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Title: How Old Is It? The Story of Dating in Archeaology - Museum of New Mexico Press, Popular Series Pamphlet No. 2
Author: Schoenwetter, James
Language: English
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IN ARCHEAOLOGY ***
HOW OLD IS IT?
THE STORY OF DATING IN ARCHAEOLOGY
MUSEUM OF NEW MEXICO PRESS
POPULAR SERIES PAMPHLET NO. 2
HOW OLD IS IT?
DATING IN ARCHAEOLOGY
_by James Schoenwetter_
There is a whole field of science devoted to the invention and
development of dating methods—or “clocks” as we may think of them. It is
called _geochronology_, the science of dating events. There are
relatively few geochronologists, scientists trained in the use of all
kinds of dating methods and in the theories upon which these methods are
based.
Geochronologists tell us that there are two major types of clocks: those
that tick at an absolute rate of speed which can be measured, and those
which tick only once in a while. A clock of the first type yields what
is called an _absolute_ date, revealing the number of hours, days,
years, centuries or millennia since an event occurred. A clock of the
second type yields what is called a _relative_ date, placing an event as
before or after another event, but does not tell us exactly how far they
are apart in time nor how long ago they occurred.
Depending upon how accurate his date must be to solve the problem he has
set for himself, the archaeologist will select absolute or relative
dating methods. Often, of course, the type of clock he wishes to use is
not available, and he must use the next best type. Probably he will try
to use a number of clocks of different kinds on the problem since each
clock will act as a check on the others.
The absolute clock utilized most widely in archaeology is the historical
record. Men have used calendars for a long time, and have often left
records with written dates. On tombstones at a site in old Virginia, on
the pedestals of statues and other monuments from classical Greece and
Rome, on the walls of the tombs of Egyptian kings, dates are clearly
inscribed which can be related to the sites dug into by the
archaeologists. These dates must often be recalculated in terms of the
Christian calendar which we use. Most calendars in use in the
Mediterranean, the Near East and China during classical antiquity have
been successfully correlated with the one we use today, and a date
inscribed or noted on such sites can be considered in our own terms.
Other calendars, such as those developed in the ancient cultures of the
Maya on the Yucatan Peninsula, have yet to be accurately correlated with
our own. Such calendars can be used on their own terms of course, and a
site which has an inscription in the Mayan calendar is known to be so
many years older or younger than another one with a different date in
that calendar. We speak of such a situation as a _floating chronology_.
That is, the sequence of events and the number of years which separate
them are known, but the dates of those events in absolute time are
unknown.
[Illustration: (uncaptioned)]
Tree rings afford another kind of absolute clock, the _dendrological_
method. Each year a tree adds a growth ring. Depending on the amount of
water the tree has available to it for cell growth, the ring will be
wider or narrower. Certain trees whose water requirements are high live
near streams or other places where their roots can tap a constant supply
of water. Such trees, referred to as _complacent_, have annual rings
which are all of about the same width. Other _sensitive_ trees live in
places where they must depend almost wholly on rainfall for their water
supply, as on the slopes of hills or in the clefts of rocks. Such trees
have annual rings which vary in width depending upon the amount of rain
they receive. In any given area, especially in arid and semiarid
regions, some years have more rainfall than others. The sensitive trees
will produce wider rings during years when there is more rainfall and
narrower rings in years when there is less. Often there are periods of a
decade or so when all of the sensitive trees will produce the same
pattern of ring growth; for example, three years of narrow rings, one
year of wide, two more of narrow and three more of wide. Such a pattern
is called a _signature_.
Signatures are the basis of tree ring chronologies. All the trees in a
region did not begin growing at the same time of course, but every time
there is a series of years which will produce a signature, all the
sensitive trees still alive will have that signature. Let us say we cut
down a sensitive tree in 1960 and, by counting back the rings, find
signatures at 1940-45, 1910-14, 1880-89, 1821-27, 1795-1800 and 1750-58.
Next we recover a beam from an abandoned Spanish Mission built in 1810.
It happens to be from a sensitive tree, and we can spot the 1795-1800
and the 1750-58 signatures near the outer rings. Now we have two records
which can be said to be _crossdated_. Let us assume that the Spanish
Mission log give us signatures as far back as 1350. An abandoned Indian
pueblo produces a log with signatures crossdating those of the Spanish
Mission log and continuing the record back to 1250. Older and older
archeological sites will yield older and older signatures with each log
crossdating some of the signatures of younger logs.
