A type of hard, grey sedimentary rock composed of sand and silt cemented together, commonly used in construction and roofing.
From German 'Grauwacke,' combining 'grau' (grey) and 'wacke' (a type of rock), a geological term borrowed from German mining terminology in the 19th century.
Greywacke is the rock that keeps showing up in geologist's hands—it forms in deep ocean trenches from underwater avalanches, and studying it helps scientists understand ancient seafloors and tectonic activity.
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