Saint Anthony Falls - Geology

Geology

Geologists say that the falls first appeared roughly 10,000 years ago several miles downstream at the confluence of the glacial River Warren (at present-day Ft. Snelling). Estimates are that the falls were about 180 feet (55 m) high when the River Warren Falls receded past the confluence of the Mississippi River and the glacial River Warren. Over the succeeding 10,000 years, the falls moved upstream to its present location. The water churning at the bottom of the falls ate away at the soft sandstone, eventually breaking off the hard limestone cap in chunks as the falls receded. From its origins near Fort Snelling, St. Anthony Falls relocated upstream at a rate of about 4 feet (1.2 m) per year until it reached its present location in the early 19th century. Tributaries such as Minnehaha Creek begot their own waterfalls as the Mississippi River valley was cut into the landscape.

When Father Louis Hennepin documented the falls he estimated the falls' height to be 50 or 60 feet (18 m). Later explorers described it as being in the range of 16 to 20 feet (6.1 m) high. The discrepancy may have been due to scope, as the current total drop in river level over the series of dams is 76 ft (23 m).

The geological formation of the area consisted of a hard, thin layer of Platteville Formation, a limestone, overlaying the soft St. Peter Sandstone sub-surface. These layers were the result of an Ordovician Period sea which covered east-central Minnesota 500 million years ago.