At present we have a tree ring calendar for certain species of trees
extending back to about 100 B.C. This is called a _master tree ring
chronology_. A log from an archaeological site may contain only one or
two signatures and be, in itself, a floating chronology, but by
comparison with the master chronology one can determine the cutting
date. Having a series of cutting dates for the construction timbers in
an archaeological site yields the probable dates at which the site was
built and occupied.
[Illustration: _Diagram to Illustrate Tree Ring Dating. Reproduced
by special permission from Stallings, “Dating Prehistoric Ruins by
Tree-Rings.”_]
C THIS BEAM CAME FROM AN OLD HOUSE
B THIS BEAM CAME FROM A HOUSE
A THIS WAS A LIVING TREE WHEN CUT BY US
THE RING PATTERNS MATCH AND OVERLAP BACK INTO TIME
SPECIMENS TAKEN FROM RUINS, WHEN MATCHED AND OVERLAPPED AS INDICATED,
PROGRESSIVELY EXTEND THE DATING BACK INTO PREHISTORIC TIMES.
Not all kinds of trees can be used for dendrochronology. Pine, fir and
pinyon are the most useful; juniper can sometimes be used. Oak,
cottonwood, willow and others are very difficult to date and frequently
cannot be used at all.
Another widely used absolute clock is that based upon the orderly
decomposition of carbon. This is the _radiocarbon_ or C-14 method of
dating. Molecules of various substances are made up of atoms. We know
now that not all atoms of a substance are precisely the same. We speak
of _isotopes_ of an atom. To clarify this let us think of the atoms of
carbon as being made up of a mass of ping-pong balls. Some atoms will
have more ping-pong balls than others, but all will have enough and in
the proper order to be carbon atoms. Each of the atoms with 14 ping-pong
balls we shall refer to as the carbon-14 isotope of carbon. There will
be other isotopes of carbon atoms too.
The carbon-14 isotope is about average in life span. It has been
determined that in any group of C-14 isotopes half of them will lose two
of their ping-pong balls and become C-12 isotopes in 5,730 years, plus
or minus 40. This is known as the _half-life_ of the C-14 isotope. The
“plus or minus 40” allows for laboratory error in terms of years.
Now all living things contain carbon atoms, and some of those atoms are
C-14 isotopes. The amount of C-14 isotopes in a living organism quickly
reaches a stable percentage after which there is no increase or decrease
while the organism is alive. After it dies, the C-14 supply is not
replenished, and with the passing of 5,730 plus or minus 40 years, it
has half the number of C-14 isotopes it had when alive. In 11,460 plus
or minus 80 years, it will have one quarter as many as it had when
alive.
The geochronologist takes a certain weight of carbon-bearing matter from
an organism which once lived. With simple chemistry he can determine the
number of carbon atoms in the material, usually charcoal, wood, bone or
shell. He places the material in a chamber equipped with geiger counters
and records the number of C-14 isotopes converting to C-12 isotopes
within a certain number of hours or days. Since he knows how many carbon
atoms there are in the specimen, he knows how many C-14 isotopes there
would be if the specimen were alive. He also knows that as the number of
C-14 atoms decreases the number of clicks on the geiger counter will
decrease too. For example, if there are 2,000 C-14 isotopes, the
decomposition of half of these over a period of approximately 5,730
years would register 1,000 clicks on the counter. In the next 5,730 year
period there would be 1,000 isotopes left, and only 500 of them would
decompose to register as geiger counter clicks.
The geochronologist does the counting and analysis of the results and
sends the information back to the archaeologist in the form of the
number of years that have elapsed since the carbon was part of a living
creature; for example, 1,500 plus or minus 150 years BP (before
present). The archaeologist, converting this to the Christian calendar
in 1964, would come up with A.D. 464 plus or minus 150 years. When was
the sample actually alive? We don’t know exactly, but statistically we
have a ninety-five percent chance of being right if we say sometime
between A.D. 164 and 764. This clock ticks in centuries. But the
radiocarbon clock doesn’t tick very long, even in centuries, before
running down. By the time 30,000 to 40,000 years have gone by, the C-14
in any sample is almost gone, and there is too little left to give
enough geiger counter clicks unless one is willing to wait a lifetime to
record two or three clicks.
The C-14 dating technique measures time by radioactive decomposition of
materials. There are other clocks which depend on the chemical
decomposition of materials. The forgery of the Piltdown Man fossil was
detected by a dating method which depends on the decomposition of bone
protein and its replacement by fluorine. Fluorine is an element which
occurs naturally as a gas, but which combines readily with other
elements to form compounds. Some of these elements are common in bones.
