Grauwacke

Greywacke (German: Grauwacke; also spelled graywacke) is a type of sandstone distinguished by its high content of clay- and mineral-rich matrix and by poorly sorted, angular to subrounded grains of quartz, feldspar, and lithic fragments. The rock is typically dark gray to gray-brown in fresh exposure, a consequence of the substantial dark matrix that binds the grains. Matrix content commonly ranges from about 15 to 50 percent by weight.

Formation and texture are linked to rapid, sedimentation-rich environments. Greywacke forms mainly in marine settings such as continental shelves and slopes, where turbidity currents deposit immature, poorly sorted sediments that include abundant clay minerals and lithic fragments. The result is a compact, matrix-rich sandstone with a characteristic coarse-grained appearance and a range of grain sizes.

Composition and classification in modern terms describe greywacke as a lithic-rich sandstone. Its grains consist of quartz, feldspar, and various rock fragments set in a muddy or clay-rich matrix. Because of its immaturity and high matrix content, it is often contrasted with more mature sandstones that have less matrix and more well-sorted grains. In many orogenic belts, greywacke is a key constituent of flysch sequences and ancient accretionary wedges, helping to reconstruct past subduction and sedimentation processes.

Occurrence and uses vary by region. Greywacke is found worldwide in old mountain belts and sedimentary basins. Some varieties are quarried as dimension stone or used as decorative rock or construction material, though their high clay content can influence weathering and stability. The term remains widely used in both German-speaking and English-language geology.