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.