Since fluorine is one of nature’s most reactive elements, it tends to
escape from the compound it is in and to form other compounds. As the
protein in a bone decays, it is often replaced by fluorine. The amount
of fluorine in old bones, then, is expected to be more than in young
bones since it has had more time to accumulate. Since fluorine does not
accumulate at a constant rate, it affords only a relative measure of
age.
In the case of Piltdown, a group of bones was discovered at a site, and
was said to contain those of one individual who lived about 60,000 years
ago. Almost 100 years after they were discovered, these bones were put
to the fluorine test. It was found that (1) some of the bones had less
fluorine than others, so not all were of the same antiquity and could
not have belonged to the same individual, (2) the younger bones had as
much fluorine in them as modern bones, and (3) the older bones had more
fluorine in them than bones known to be 60,000 years old.
[Illustration: (uncaptioned)]
Another form of absolute dating of importance to archaeology is that
called the _varve_ method. This can only be utilized as an absolute
clock under very special circumstances however. Varves are like tree
rings in a way. In a lake which is sufficiently deep, or at the edge of
a glacier, particles of sediment are being deposited continuously as a
sort of fallout from the water. During the winter, when the glacier
freezes or the density of the water in the lake increases because of the
cooler temperature, less particles are deposited, and those which are
deposited are usually of a characteristic color or texture. During the
summer, when the glacier melts or the lake warms up, more and different
particles are deposited. The bands of deposited sediment are called
varves; every year two varves are formed. Starting from the top, one can
count back the number of years in a varve series. If the top varve is of
known date such as the present year, one has a calendar with each varve
having a known date. Attempts are made to correlate one varve series
with another in order to recover even longer series. If the
archaeologist is lucky, and it is not rare in Europe, there will be
materials from a site buried in the local varve sequence. Counting back
gives an absolute age for the artifacts embedded in the site and thus an
approximate age for the site. The European varve chronology is believed
to extend back to about 9650 B.C.
Most of the clocks which the archaeologist uses to produce relative
dates, the before-or-after kind, have as their theoretical basis the
principle of _stratigraphy_. In effect the principle of stratigraphy
assumes two things: that the rocks of the earth are constantly wearing
down by erosion, and that things which appear to be alike actually are
alike and are probably more or less the same age.
If rocks are constantly wearing down, it follows that the surface of the
ground is constantly building up. Thus the surface we walk on is a
younger, higher surface than that which our ancestors walked on. When we
dig below the surface, those things we find which are at higher levels
are younger than those which we find at lower levels. The deeper we dig,
the older things get.
There is no reason to believe that the rate of deposition on the surface
is the same everywhere. If we dig two feet in one place we may be at a
level which is now five feet below the surface in another location. If
we find a particular object, say a type of pottery, on the surface at
site A and the same kind of pottery five feet below the surface at site
B, we can use the second of our assumptions and maintain that both
pieces are of the same age. Then any objects found at higher levels than
five feet at site B are younger than the piece of pottery and are
younger than anything found at site A. This is the principle of
stratigraphy.
Like tree rings, objects in _stratigraphic sequence_ can be crossdated.
These sequences may be of various kinds as any object will do.
Distinctive bands of sediment, distinctive artifacts, types of fossils,
specific details of chemistry or any other phenomenon may be used with
varying amounts of success. Suppose we have the following sequences of
objects at sites A and B:
[Illustration: (uncaptioned)]
A B
_Black earth_ _Brown dust_
_Caliche_ _Black earth_
_Eroded layer_ _Caliche_
_Cobbles_ _Yellow silt_
_Soil_ _Brown silt_
_Brown silt_ _Eroded layer_
_Cobbles_
Now there are some things that are similar about these two _profiles_
and other things that are different. Both profiles contain layers of
black earth, caliche, an eroded layer and cobbles. Both profiles contain
pottery, and both contain arrow points. The type of pottery in profile A
is the same as that in profile B, but in B there is brown dust above the
black earth. In profile B the brown silt is above the eroded layer,
while in A it is below the eroded layer. In profile A the arrow points
have different shapes than the ones in profile B. What we need to
correlate the sequences are _horizon markers_, objects that are enough
alike to be in the same time range.
The pots in both cases look the same and are embedded in the same kind
of sediment, the black earth. They form one horizon. The caliche may be
a horizon marker, but this is not positive since at profile A it is
above the eroded layer, while at profile B it is above the yellow silt.
