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Noun: graywacke
Usage: N. Amer (elsewhere: greywacke)
  1. Hard dark-colored sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix
    "Geologists found extensive graywacke formations in the coastal cliffs";
    - greywacke

Derived forms: graywackes

Encyclopedia: Graywacke