Bonny Lake State Park Geology
Bonny Lake State Park features a 1,900 surface-acre reservoir (at normal lake levels), lies in the South Fork of the Republican River Valley of the western Great Plains, and fed by the S. Republican River and Landsman and Armknecht creeks. The park occupies a gently rolling plain recently mantled with thin, silty loess and thick eolian sand deposits. Pleistocene loess deposits, known as the Sanborn Formation, cap underlying Ogallala Formation silt, sand, gravel, caliche and clay beds. In some areas of the park, the Sandborn Formation is buried under dune sands, which form a series of low hills and terraces as much as 200 feet high.