| 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.
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.