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.
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:
