This report summarizes regional geologic data pertinent to construction of a dam and reservoir on the Patoka River near Ellsworth, eastern Dubois County, Ind. The river occupies a valley that in its lower (western) reaches is broad by comparison with most valleys in the dissected upland area through which the river flows. The broad valley is underlain by alluvial and lacustrine deposits that in the vicinity of the damsite are 50 to 80 feet thick. This stage of valley filling was caused by blocking of the mouth of the old upper Patoka valley by glacial and glaciofluvial deposits. Bedrock in the area includes one group of formations of Pennsylvanian age and four groups of Mississippian age. The Pennsylvanian rocks are principally sandstones and shales; Mississippian rocks include approximately equal amounts of sandstones, shales and limestones. A disconformity separates Pennsylvanian from Mississippian rocks, and all of these formations dip almost due west at about 25 feet per mile. The only serious geologic problems involved in construction of a dam and reservoir at Ellsworth are centered around the damsite. Two beds of cavernous limestone, 14 feet and 30 feet thick, occur in the valley walls at the damsite and in adjacent parts of the reservoir area. Detailed engineering studies must be made to determine the extent and nature of the treatment that will be necessary to prevent serious leakage through these limestones. Other engineering studies will be needed to determine the bearing capacities of the thick valley fill and the suitability of available unconsolidated deposits for earth-fill in the dam. Limestone for rockfill and concrete aggregate is in adequate supply. The flood pool should not be raised above the proposed 550-foot altitude to avoid seriously increasing leakage and bearing problems.