The Falls Evolve

The Falls Evolve. . , Geology , The river flows over land, once a seabed, laid down 500 million years ago. The top layer, covered by soil left much later by glaciers, is hard Platteville limestone, which is up to 35 feet thick beneath much of Minneapolis but thins near Saint Anthony Falls. Beneath this is a narrow band of Glenwood shale, and then a thick bed of very soft, easily eroded Saint Peter Sandstone., Cross-section just above the falls. George M. Schwartz, Geology of the Minneapolis-St. Paul Metropolitan Area, 1936, courtesy of University of Minnesota Press, Although this area was ice-free by at least 14,000 years ago, runoff from receding glaciers etched the future course of the Mississippi. A massive cataract in present-day Saint Paul advanced upstream as its weight fractured the limestone and eroded the underlying sandstone. For 10,000 years, the waterfall moved about 4' to 5' annually. But as the limestone layer thinned and industry sped up its destruction, the annual change accelerated to as much as 35' in the 1850s and 100' in the 1860s. As the deterioration neared the edge of the limestone layer a short distance above Hennepin Island, the falls were close to extinction., These different impressions reflect substantial changes in the falls over the past centuries, as well as exaggeration by visitors.,

"The Falls of St. Anthony have no considerable heights, and strike me merely as the cascade of a great mill-dam." , –Fredrika Bremer, 1853

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"I measured the heights of the Falls and found them to be a little short of 15 feet. Concerning the height of the fall and breadth of the river, much incorrect information has been published. Hennepin, who was the first European that visited it, states it to be fifty or sixty feet high. This is, by Carver, reduced to thirty feet." , –James E. Colhoun and William H. Keating, 1823

, Newton Winchell and Warren Upham, The Geology of Minnesota, vol. 2, 1882-1885.,

"The murmuring of the cascade, the roaring of the river, and the thunder of the cataract, all contributed to render the scene the most interesting and magnificent of any I ever before witnessed. The perpendicular fall of the water at the cataract is 16 1/2 feet by actual measurement. To this height however 4 or 5 feet may be added for the rapid descent which immediately succeeds the perpendicular fall within a few yards below. , –Major Stephen H. Long, 1817

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"We could distinctly hear the noise of the water full fifteen miles before we reached the Falls. This amazing body of waters form a most pleasing cataract; they fall perpendicularly about thirty feet." , –Jonathan Carver, 1766

, Trying on Aprons at the Falls , The horseshoe dam (to your right), built by 1858, directed the river's flow to the mills, any water over the falls was "wasted." But during floods, there was more water than the mills could use. Excess water, along with logs and ice, pounded the fragile limestone ledge, hastening the deterioration of the falls. By 1870, its crest had moved 300' upstream., Fixing the apron in 1895 Hennepin History Museum, The waterpower companies tried to protect the falls with a $25,000 timber "apron" in 1866, only to see it washed away by a flood the next spring. Another flood foiled a second attempt in 1869. When the Eastman tunnel collapsed later that year, the U.S. Army Corps of Engineers intervened. Between 1870 and 1884, the corps led the effort to stabilize the falls, until Congress refused to appropriate more funds. The waterpower companies reconstructed the apron again in the 1890s. Patches of timber and concrete held it together until a record-breaking flood in 1952. The existing apron, this time constructed of sand fill with a reinforced-concrete cap, was completed by 1955., Sand fill being placed at north end of apron, 1954 Don Berg, photographer