[grey-wak, -wak-uh]
Geology. a dark-gray coarse-grained wacke.
Also, greywacke.

1805–15; partly translation, partly adapted from German Grauwache; see wacke Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
greywacke or (US) graywacke (ˈɡreɪˌwækə)
any dark sandstone or grit having a matrix of clay minerals
[C19: partial translation of German Grauwacke; see wacke]
graywacke or (US) graywacke
[C19: partial translation of German Grauwacke; see wacke]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Science Dictionary
graywacke   (grā'wāk', -wāk'ə)  Pronunciation Key 
Any of various dark gray, coarse-grained sandstones that contain abundant feldspar and rock fragments and often have a clay-rich matrix. Graywackes are thought to originate in environments where erosion, transportation, and deposition happen so quickly that minerals and rock fragments do not have sufficient time to break down into finer constituents.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Primarily fine-grained pelitic rocks and graywacke, variably metamorphosed.
Schist and gneiss in alternating layers suggest a turbidite sequence of shale and graywacke.
The regolith weathered from gray and brown acid sandstone in places interbedded with shale and graywacke.
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