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.