The eroded layer sits above cobbles at both profiles, which makes the
eroded layer-cobbles complex a pretty good horizon marker. The brown
silt is not a horizon marker because it is above the eroded layer at one
profile and below it at the other. Furthermore, the arrow points which
it contains are not all of the same kind. The correlated sequence, using
the horizon markers, must be as shown in the accompanying chart.
The stratigraphy proves that the arrowheads in the upper brown silt must
be younger than those in the lower brown silt. The next time we find
arrowheads of the types recovered in the upper brown silt we will know,
regardless of the stratigraphic sequence in the new locality, that they
must be younger than the types found in the lower brown silt, and they
must also be older than pottery of the type found in the black earth. We
won’t know how many years old they are, but we have dated them in the
sense that we know they are older than some things and younger than
others.
The principle of stratigraphy is one of the archaeologist’s most useful
theoretical tools as it allows one site to be compared with others. Our
example has shown how artifacts (pots and arrowheads in this case) and
sediment types can be placed in stratigraphic order to provide a clock
for archaeological dating. Other objects which are commonly used as
horizon markers in stratigraphic sequences are fossils. Remains of
extinct animals are often used to prove extreme antiquity. Remains of
plants and animals will often indicate similarity of ecological
conditions at two sites and allow them to be crossdated.
[Illustration: (uncaptioned)]
_Brown dust_
_Black earth_
_Caliche_
_Yellow silt_
_Brown silt_
_Eroded layer_
_Cobbles_
_Soil_
_Brown silt_
The _geologic-climatic_ method of dating combines the principle of
stratigraphy with an interpretation of the meaning of natural
occurrences. When geologists demonstrated that the earth had recently
gone through a period when enormous glaciers advanced and retreated
across the northern and southern hemispheres, they began to speculate on
the effects of such conditions on the landscape. The surface would be
scraped down to bare rock by the advancing ice sheet. When the ice
retreated, its load of rocks and cobbles would tend to be left behind,
while the smaller particles of dirt would be flushed away in the rivers
formed by the melting ice. If the archaeologist found tools among the
boulders and cobbles, those tools should date to the period when the
cobble and boulder stratum was formed, the period when the glacier was
retreating. Here the geology gave clues to the climate, and if the age
of the climatic event was known, artifacts associated with the geology
could be dated.
It is through the geologic-climatic method of dating that archaeologists
discovered that human beings lived in southern Europe at the margin of
the last great glacier, and therefore the date of their occupation was
on the order of 20,000 years ago.
It is not necessary to use the geologic aspect of geologic-climatic
dating if one has other clues to the climate. Plant and animal fossils
are clues to ancient climates, since living organisms have a tendency to
live in climates to which they are best adapted. If we find the fossil
bones of a giraffe or an ostrich in the Sahara Desert, we can conclude
that at some time in the past the Sahara was not a desert but a veldt,
since giraffes and ostriches live in the veldt today. If stratigraphy
allows us to relate artifacts to those fossils, we can maintain that the
makers of those artifacts lived at the time the veldt existed in what is
now the Sahara area. Now if we can relate the fossils to a date when
such a climate could have obtained in the area, we have a date for the
artifacts.
Animal fossils are relatively rare, and fossil remains of plants which
can be seen with the naked eye are even rarer. But microscopic pollen
grains are not particularly rare in the stratigraphic sequences of
sediments, and are directly associated with artifacts in many kinds of
sediment. The pollen of most plants is protected by a tough coat like
that on many seeds. Millions of pollen grains are produced by local
vegetation each year, and a percentage of them are buried in the yearly
accumulation of sediment. Once they are buried, the tough outer covering
is preserved.
[Illustration: (uncaptioned)]
When pollen grains are extracted from their sediment matrix, the
different types of plants which grew in the area at the time the
sediment was laid down can be recognized from the distinctive
characteristics of their pollen grains. The _pollen analyst_ has the job
of interpreting this information to discover the nature of the
vegetation patterns in the past and the climates associated with them.
Through dating those climates, the artifacts with which the pollen
sample was associated are dated.
Some archaeological sites do not allow dating by any of these methods.
The farmer plowing a field may pick up an arrowhead and wonder how old
it is. Can the archaeologist date the arrowhead? The answer is a
qualified yes. What the archaeologist will try to do is give an educated
guess as to the age of the arrowhead. He might, for example, know that
arrowheads of that type are found at a site which is dated about A.D.
1000 by various methods. By correlation, he could apply that date.
If the arrowhead is not of a type which has been reliably dated, the
archaeologist may rely upon the method of dating known as _seriation_.
