Lake Pepin fills the Mississippi Valley above this point for 22 miles. It was created by the delta of the Chippewa River, which enters the Mississippi directly east of this site. Although the Chippewa is a relatively small river, its steeper gradient let it carry more sand and coarser gravel than the Mississippi could remove, damming the larger river back in the gorge and forming the lake. The lake surface stands 664 feet above sea level and 450 feet below the tops of the bluffs along its shores. The sand and limestone walls of the gorge consist of material deposited in Cambrian and Ordovician seas when the continent was submerged about 400 million years ago. The gorge bottom lies 150 feet below the lake surface, having filled to its present elevation as the river's carrying power declined.