Monday, March 12, 2012

Minnesota Geology Monday - Minnehaha Falls

Minnehaha Park (http://www.minneapolisparks.org/default.asp?PageID=4&parkid=252) is home to Minnehaha Falls.  The first Google Earth image shows the waterfall's location in comparision to the Mississippi and Minnesota river (which enters the picture from the bottom left).  The second image gives a sense of the changes in elevation in the immediate area around the falls.





Minnehaha Creek flows approximately 22 miles from it's source, Lake Minnetonka, to the Mississippi River.  Near the confluence of the creek and Mississippi River is Minnehaha Falls, which is a 53-foot high waterfall.









Geology of the Twin Cities area provides an excellent record of the transgression and regression of Paleozoic oceans.  The three Ordovincian rock formations (from top to bottom) found at the Minnehaha Falls are the Platteville Limestone, the Glenwood shale and the St. Peter Limestone.  The Platteville Limestone is fossiliferous, including numerous brachiopods, bryozoans, corals, etc.  The Glenwood Shale is found in a thin layer throughout the region.  The St. Peter Sandstone is world famous as a well sorted, nearly pure quartz sandstone.




As water of Minnehaha Creek flows over the resistant Platteville Limestone, it erodes the easily erodable St. Peter Sandstone.  In the valley below the falls, large blocks of Platteview Limestone can be found that have fallen as the sandstone was eroded away, effectively undercutting the limestone.   This results in the gradual movement of the waterfall upstream.





Minnehaha Falls formed when Glacial River Warren (an outlet of Glacial Lake Agassiz) suddenly discharged carving an enormous valley through southern Minnesota.  In the Twin Cities area, Glacial River Warren exposed the resistant Platteville Limestone six miles south of present Fort Snelling forming a waterfall nearly one mile wide.  As this waterfall moved six miles upstream to Fort Snelling, the much smaller post-glacial Mississippi River flowed into the larger channel, continually eroding the sedimentary rocks, now forming St. Anthony Falls.

St. Anthony Falls moved upstream at about 2.5 feet per year, and when it reached the confluence of the Minnehaha Creek, the waterfall began to move up both the river and the creek.  Minnehaha Falls continues to move upstream, while further movement of St. Anthony Falls on the Mississippi River was prevented with the construction of a concrete spillway in 1956.








The pictures above show the Minnehaha Falls during June of 2009 when the outlet on Gray's Bay (Lake Minnetonka) was closed due to low water of the lake.