Feb 27, 2013· Siltstone is a siliciclastic sedimentary rock composed of silt-sized grains (between 0.0625 mm and 0.00390625 mm in size; i.e., 1/16 mm to 1/256 mm in size). These feel barely rough to the touch, whereas sandstones feel fairly rough & gritty. Siltstones are often quartzose in composition, and they vary in color from black to gray to brown to reddish.
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