Wacke

Greywacke or graywacke (German: Grauwacke'a grey, earthy rock') is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoicstrata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15 percent of the rock by volume.