Lake Minatare Reservoir is within the High Plains physiographic province, near the northern edge of a terrace of the North Platte River. A veneer of wind- and water-intermixed and interlayed sand, silt and some gravel, cover the terrace. Their surficial materials are noncohesive and have little resistance to erosion by wind or water. The terrace is underlain by the Brule Formation, a Tertiary siltstone to sandy siltstone of continental origin, which forms the foundation for Minatare Dam and Reservoir. The Brule is a soft but well-compacted tan to yellowish-brown sedimentary rock. The sand content is reported to gradually increase from the base toward the upper contact of the formation. The Brule is weakly calccareously cemented and tends to break down when exposed.