The story of St. Anthony and the Twin Cities began over four hundred and fifty million years ago when a sea covered Minnesota. This ancient sea led to the creation of two rock layers: the St. Peter’s Sandstone, an easily eroded rock, and the Platteville Formation, a yellow hard rock that does not erode easily. These two layers are essential to the formation of a waterfall. Twelve thousand years ago, the falls were near what is now downtown St. Paul, and have eroded away to its current location.
|1. The Geology of the Twin Cities|
|2. The Retreat of the Falls|
The story of St. Anthony and the Twin Cities began over four hundred and fifty million years ago with a very different Minnesota. First, it was nearer the equator, giving the state a tropical feel. Second, an ancient sea covered most of the land. Trilobites scuttled along the bottom of the ocean while cephalopods swam above them.
It does not take much to find evidence of Minnesota’s past. Around twenty thousand years ago, glaciers melted away and deposited sediment that became our present top soil. Flowing streams easily erode these glacial soils to uncover the underlying bedrock layers and the fossils they hold.
The uppermost rock layers in this area are part of the Platteville Formation, a yellow or gray hard rock (limestone and dolostone) known for its marine fossils, like trilobites and cephalopods, and many trails of burrowing organisms. It formed over four hundred million years ago. Beneath the Platteville lies a thin layer called the Glenwood Shale, which formed from mud deposited on the ancient seafloor. Less than a meter thick and in some areas all but absent, the Glenwood only plays a minor role in the falls’ story. Beneath the Glenwood lies the Saint Peter Sandstone, a rock which was once beach sand. As the name suggests, ‘sandstone’ is a rock composed of sand grains cemented together. In the case of the St. Peter Sandstone, its sand grains are only weakly held together and the rock is easily eroded or worn away.
In the picture above, taken near the bluffs by St. Anthony Falls, vertical fractures cut into the Platteville formation. Soft, easily eroded Saint Peter Sandstone forms a gentle vegetation-covered slope beneath the Platteville. The vertical fractures break the Platteville up into large blocks, easily separated from one another as the underlying sandstone erodes away. The large block in the above photo is almost ready to fall.
St. Anthony Falls did not always stand where it stands today. Twelve thousand years ago, the falls were near downtown St. Paul stands today. So, how did St. Anthony end up where it is today? The answer lies in the area’s geology.
Waterfalls in the Twin Cities rely on four essential components. First, they need a hard rock layer. Second, that hard rock layer must rest on another layer that is soft and easily eroded. Third, vertical fractures must cut the hard rock. Finally, a river must flow over the hard rock and through the fractures and wash away the easily-eroded rock layer beneath.
Below is a model of the falls area. Note the similarities between it and the picture above. Layers of ceramic tiles, representing the Platteville formation, rest on a sand layer, representing the St. Peter’s Sandstone. The vertical breaks between the stacks of tiles are similar to the fractures found in Platteville.
In the photo below, water flows across the tiles in the model. This water represents the Mississippi River. As flowing water makes its way through a fracture in the Platteville Formation, it erodes the underlying Saint Peter Sandstone to form a cliff. If water flows over this cliff edge, a waterfall forms. However, the waterfall does not necessarily stay in one place. Vertical cracks, or fractures, which run through the Platteville Formation, allow water to seep down through the Platteville Formation to erode the underlying Saint Peter Sandstone. This causes the large blocks of limestone to collapse and fall, causing the waterfall to retreat upstream.
The above picture demonstrates this process. Water seeps through the fractures and erodes the sandstone underneath. You can click on the link below to see a video of a model demonstrating the erosion of this process.
Typically, textbooks explain that waterfalls retreat through the erosion of the hard layer of rock, where the falls retreats by a few millimeters a year. However, as the model demonstrates, Saint Anthony Falls did not retreat millimeter by millimeter, but in spectacular collapses as meter-wide blocks of rock fell, crashing into the river below.
The image at the left comes from the Minnesota Historical Society and shows huge limestone blocks that have broken free from the falls. (Photographer: Beal's Gallery, ca. 1865)
Huge blocks of collapsed rock from the falls’ retreat used to lie in the riverbed from St. Anthony Falls all the way to St. Paul, preventing the millers from shipping their products directly from the falls. As a result, they had to rely upon St. Paul, which became the shipping center for most of the products produced at the falls. This was a sore point for Minneapolis who had to share its milling profits with St. Paul. As early as the 1850s, Minneapolis had tried removing boulders to improve river traffic. However, it was not enough. Minneapolis had to wait until the 1960s, when the Army Corps of Engineers built two locks near the falls. Finally Minneapolis could ship its products downriver from the falls. Ironically, only two mills still operated at the time.
The retreat of the falls from St. Paul to Minneapolis. Picture from the "Falls of St. Anthony Historical Marker" located in Father Louis Hennepin Bluffs Park. Marker apart of the St. Anthony Falls Heritage Trail.
Click the footnote number to return to the paragraph or click here to return to the top of the page.
1. Thomas Madigan, “The Geology of the MNRRA Corridor” in John O. Anfinson, River of History: A Historic Resources Study of the Mississippi National River and Recreation Area (St. Paul: Army Corps of Engineers, 2003), p. 23
2. Madigan, “The Geology,” p. 26
3. Madigan, “The Geology,” p. 23 -25
4. John O. Anfinson, River of History: A Historic Resources Study of the Mississippi National River and Recreation Area (St. Paul: Army Corps of Engineers, 2003), p. 117
5. Kent Kirkby, Professor of Geology and Geophysics, University of Minnesota-Twin Cities. Personal Interview. July 15, 2011
8. Demonstration of retreat of the Falls recorded June 21, 2011 at Ford Hall, University of Minnesota-Twin Cities