In geology and related fields, a stratum (plural: strata) is a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. They may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are typically seen as bands of different colored or differently structured material. Individual bands may vary in thickness from a few millimeters to a kilometer or more. Each band represents a specific mode of deposition -- river silt, beach sand, coal swamp, sand dune, lava bed, etc. Eozoic strata pertains to rocks or strata older that the Paleozoic, in many of which the eozoön has been found.