault system extends about 15 miles farther east.


(In Long Creek turn right—south—on U. S. Highway 395).

[Illustration: Fig. 11.—View of Long Creek Mountain and Round Basin, and
diagram of the geologic structure.]

[Illustration: Fig. 12.—North-south section across Fox Valley, showing
the faulted basin structure.]


                                R. 99.5

_Fox Valley._ Down-warped flows of Picture Gorge Basalt dip toward Fox
Valley from all sides to form a basin (Fig. 12). The valley is eroded
out of ashy beds and gravels of the Mascall Formation which fill the
center of the basin to an estimated depth of 1000-1200 feet. Faults form
parts of the northern and southern borders of the basin. The straight,
timbered, northward-facing steep slope less than a mile southeast of the
viewpoint marks a fault.


                                S. 180.3

_Strawberry Range._ This range and the Aldrich Mountains form a mountain
range 50 miles long; Strawberry Mountain, altitude 9038 feet above sea
level, is its highest peak. The eastern two-thirds of the Strawberry
Range (Fig. 13) was raised as a great block by uplift on the John Day
fault, which follows the northern base of the mountains. The rocks in
Strawberry Mountain and to the east are mostly lavas which poured out
over the land, whereas the Canyon Mountain part of the range consists of
gabbro and peridotite which were intruded at great depth, like granite.

The valleys in the higher parts of the range, above about 5000 feet,
were widened from narrow V’s to their broad U profiles by glaciers
during the Pleistocene Epoch, or Great Ice Age. The alluvial fans
(Rattlesnake Formation) in front of the mountains were built up of
bouldery gravels and finer sediments. These materials were eroded from
the mountains, carried by streams down the steep narrow canyons, and
spread out on the valley floor. Because much more material came into the
John Day River from the Strawberry Mountains than from the lower
mountains to the north, the river was pushed to the north side of its
wide valley. Faulting and erosion have completely destroyed the cones of
the volcanoes from which the volcanic rocks were erupted in Miocene and
Pliocene time.

[Illustration: Fig. 13.—Panorama of the Strawberry Range and the John
Day River valley from the north.]

[Illustration: Fig. 14.—Section through the Strawberry Mountain, along
Strawberry Creek.]

                    [This image in higher resolution]

_Strawberry Lake and Vicinity._ At Strawberry Camp, about 12 miles south
of Prairie City, the broad floor and steep walls of Strawberry Creek
valley indicate that the valley has been glaciated. The precipitous
cliffs and rounded valley bottom above Strawberry Lake are
characteristic of glaciated mountains (Fig. 15). Strawberry Lake is
dammed by landslides which probably came from the west wall of the
valley after the glacier melted and left the valley wall over-steepened.
The hummocky surface and blocky material in the slide are well shown
along the last half mile of the trail to Strawberry Lake. Strawberry
Falls mark the front of a glacial step over a massive flow of platy
andesite. Little Strawberry Lake is dammed by a low glacial moraine.

[Illustration: Fig. 15.—Strawberry Lake, the glaciated valley of
Strawberry Creek, and cirque walls formed by the Strawberry volcanic
plug.]

Most of the lavas in the Strawberry Mountains were erupted from a
central vent about 4000 feet in diameter which is exposed in the cliffs
above Little Strawberry Lake. The pinnacles known as “Rabbit Ears,”
above the prominent talus in figure 15, are of vent breccias that
consist mostly of welded blocks of scoriaceous basalt, but also contain
volcanic bombs which were blown out as blobs of fluid lava. Huge blocks
of the breccia have fallen onto a gentle bare slope west of Little
Strawberry Lake. The massive, vertically-jointed cliffs are formed of
basalt which cooled slowly and formed a plug in the throat of the
volcano after the eruptions ceased. The thin irregular scoriaceous
andesite flows, which are exposed in the cliffs east of Little
Strawberry Lake adjoining the plug, contrast strikingly with the massive
even flows of the Picture Gorge Basalt.

Tilting of the Strawberry Mountain block is shown by the southward dip
of all the flows in the area. The flows in the cliffs west of Strawberry
Lake, for example, originally must have sloped northward away from the
vent where they erupted. Their present southward dip of about 15°
therefore indicates that they have been rotated more than 15° by
faulting, partly along the northern edge of the mountain range. (Fig. 14
).



                Selected References To Detailed Reports


  U. S. Geological Survey Maps:
  No. 1-447 Geologic Map of the Canyon City quadrangle, northeastern
          Oregon, by C. Ervin Brown and T. P. Thayer, 1966. The map
          covers the entire region at a scale of one inch equals 4
          miles.
  GQ-438 Geologic Map of the Aldrich Mountain quadrangle, Oreg., (with
          text) by T. P. Thayer and C. Ervin Brown, 1966.
  GQ-548 Geologic Map of the Mount Vernon quadrangle, Oreg., (with text)
          by C. Ervin Brown and T. P. Thayer, 1966.
  MF-51 Preliminary Geologic Map of the John Day quadrangle, Oreg., by
          T. P. Thayer, 1956.
  U. S. Geological Survey Professional Papers:
  No. 550-C Local thickening of basalts and silicic volcanism in the
          Canyon City quadrangle, Oreg., by T. P. Thayer and C. E.
          Brown, pages C73-C78, 1966.


             _(From material provided by Thomas P. Thayer)_

[Illustration: ]


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.



                          Transcriber’s Notes


--Silently corrected several typos.

--Provided all images resized and oriented for use on a portable eBook
  reader.

--Provided clickable links to higher-resolution versions of detailed
  images.





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