How are clastic sedimentary rocks formed?
Clastic sedimentary rocks are formed from the erosion and redeposition of particles of older rocks in water, ice, or air. These clasts may be of any rock type, even older clastic sediments. Clasts are found in many different sizes, some being too small to see without the help of a microscope. Very small clasts (clay- and silt-sized particles) make up the black shale hills near Chamberlain and Pierre (Fig. 1). Clastic fragments may also be very large, up to several inches across. Rocks containing large fragments can be seen along the Fall River near Hot Springs and other places in the Black Hills. Clastic fragments that have been churned or moved around by water for a long time are usually smooth, round, and similar in size. The sand that makes up the sandstone in many of the steep ridges in and near Rapid City are examples of these types of clastic sediments. A similar sandstone with well-rounded and smooth clasts can be found in the banks of Firesteel Creek near the city of Mitchell.
