Duricrust

Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters. It is a general term (not to be confused with duripan) for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic and (or) non-pedogenic p...
Found on http://en.wikipedia.org/wiki/Duricrust

duricrust

surface or near-surface of the Earth consisting of a hardened accumulation of silica (SiO2), alumina (Al2O3), and iron oxide (Fe2O3), in varying ... [1 related articles]
Found on http://www.britannica.com/eb/a-z/d/85
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