Stratigraphy is the fundamental principle, well known to geologists, and applied here to anthropic formations instead of natural formations.
The ground at an archaeological site is not homogeneous but rather made up of superimposed stratum : layers of unequal thickness and sediments of different substance, texture, and color.
The deepest layers are, of course, the oldest. Each layer is a result of and is composed of the human activity, which produced it. Understanding the history of a site consist, therefore, in reconstructing time, by identifying each of these layers, and establishing the elements which relate one layer to another. Each layer is removed carefully and the archaeological artifacts, i.e. the objects that it contains (ceramic, bone, flint tools, etc.), are collected separately. These artifacts will be used to date the layer.