Part 2

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.

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.

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.