A type of sedimentary rock consisting of medium to coarse sand grains mixed with clay and silt, typically gray in color.
From German 'Grauwacke,' a geological term: 'grau' (gray) plus 'Wacke' (a type of rock). The German term was adopted into English geological vocabulary in the 19th century.
Graywacke is the geologist's bread-and-butter rock—it forms from rapid underwater sediment flows called turbidites, and entire mountain ranges are built from it, yet most people have never heard of it!
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