The Big Horn Basin is an elliptical lowland, limited on the west by the Absaroka and Beartooth units of the Rocky Mountain system, and on the south, east, and northeast by the crescentic arch of the Owl Creek, Big Horn, and Pryor Mountains (Fig. 1). The geomorphic history of the basin is divided into two, more or less distinct periods, the first characterized by relative uplift of the surrounding mountain ranges and by widespread fluvial aggradation of the resulting structural depression, and the second by production of the present topographic lowland through partial re-excavation of the Basin fill. The field work on which this paper is based was confined almost entirely to the broad, gravel-capped terraces and interstream benches formed during the second period by degrading streams, and the greater part of the paper is devoted to an analysis of the manner of development and the geological . . .