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_