Oligocene rocks on the eastern side of the central Rocky Mountains are part of a broad expanse of such deposits extending from central Colorado to Saskatchewan. Resting unconformably on older rocks, they record renewal of sedimentation following an erosional episode. Only in western South Dakota and Nebraska and southeastern Wyoming do the succeeding Miocene beds lie conformably on the Oligocene; however, with few exceptions, the Oligocene strata have not been disturbed from their original attitude. Heavy mineral suites of the early Oligocene beds indicate derivation from local sedimentary rocks, from volcanic sources, and from the Black Hills, the Laramie Range, the Front Range, and perhaps other mountain uplifts. Late Oligocene strata in mapped channels were derived from about the same sources, whereas finer sediments of this age are mostly of pyroclastic origin. Consideration of these beds and others previously mapped in central and western Wyoming permits reconstruction of the major elements of a probable Oligocene drainage system. Most of the streams flowed generally eastward, but local exceptions are indicated, and the drainage pattern differed in details from the present-day network. The development of an aggrading fluvial regime, extending from central Colorado to Saskatchewan in Oligocene time, can scarcely have resulted from simple transgressive overlap, from basin downwarping near source mountains, from downwarp along a regional hinge, nor solely from overloading of the streams by volcanic ash. Instead, the evidence points to progressively drying climate with increased erosion as the major factor causing widespread fluvial deposition.