Where to see metro geology, part 1: Lock and Dam No. 1

As the snow and ice begin to melt away and the temperature rises above freezing, it once more becomes possible to see the geology of the metro without outfitting a polar expedition. While there's something to be said for iced-over waterfalls and the peace and stillness of a fresh winterscape, it's darned hard to take field notes with choppers on or fogged-up glasses, and frankly if you're doing it to impress people, it won't work. Spring is not a bad time to do some exploring; you don't have to contend with the bugs, the generous Minnesota humidity isn't in full swing yet, and the ground vegetation hasn't obscured many areas (and given ticks places to loiter). The most obvious drawback to springtime geology is the saturation of the ground, particularly in places where the Decorah Shale is at or near the surface. The Decorah likes to form a thick gray-green mud that both cakes footwear and makes the ground admirably slippery, particularly on slopes where you'd just as soon prefer that it wasn't slippery. Throw in the leaf litter from the previous fall, and your April hike can get a lot more interesting. A less-obvious hazard is the insidious action of freeze and thaw cycles. In short, rocks that weren't fractured and liable to fall or slide in the autumn can get that way by the spring.

If you've never gone out to see the bedrock geology of the Twin Cities in person before, the best place to acquaint yourself with the rocks is at Lock and Dam No. 1, near the Ford Bridge. In terms of logistics, it's near major roads and Minnehaha Park, and no hiking is required. The first place to go is the observation area on the west side of the dam, south of the bridge. Parking is available along the access road, which splits off of West River Parkway South about where it turns west and changes its name to Godfrey Parkway, and in Minnehaha Park, although you'll need to walk down a bike path to get to the observation area from there. (Incidentally, the bedrock geology of the Twin Cities is very bike-friendly, and the general areas of interest can all be reached by bike. You'll want to park it somewhere once you get to Location Whatever, of course, but it beats driving around the cities and trying to find a place to park a car all the time.) The attraction of the observation area is that there is a nice cut through the upper St. Peter Sandstone, Glenwood Formation, and most of the Platteville Formation right there for you to see, all together, without having to worry about rocks falling on your head, traffic, or how you're going to get back up the outcrop. It's an excellent place to take pictures or make sketches, should you want visual aids. The photograph below shows it better, but you can quickly recognize the generally uniform off-white sandstone of the St. Peter, the thin recessive gray and brown beds of the Glenwood, and the resistant cliff-forming beds of the Platteville. If you remember the post on the members of the Platteville, you can probably start to pick out the individual members as well: the thin chunky Pecatonica Member at the base, the wavy-thin-bedded flat-walled Mifflin Member above it, the thin recessive Hidden Falls Member next up, and finally at least part of the thick, chunky Magnolia Member.

This is what you're looking for: most of the Platteville (plain gray) over the Glenwood (the thin light gray bed over the thin brown bed) over the St. Peter (off-white).

Once you have seen enough of this cut to familiarize yourself with the rocks, I recommend getting a long view. You can do this one of two ways: ascend the bike path that joins the access road near where the Ford Bridge crosses over (and pass some exposures of the Platteville Formation on the way), get to the top of the hill, and look east across the river to the similar cuts on the St. Paul side; or cross the Ford Bridge into St. Paul and find a place to look back across the river to where you just were. The advantage of this is that it becomes easier to spot the Hidden Falls Member, which presents as a thin gray featureless strip recessed between the Mifflin and Magnolia members; up close it tends to get lost between them. A trip to the Lock & Dam is also a good thing to work into a trip to Minnehaha Park; the same rocks are exposed all through the park, although without one spot that has as much of the section exposed.

Familiar, isn't it? You're looking from the St. Paul side over at the observation area on the Minneapolis side. The St. Peter, Glenwood, and Platteville can be distinguished, and the thin light gray strip within the Platteville is the Hidden Falls Member. (I'm told this is also a great place to watch birds of prey.)

From the Minneapolis side to the St. Paul side, with the sign for the old Ford Plant in the background. The same formations are exposed here.

Because we've got the Platteville here, it shouldn't surprise you to find that fossils have documented from the dam exposures and surroundings. Clinton R. Stauffer, making his triumphant reappearance after two posts without him, did a fair bit of collecting in this area during the 1920s and 1930s, as mentioned in the worm post. At that time, this area was being extensively developed. The now-defunct Ford Plant opened in 1925, the Ford Bridge was built from 1925 to 1927, and an expansion to the dam was completed in 1932. As any good geologists knows, projects that move a lot of earth can provide great windows into otherwise buried geology, windows that are usually destroyed quickly as construction progresses. Detailed stratigraphic sections from both sides of the dam are published in Stauffer and Thiel (1941). Interestingly, the sections also include the Carimona and shale members of the Decorah Shale, above the Platteville, neither of which are visible at this time on either side of the dam. There is surficial cover now, of course, but it's also possible that at least some of those younger rocks were removed during the various construction projects. The Platteville was found to have the usual selection of horn corals, brachiopods, bivalves, snails, trilobites, and ostracodes, with bryozoans, nautiloids, and crinoids joining the party in the overlying Decorah units. If you're interested in the naming game, Stauffer ended up describing 40 species of worms from scolecodonts (Stauffer 1933), 15 species of conodonts (Stauffer 1935), and a chitinozoan (an enigmatic microfossil; Stauffer 1933) from the Decorah Shale near the bridge. Don't expect to find any of them yourself, unless you are extremely myopic: they're all microfossils. Also, August Foerste (1932) named the nautiloid Metaspyroceras perlineatum from the Ford Plant.

Platteville block (Magnolia Member, probably) with molds and casts of brachiopods and snails, in the vicinity of the bike path, with an extremely shiny quarter for scale.

References:

Foerste, A. F. 1932. Black River and other cephalopods from Minnesota, Wisconsin, Michigan and Ontario (Part 1). Denison University Scientific Laboratories Journal 27:47–137.

Stauffer, C. R. 1933. Middle Ordovician Polychaeta from Minnesota. Geological Society of America Bulletin 44(6):1173–1218.

Stauffer, C. R. 1935. The conodont fauna of the Decorah Shale (Ordovician). Journal of Paleontology 9(7):596–620.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Bulletin 29.