Where to see metro geology, part 2: Minnehaha Regional Park

When we left off in Part 1, we were hanging around the observation area of Lock & Dam 1. Directly west of the observation area is Minnehaha Regional Park, famous as the home of Minnehaha Falls. Minnehaha Falls is one of a series of cataracts that have formed along the Mississippi in the Twin Cities. The basic processes went like this: each time the glaciers rolled through, they filled up the preexisting river system with glacial debris, and each time they retreated, the river system reestablished drainage. The last time the system was reset, the Minnesota–Mississippi network was grinding its way through the Platteville until it scoured into an old channel in the vicinity of Dayton's Bluff. It's a lot easier to wash out loose debris than it is to erode the Platteville, and a waterfall (proto-St. Anthony Falls) formed where the two met. Over the years, the waterfall eroded upriver, and every time the falls passed a tributary, a smaller waterfall ended up forming on that creek or river, too. This is how we got Hidden Falls, Shadow Falls, and a host of falls that are now mostly to well and truly extinct (human intervention in the drainage systems), as well as Minnehaha Falls. There is something of a cottage industry among Minnesota's geologists of reckoning the elapsed time since deglaciation by calculating the rate of retreat of St. Anthony Falls, using historical observations of its locations before the 1870s, the properties of the underlying formations, and the starting point, but that is for another time.

Minnehaha Falls, known somewhat more prosaically as Brown's Falls in the early 19th century.

Minnehaha Falls is the most famous of the secondary falls. It is not far from Fort Snelling, and both Keating (1824) and Featherstonhaugh (1836) visited it during their expeditions through the area. Granted, it doesn't look the same and was going by "Brown's Falls" at the time, but most of us make changes over two centuries. Like the other falls along the Mississippi, Minnehaha Falls is held up by the Platteville Formation (well, and a bit of engineering is added nowadays). It has actually gotten through part of the formation, as you can see from the photo below: Minnehaha Creek has cut down through the Magnolia and Hidden Falls members by the time it has reached the falls, probably via erosion of the weaker Hidden Falls beds. As Longfellow might have put it:

"By the flowing Mississippi,
Cuts the creek of Minnehaha,
Descending through the upper Platteville,
Tumbles off thin-bedded Mifflin,
Over chunky Pecatonic,
And a shale of least resistance,
Into sandstone of St. Peter,
Falls the creek of Minnehaha."

(I couldn't resist it. Do you know how addicting it is to write in trochaic tetrameter?)

"A head-on view to make it clearer/where erosion has proceeded..." (Augh! I'm doing it again!).

Anyway, the valley of Minnehaha Creek below the falls is shaped by an earlier history as a part of the Mississippi itself. Adjacent to the creek on the northeast is a north-south valley, which includes the "deer pen" area. This is an extension of a diversion around a former island (Grant 1890), similar to the channel to the east of Nicollet Island today. Because there was so much more water moving through the rest of the river, the diversion couldn't keep up when St. Anthony Falls went through and became abandoned, with Minnehaha Creek taking over part of its former channel.

The Platteville Formation is exposed to good effect around the perimeter of the valley. Obviously, it's easiest to get a good look at the upstream end, where you don't have to schlep as far up the side of a ravine. Just below the falls, near the walking bridge, the Platteville is probably at its most accessible in the park. Look for the enormous fallen blocks of Mifflin Member rock.

"Block of Mifflin that eroded/With a level book for scaling/Seven inches and one half inch/Spans the orange book, low center."

The outcrop above this heap shows the Pecatonica and Mifflin members quite well, and would also show the Glenwood Formation if it wasn't quite so shy.

"Here beneath the Pecatonic/Lies the Glenwood undiscovered/If you try to make it clearer/You will sore displease the park staff."

Otherwise, you're looking at something more akin to this:

"Far above the gray block talus/Dwells the Platteville, which is waiting/Waiting just to watch you mis-step/Slide back down upon your rear end."

As you might expect, you can see various Platteville Formation fossils throughout the park, typically in the talus. Like any good area of Platteville Formation, the fossils are perhaps 85 to 90% molds and cast of brachiopods, 5 to 10% snails, and a small fraction of everything else (pers. observation). Minnehaha Regional Park is, of course, a city park (within a National Park Service unit to boot), so you're welcome to take pictures, but leave the fossils behind.

"From the dolomitization/We are left with just the snail molds/And more brachs than can be counted/(Not included in this photo)."

Down toward the mouth of the creek, you begin to run into outcrops of the St. Peter Sandstone. They are exactly as interesting and unusual as any outcrop of the St. Peter Sandstone that exists anywhere. (You are free to take that in the complimentary or the negative sense, depending on your mood. Either way, one of the things the St. Peter is famous for is being consistent.)

By now, you have probably scrambled up and down valley sides a few times, and seen numerous brachiopod molds and casts, and people are probably looking at you funny. This is a good day's work, and you may want to call it a day. If you're not tired yet, and you brought a bike (and you're not parked at a meter), you could also hit the bike paths. It's only a couple of miles to Fort Snelling from the falls...

References:

Featherstonhaugh, G. W. 1836. Report of a geological reconnaissance made in 1835, from the seat of government, by the way of Green Bay and the Wisconsin Territory, to the Coteau de Prairie, and elevated ridge dividing the Missouri from the St. Peter’s River. Gales and Seaton, Washington, D.C.

Keating, W. H. 1824. A narrative of an expedition to the source of St. Peter’s river, lake Winnepeek, lake of the Woods, etc., performed in the year 1823, by the order of the Hon. J. C. Calhoun, Secretary of War; under the command of Stephen H. Long, U. S. T. E. Volume 1. H. C. & I. Lea, Philadelphia, Pennsylvania.

Grant, U. S. 1890. Account of a deserted gorge of the Mississippi near Minnehaha Falls. The American Geologist 6(1):1–6.