greywacke

 
Pronunciation: /ˈgreɪwakə/
(US graywacke)

noun

[mass noun] Geology
  • a dark coarse-grained sandstone containing more than 15 per cent clay: a 120-foot pyramid of battered greywacke a quarter mile offshore [count noun]: the shales and greywackes of the Skiddaw Group

Origin:

late 18th century (as grauwacke): from German Grauwacke, from grau 'grey' + wacke. The anglicized form dates from the early 19th century