What is Greywacke?
[[Image:Mineraly.sk - kremenec.jpg|right|thumb|250px|A sample of greywacke rock]] '''Greywacke''' <ref>[[German language|German]] ''grauwacke'', signifying a grey, earthy rock</ref> is a type of [[sandstone]]. It is hard, and dark in color. It has different sized grains of [[quartz]], [[feldspar]], and small rock fragments set in a compact, fine [[clay]] matrix. It is mostly found in [[Palaeozoic]] [[strata]]. The larger grains can be sand-to-gravel-sized, and matrix materials make up more than 15% of the rock by volume. The origin of greywacke was a problem. [[Gravel]], [[sand]] and [[mud]] should not be laid down together, because river outflow drops heavier grains first. Now geologists think greywacke is formed by [[submarine]] [[avalanche]]s or strong turbidity currents. These actions churn sediment and cause mixed-sediment [[wikt:slurry|slurries]] to occur. When this happens, the resulting rocks show various sedimentary features. Support for the turbidity current origin is the fact that deposits of greywacke are found on the edges of the [[Continental shelf|continental shelves]], at the bottoms of [[ocean]]ic trenches, and at the bases of [[mountain]] formational areas. It also occurs in association with black [[shale]]s of deep sea origin. == References == {{Reflist}} [[Category:Sandstones]]