cap rock

or cap·rock

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noun Geology.

a mass of anhydrite, gypsum, or limestone immediately above the salt of a salt dome.
an impervious stratum overlying an oil- or gas-bearing structure.

Origin of cap rock

An Americanism dating back to 1865–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for cap rock

cap rock


noun

a layer of rock that overlies a salt dome and consists of limestone, gypsum, etc
a layer of relatively impervious rock overlying an oil- or gas-bearing rock
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012