Greywacke
Sandstone with angular grains in a clay-fine matrix
Greywacke
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 Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15 percent of the rock by volume.
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