 thumb|photomicrograph of a feldspathic wacke (sandstone). top image is in plane polarized light (ppl); bottom image is in cross polarized light (xpl). blue epoxy fills pore spaces. left|thumb|photomicrograph of a lithic wacke (sandstone). top image is in plane polarized light (ppl); bottom image is in cross polarized light (xpl). blue epoxy fills pore spaces. greywacke or graywacke (german grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. it is a texturally immature sedimentary rock generally found in palaeozoic strata. the larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. the term "greywacke" can be confusing, since it can refer to either the immature (rock fragment) aspect of the rock or the fine-grained (clay) component of the rock. the origin of greywacke was problematic until turbidity currents and turbidites were understood, since, according to the normal laws of sedimentation, gravel, sand and mud should not be laid down together. geologists now attribute its formation to submarine avalanches or strong turbidity currents. these actions churn sediment and cause mixed-sediment slurries, in which the rocks may exhibit a variety of sedimentary features. supporting the turbidity current origin theory is that deposits of greywacke are found on the edges of the continental shelves, at the bottoms of oceanic trenches, and at the bases of mountain formational areas. they also occur in association with black shales of deep sea origin. greywackes are mostly grey, brown, yellow or black, dull-colored sandy rocks which may occur in thick or thin beds along with shales and limestones. they are abundant in wales, the south of scotland, the longford massif in ireland and the lake district national park of england; they compose the majority of the main alps that make up the backbone of new zealand. they can contain a very great variety of minerals, the principal ones being quartz, orthoclase and plagioclase feldspars, calcite, iron oxides and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as felsite, chert, slate, gneiss, various schists, and quartzite. among other minerals found in them are biotite, chlorite, tourmaline, epidote, apatite, garnet, hornblende, augite, sphene and pyrites. the ce
Sandstone Sedimentary_rocks