Great River Bluffs State Park - Geology

Geology

The region lies on sedimentary rock layers deposited on the floor of a shallow sea 500 million years ago. The lower, older layer is sandstone overlain by more recent dolomite rock. Much later a deep channel was carved into this stone by the ancient Mississippi River, which 10,000 years ago was greatly swollen from the melting glaciers to the north. The softer sandstone was eroded more easily, undercutting the dolomite on both banks of the river. The dolomite fractured off along vertical joints, leaving sheer rock faces still visible today. Side streams flowing into the Mississippi cut gullies into the river banks, creating a series of bluffs capped with erosion-resistant dolomite. Located in the Driftless Region of southeast Minnesota, the area was missed by the most recent Wisconsonian glaciation, and therefore is a region of deep river valleys dissecting the upland plains.