graywacke

Noun

greywacke (countable and uncountable, plural greywackes)

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