Stratum
From Wikinfo
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. The stratum is the unit that forms the basis of the study of stratigraphy.
References
- Adapted from the Wikipedia article, "Stratum" http://en.wikipedia.org/wiki/Stratum, used under the GNU Free Documentation License
