hardpan

[hahrd-pan]
noun
1.
any layer of firm detrital matter, as of clay, underlying soft soil. Compare caliche, duricrust.
2.
hard, unbroken ground.
3.
the fundamental or basic aspect of anything; solid foundation; underlying reality: the hardpan of mathematical theory.

Origin:
1810–20, Americanism; hard + pan1

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
hardpan (ˈhɑːdˌpæn)
 
n
a hard impervious layer of clay below the soil, resistant to drainage and root growth

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
hardpan   (härd'pān')  Pronunciation Key 
A hard, usually clay-rich layer of soil lying at or just below the ground surface, in which soil particles are cemented together by silica, iron oxide, calcium carbonate, or organic matter that has precipitated from water percolating through the soil. Hardpans do not soften when exposed to water. Also called caliche.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

hardpan

calcium-richcalcium-rich duricrust, a hardened layer in or on a soil. It is formed on calcareous materials as a result of climatic fluctuations in arid and semiarid regions. Calcite is dissolved in groundwater and, under drying conditions, is precipitated as the water evaporates at the surface. Rainwater saturated with carbon dioxide acts as an acid and also dissolves calcite and then redeposits it as a precipitate on the surfaces of the soil particles; as the interstitial soil spaces are filled, an impermeable crust is formed.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
When small lakes dry up, they leave a salt crust or hardpan.
The hardpan layer, consisting of cemented limestone and slag, was located
  between the second granular layer and the subgrade.
The subgrade that was visible at the edge of the hardpan was moist clay.
Small, light equipment was used to avoid damaging the hardpan layer located
  below the vernal pools.
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