This method depends on the second assumption basic to the principle of
stratigraphy, that similar objects tend to be about the same age. The
farmer’s arrowhead may not be exactly like any which has ever been
discovered, but it probably will be more like some known ones than
others. Observing the general style of the arrowhead and the way in
which it is made, the archaeologist can make a pretty good estimate of
when it was made.
Let us assume that the archaeologist goes into an unexplored area where
no absolute or relative dating techniques are available. There he finds
a number of sites with potsherds of types new to archaeology. Can he
date these sites? Again he uses seriation, but this time he inverts the
logical proposition. If things which look alike tend to be of the same
age, things which do not look alike should tend to be of different ages.
His first task would be to separate all the different types of
potsherds. Taking a specimen of each type, he lays them out in a row. If
there is any difference in the sites through time, the styles of pottery
will change correspondingly. But there will be some styles which change
slowly and some which will influence others. For example, if we were to
seriate the style of automobile rear fenders for the period 1956 to
1964, we would observe that they began to sprout taller and taller fins;
then the size of the fins became reduced more and more. Some of the
style aspects of pre-1956 rear fenders went along with the development
of fins and others dropped out. Some of the style aspects which were
developed with the fins were retained after the fins decreased in size.
Seriation of the design styles on the potsherds will result in a series
of developments in style which are probably in chronological order.
Broad straight line designs may give way to mixed broad and narrow
straight lines, then to narrow straight lines, then to narrow wavy
lines. The archaeologist could then maintain that sites which have
potsherds with broad straight lines are separate in time from those with
sherds having narrow wavy lines. He won’t know which is the older,
because the sequence could work either way. Stratigraphy or the cultural
similarity between other artifacts at the sites will probably resolve
this problem.
The reader will now have realized that many of the clocks used by
archaeologists are interrelated. Relative dating clocks such as
stratigraphy, pollen dating and geologic-climatic dating are utilized
together where possible, and all are dependent upon the principle of
stratigraphy. Crossdating is thus of vital importance and is constantly
undertaken. The archaeologist tries to employ both absolute and relative
clocks to find out the age of a site. Stratigraphy yields a series of
relative dates for artifacts within the site; geologic-climatic and
pollen dating yield a series of dates for types of sediment and samples
collected in association with those artifacts; tree ring dates yield the
absolute age of the site which allows the pollen, stratigraphic and
geologic-climatic dates to be comprehended in terms of absolute age.
Radiocarbon dates act as a check on the tree ring dates and, if they
agree, lend support to the pollen and geologic-climatic dates. The
pollen and geologic-climatic dates from the site are compared with
similar dates from other sites as additional checks. Since the clocks
used by the archaeologist tick at different rates of speed, and since
not all of them are dating the same thing, the archaeologist usually
ends with a series of dates for any given site.
New geochronological techniques are always being invented and perfected.
Here is a list of some that are expected to become available in the next
few years:
1. The _obsidian hydration_ method. Obsidian, a naturally formed glass,
is so constituted chemically that it takes chemicals from its
environment at a slow rate. As it does so, the outside layer changes
from transparent to translucent or opaque. The depth of the opaque
layer, it is hoped, can be used as a measure of the amount of time since
the surface was exposed. As many artifacts were made by chipping
obsidian to form a sharp edge, this method may reveal the time since an
obsidian artifact was made.
2. The _thermoluminescence_ method. The chemical properties of certain
minerals change when the minerals are heated to high temperatures. If
they are heated again they will glow, and the amount of time they glow
upon reheating depends on the amount of time that has elapsed since they
were heated originally. As yet the rate has not been accurately
calculated, nor is it yet understood what effect different kinds of
soils and atmospheres may have on this rate. If the method is proven, it
will be invaluable to archaeology since it will afford a way of dating
pottery directly on an absolute time scale.
3. The _paleomagnetism_ method. When an object containing particles
which can be magnetized is heated, the magnetic particles line up
according to the earth’s magnetic field. When the object cools, the
particles are trapped in this position. We know that the earth’s
magnetic field is changeable, and that at different times it has been
oriented in different directions. Work is now in progress to determine
how the field has varied through time and how successfully one can date
materials such as pottery or hearths which were exposed to high
temperatures in the past by the difference between the present magnetic
field and that trapped in the object.
Museum of New Mexico Press
Santa Fe 1965
_Illustrations by Mary Spencer
and Phyllis Hughes_
Transcriber’s Notes
—Silently corrected a few typos.
—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
_underscores_.
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