A well-indurated dark gray sandstone that is characterized by abundant dark-colored detrital rock fragments and more than 15% clay matrix minerals between sand grains. Graywacke sands were deposited chiefly in marine basins near the edge of continental margins where plate subduction was taking place. Subsequent compressional deformation and uplift of rocks in the sedimentary basins results in the occurrence of most graywackes in Alpine-type (compressional) mountain ranges.