In geology and related fields, a stratum (plural: strata)'s a layer of rock or soil with internally consistent characteristics that distinguishes it from contiguous layers. Each layer's 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 exposed in cliffs, road cuts, qua… (More on Stratum)