|Three geologic units are exposed at Keyhole damsite:|
1)Lakota sandstone which is soft to medium hard with considerable bedding and has a system of vertical joints that separates the bedrock into blocks or large masses. The lower portion of the Lakota sandstone across the valley floor.
2)Fuson shale lies above Lakota sandstone. It is dark-gray shale with sandy shale and sandstone zones.
3)Dakota sandstone (similar to Lakota) lies above the Fuson shale on both banks
Lakota sandstone forms the basal portion of both dam abutments with Fuson shale lying above the Lakota formation. Dakota sandstone lies high on the abutments above the Fuson shale. Dips of these strata are essentially horizontal at the damsite. Over burden near the dam axis consists of clay, silt, sand, and minor quantities of fine gravel.