rimrock

[ rim-rok ]
/ ˈrɪmˌrɒk /
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noun Geology.

rock forming the natural boundary of a plateau or other rise.
bedrock forming the natural boundary of a placer or of a gravel deposit.

Origin of rimrock

An Americanism dating back to 1855–60; rim + rock1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for rimrock

rimrock

/ (ˈrɪmˌrɒk) /

noun

rock forming the boundaries of a sandy or gravelly alluvial deposit
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