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.