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Title: Description of the Geology of the Falls
Under the limestone is a soft sandstone called the St. Peter, so soft as to be easily excavated Dug out. with a pick. The recession Moving back upstream. of the falls up stream before any protecting apron Structure built under the water to protect the falls from further erosion. was constructed was quite rapid, and especially so when the owners of the water power built dams jutting out into the river above the falls and concentrated its whole flow into a width of not more than 300 feet. This stream of water washed out the soft sandstone under the limestone, and the latter fell down and was soon disintegrated Fell apart. by the rushing waters and carried away, and fresh undermining commenced. At the Falls of St. Anthony the river is divided into two channels by Hennepin Island. The east channel is entirely closed by dams, and the whole flow of the river, except what goes through the turbine Engine that can convert water power into mechanical power. or other wheels, passes over the main falls. The ordinary high water discharge of the river is about 50,000 cubic feet per second, and at extreme low water about 4200 cubic feet per second.
By the building of dams the fall of river is increased to 50 feet. The limestone forming the bed of river only extends up stream some 900 feet above the crest of the falls, and at its upper edge is only 18 inches thick.