Minneopa is a Dakota word meaning 'water falling twice' representing the two falls within the state park. Minneopa State Park is a short drive from Mankato (for a little bit of United States history, Mankato is the site of the largest mass execution in the U.S., but that's for another story).
The falls formed when Minneopa creek flows over the Cambrian-aged Jordan Sandstone. Because of differential cementation of the sandstone, areas with more cement form the top level of the double waterfall.
The upper falls is small, falling six or seven feet. Once over these first falls, Minneopa creek flows approximately seventy feet before reaching the lower falls.
At the lower falls, the creek falls 40 feet. This particular waterfall on the creek is the second highest waterfall in southern Minnesota. The highest waterfall in the region is only slightly higher and is also found nearby.
Minneopa Falls was formed when Glacial Lake Agassiz discharged catastrophically via Glacial River Warren. Glacial River Warren carved a large valley (250 feet deep and up to 5 miles wide in places) that created various knickpoints that caused rivers or creeks to adjust their gradients to reach this new baselevel. Glacial River Warren created this new knickpoint at Minneopa Creek approximately 9,700 years before present and during that time, the falls have migrated upstream almost 0.75 miles.
Beneath the lower falls, if the waterlevel in the creek is low enough, you are able to walk quite near falls themselves and examine large sandstone blocks that have fallen from overhead.