greywacke, pronounced ‘graywacky’, from the German Grauwacke, is a tough, dark grey sandstone, with a high clay content. Most sedimentary rocks (see geology) show a fairly uniform grain size, whereas greywacke, formed in turbid deep-sea water, shows characteristically jumbled grain sizes, with thick accumulations of coarse material (typically of quartz, feldspar, and rock fragments) closely intermixed with fine clay. On land it weathers slowly, giving stony, free-draining soils.
Greywacke is found in South Africa’s Western Cape; in California’s russian river valley; and in Germany’s mosel, ahr
, and mittelrhein; but perhaps most famously in new zealand, where the bedrock spine of both islands is composed largely of greywacke, and detritus derived from it dominates the gravels of regions such as Hawke’s Bay, Marlborough, and Waipara.
