gray-wacke

graywacke

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


Origin:
1805–15; partly translation, partly adapted from German Grauwache; see wacke

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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.
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