Elk Creek is 120 stream miles (70 airline miles) in length and flows eastward from the northern Black Hills to join the Cheyenne River. The drainage basin encloses 601 sq. mi., of which 95.1 percent is underlain by Paleozoic and Mesozoic sedimentary rocks, 2.3 percent by Precambrian metamorphic rocks, and 1.8 percent by Tertiary felsitic siliceous hypabyssal rocks ("volcanics"). Cenozoic terrace gravels overlie approximately 25 percent of the sedimentary rocks. A detailed petrographic examination of sand sized stream sediment in Elk Creek reveals: (1) most schist fragments are mechanically destroyed by less than 15 miles of transport (8 airline miles) in the high gradient portion of the stream; (2) "volcanic" rock fragments are durable and there is no noticeable loss from mechanical abrasion during the initial 100 miles of transport; (3) rounding of "volcanic" rock fragments due to mechanical abrasion occurs only in sediment coarser than 1mm.