Geology
Visitors to the park can view the ancient geological history of Minnesota as they walk the path leading from the upper falls down to the Mississippi River. The uppermost layer of soils and gravels of Minnesota were deposited by the most recent Ice Age, about 10,000 years ago. Four great ice ages have swept away all traces of the more recent Mesozoic and Cenozoic eras in the Twin Cities area, however standing at the upper falls, one is standing directly on the Platteville Limestone Formation laid down during the Ordovician Period of the Paleozoic Era. During the Ordovician Period, about 450 million years ago, the North American continent was situated along the equator, and a warm shallow sea covered much of Minnesota. Sea life was abundant and a large number of marine fossils including corals, bryozoans, brachiopods, clams, snails, cephalopods, and trilobites can be found in the limestone and shale sediments at several areas in the Twin Cities and along the Mississippi River in the park and elsewhere.
Walking from the upper falls to the base of the falls one passes through the thin layer of the Glenwood Shale Formation, and just below the base of the falls, one enters the level of the Saint Peter Sandstone Formation of pure white quartz sandstone. The Minnesota St. Peter Sandstone is "famous the world over as the example of a well-rounded, sell-sorted, pure quartz sand. It is the Ivory Snow of sediments, because it is close to 99.44% pure". This sand was first deposited as beach sand, probably eroded from earlier Cambrian sandstones, along the shores of the Ordovician sea. Walking along the creek to the area at which the foot bridge crosses, the Glenwood Shale Formation, a thin layer of grey-green rock probably deposited in a deeper-water offshore from the beaches, can be seen where it has been exposed by erosion. Shale was formed from ancient mud deposits and the Glenwood shale in the park is filled with numerous small fossils, appropriately termed "fossil hash". Together, these three formations that are visible as one walks from the upper falls to the river represent a sequence of sea-level rise during Ordovician time.
Minnehaha Falls is linked geologically to Saint Anthony Falls, which today is several miles upriver. The falls first appeared roughly 10,000 years ago several miles downstream of the Mississippi River at the confluence of the glacial River Warren (at present-day Ft. Snelling). They have since relocated upstream at a rate of about 4 feet (1.2 m) per year. The water churning at the bottom of the falls ate away at the soft underlying sandstone, eventually breaking off the hard limestone cap in chunks as the falls receded. As St. Anthony Falls moved past Minnehaha Creek and formed a deep river bed, they caused a new waterfall to form, the Minnehaha Falls which continued to move upstream in Minnehaha Creek.
There used to be an island in the Mississippi River near Minnehaha Creek. The receding St. Anthony Falls divided into two as it passed around this island. The falls in the channel farthest from Minnehaha reached the upstream end of the island first, cutting off water to the west channel and resulting in an "abandoned waterfall" at the north end of that channel. The abandoned west channel is now a grassy cul-de-sac known as the "Deer Pen". Locating the abandoned waterfall has been made difficult in recent years since the Deer Pen was partially filled with tons of fill dirt from nearby construction projects. The lower portion of Minnehaha Creek now flows through a wide and deep channel once belonging to the larger river. The end of Minnehaha Creek where it joins the Mississippi River is the lowest surface point in the city of Minneapolis at 686 ft (209 m) above sea level.
Erosion within the last century has resulted in a falls that is fairly narrowly channeled and vigorous, notably after a heavy rain. Photographs of the waterfall from the 19th century (such as the one from 1860) show a much wider, curtain like character to the falls. When the creek is dry, the older, much-broader ledge can be observed. If there were sufficient interest and funding, some remedial work could theoretically restore the 19th century appearance of the falls.
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