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clay stone

or clay·stone

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noun Geology.
  1. argillite.

Origin of clay stone

First recorded in 1770–80
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
claystones in Science

claystone

[klāstōn′]
  1. A fine-grained, dark gray to pink sedimentary rock consisting primarily of compacted and hardened clay. Claystone is similar to shale but without laminations.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.