caliche - tan hardpan caliche from the Mojave Desert - large hand/display specimen
Caliche underlies desert soils in a large part of the Western U.S. and in other arid areas. It is composed of calcium carbonate and can be tens of feet thick. An ideal caliche profile would have a hardpan at the surface progressing downward into a platy or laminar layer which overlies a nodular zone, and then intercepts the bedrock. Since a caliche takes tens to hundreds of thousands of years to form, changes in climate disrupt this sequence and an ideal profile is seldom seen.
The term caliche is Spanish from the Latin calx for lime. It forms when calcium carbonate is leached from the overlying soil and accumulates some distance below the surface.
In the Mojave desert, a major period of calcium carbonate deposition appears to have occurred 20,000 years ago, forming a horizon below a depth of 100 cm during a period of greater rainfall than at present. As the climate dried, CaCO3 precipitated to form caliche.
The mill for the Copper World Mine was at Valley Wells. The caliche there served as the roof of the mill workers' dugout cabins during the first 20 years of the 1900s, where, comfortable underground, they simply excavated under the caliche from the side of a ravine. In other parts of the Southwest, caliche posed a challenge for pioneers when they attempted to dig wells.
This is a substantial example of this culturally interesting rock. The pencil is 5 1/2 inches long, for scale.
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