Black Hills Terrace Gravels: A Study in Sediment Transport

Terraces were mapped along three streams which flow eastward from the Black Hills of South Dakota. Five erosional surfaces were found which represent major pauses in the downcutting activity of the regional drainage. The presence of knickpoints in the present-day stream profiles offers proof that the more recent terraces of this region are the result of factors controlling downcutting downstream in the Cheyenne and Missouri rivers. These factors are best explained on the basis of Pleistocene glaciation in eastern South Dakota. Twenty-three channel samples were taken from the terraces of Bear Butte, Rapid, and Battle creeks. In addition, sand samples 1-2 mm. in size were collected from the headwaters of Battle Creek near Harney Peak to the lower end of the Cheyenne River where it flows into the Missouri River, a distance of about 200 miles. The samples were analyzed for size, shape, roundness, and lithology. Systematic changes as a function of distance were observed for mean size, skewness, lithology, shape, and roundness. The rate of rounding was found to be directly proportional not only to the difference between the roundness at some point and a limiting roundness but also to some power of the distance transported. An attempt is made to express quantitatively the effects of stream transport on roundness and lithology in respect to "indices of maturity."