The River Bluffs along the east bank of the Mississippi River are really quite old. The upper layer has been dated via a thin layer of volcanic ash as being ~ 450 million years old. The river cut into the rock not removed by glaciars some time within the last 10,000 years. These rocks were deposited as sediment in an environment that was either marine or very near to a marine environment (like a beach). Zoom in to learn more.