Frozen Minnehaha Falls

A week ago, a colleague of mine visited Minnehaha Falls and took the picture above to show me.  The bedrock of the Minnehaha Falls area consists of the resistant Platteville Limestone, the Glenwood Shale and the St. Peter Sandstone.  As water falls over the more resistant limestone, it erodes the weakly cemented St. Peter Sandstone at the base of the 53-foot waterfall.

What's interesting to me in the picture isn't how the water has slowly frozen in the waterfall through the course of a Minnesota winter, it's all of the other frozen water to the right and left of Minnehaha Falls.  In between the limestone and sandstone is the Glenwood Shale.  As water soaks downward through the upper layers of soil it comes to this impermeable layer of shale and flows laterally until eventually reaching the eroded valley carved by Minnehaha Creek.  At this point the water cools to the point of freezing, forming a ring of large icicles.

On September 29, 2012, I took a gigapan of Minnehaha Falls (with a very low amount of water).  For a full size version of the gigapan, please click here.

For a more detailed post on the Minnehaha Falls, please click here.

Minnehaha Falls - In a dry year

Minnehaha Falls is a 53-foot waterfall found near the confluence of Minnehaha Creek and the Mississippi River.  The falls form as the creek flows over the Platteville Limestone caprock to erode the underlying and weaker St. Peter Sandstone.  Between the limestone and sandstone is a thin layer of the Glenwood Shale.  The Platteville, Glenwood and St. Peter are all Paleozoic in age.  An earlier write-up of the geology of the area can be found here.

The month of September 2012 was the second driest month of September on record for the Minneapolis-St. Paul area.  Officially, just 0.30" of rain fell (the driest month of September was in 1882, when 0.27" of rain fell).  Ironically, September 2011 was the third-driest month of September with 0.36" of rain.

The lack of rain has had an effect on Minnehaha Falls, it is nearly dry now.  The source of Minnehaha Creek is Gray's Bay on Lake Minnetonka and in periods of low rain, the outlet to the creek is closed to preserve the lake level.

Without the flowing water, it is much easier to see the sandstone, shale and limestone layers.

See the full size version of the gigapan here.

Minnesota Geology Monday - St. Peter Sandstone

The St. Peter Sandstone outcrops at numerous locations around Minneapolis and St. Paul, including Minnehaha Falls and throughout the Mississippi River valley.  The sandstone is world famous as a nearly pure sandstone, it is over 99% quartz.  The sandstone is not well cemented and as such, can be easily eroded.  The sandstone was deposited 458 million years ago during the Ordovician Period.  The sandstone was named for outcrops near the confluence of the Mississippi and Minnesota rivers at Fort Snelling.  An early name for the Minnesota River was the St. Peter River, this name was later given to the sandstone.

In the Twin Cities, the sandstone is overlain by the greenish Glenwood Shale and the Platteville Limestone.  The St. Peter Sandstone was deposited over a large area of the midwest United States, in Minnesota it is at the northernmost extent.  The average thickness of the sandstone, in the Twin Cities, is 155 feet.

Nearly all the sand grains within the sandstone are rounded, the result of the mechanical weathering process of abrasion.  Predominantly made of the resistant mineral quartz, four other minerals are present in the sandstone in small quantities, tourmaline, zircon, rutile and garnet.  Radiometric dating of the zircon indicates that they are derived from rocks 2,600 to 1,000 million years in age.  These ages leave plenty of time for the mineral grains to be weathered into the nearly round shapes they are in today.

Because the sandstone is not well cemented, it is easily excavated by humans and animals.  Birds use the sandstone as nesting burrows, humans have used it for numerous purposes.  From 1926-1959, the (now closed) Ford Twin Cities Assembly Plant excavated the sand to produce window glass for their vehicles.  In St. Paul, humans excavated caves in the sandstone to be used as storage spaces, growing mushrooms and during Prohibition, a restaurant and nightclub known as the Wabasha Street Speakeasy.

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.