greywacke

• graywacke

Anglicised form of grauwacke.

• (UK) IPA^(key): /ˈɡɹeɪwakə/

greywacke (countable and uncountable, plural greywackes)

1. (geology) A hard dark sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix.
   • 2004, Richard Fortey, The Earth, Folio Society 2011, p. 177:

     Endless bleak, rounded hills composed of shales and ribs of tough ‘greywackes’ make up the Southern Uplands – all of such rocks were originally deposited under marine conditions.

• metagreywacke, metagraywacke