Out in the fall: the southeastern metro

There is a brief period of time during the fall in the Twin Cities metro after the heat, humidity, insects, and plant growth of the summer have subsided, but before the first significant snowfall has hidden the ground. It usually starts around the middle of October and lasts until about Thanksgiving, depending on temperature and moisture. It's a great time to be outside, if you don't mind the increasing chill, and it gives you one last chance at the rocks. There's one area of bedrock outcrops on the Mississippi before we get to the confluence with the St. Croix, from about Grey Cloud Island and downstream, and we've only really stopped there once, although the rocks are covered here as well. However, there's a reason it hasn't been discussed much: there's not as much to see. If you're down in the area, though, here's the rundown:

Going southbound on 61 out of St. Paul, you may notice the surround topography begin to change. (If you are not driving, pull out your copy of Mossler 2013 so you can follow along on with the geologic map). Dayton's Bluff is about the last stand for steep, Platteville-topped bluffs near the Mississippi River. The bluffs have been receding from the Mississippi since the confluence with the Minnesota River near Fort Snelling. The Platteville doesn't go away completely, but on the south and west side of the Mississippi it is confined to a buried platform in the Mendota Heights area, and on the east and north side it is mostly limited to a buried ridge between the Mississippi and a parallel old channel that leads into Cottage Grove Ravine Park. (That glacial drift really buries things.) Here's the dime tour explanation for what's going on: glaciation. The Minnesota was once a much more formidable river, because it drained glacial meltwater from enormous Lake Agassiz to the north. Every so often, an ice dam or other barrier would fail on the lake, and a catastrophic outwash would sweep through the floodplain, further scouring it. This more active incarnation of the river is known as Glacial River Warren, and it left behind a floodplain that dwarfs the Minnesota. The Mississippi was just a small tributary. Another glacial factor at work here is the phenomenon of buried channels. Minnesota was glaciated multiple times, and each time the new glaciers buried channels carved after the previous glaciers. After a glaciation, drainage would reform, sometimes in or near old channels, sometimes in completely different places. The modern Mississippi is in a new post-glacial channel up to about Dayton's Bluff, where it encountered an old channel and settled back into familiar digs. Pushing out loose debris is faster than cutting through bedrock, so a step began to form, which turned into a waterfall, which began retreating upstream and became St. Anthony Falls and all the smaller falls (as well as a long-extinct branch that went up the Minnesota valley until it ran out of Platteville caprock, in the vicinity of the airport).

Anyway, the river cuts down deeper through bedrock beyond Dayton's Bluff, all the way through the Prairie du Chien and into the Jordan Sandstone. If you were taking a boat down the Mississippi, you'd start to encounter outcrops of the Prairie du Chien around Newport. They don't become extensive until you reach Grey Cloud Island. Once you get to about the south end of Spring Lake, the Prairie du Chien bluffs become quite steep and tall. For the explanation of this new deviltry, we have to go back somewhat farther in time than the Ice Age. In the Hastings area, a series of ancient faults related to the Midcontinent Rift of about 1.1 billion years ago trace the southeastern margin of a geologic feature called the Twin Cities Basin, discussed back in this post. The Twin Cities Basin is an area of bedrock that is down-dropped relative to the surroundings because of faults and folds. It is because of this that Late Ordovician rocks make up the bedrock of the Twin Cities, while all around it, the bedrock found at similar elevations is older. My favorite explanation for the basin is that the thick pile of volcanic rocks in the rift did not cool and settle evenly, and the Twin Cities Basin is one such area where the contracting rocks took advantage of the rift faults and formed a geologic "pockmark". The upshot of the presence of the faults is that the durable Prairie du Chien has become the rock unit next to the river, and it holds up much more like the Platteville than the St. Peter (probably better than the Platteville, because it's substantially thicker).

Grey Cloud Island

The history of scientific description of the rocks along the Mississippi in Minnesota goes back to well before statehood, and in the late 1840s to early 1850s, we had David Dale Owen and later Benjamin Franklin Shumard. Both of these men are important figures in geology for several states; unfortunately, neither made it to 55, and their work is obscured by the Civil War (and the fact that fire and depredations destroyed much of Shumard's collections in the 1860s). Shumard, working for Owen, reported fossils in the "Lower Magnesian Beds" (early name for the Prairie du Chien Group, "magnesian" being another way of describing dolomite) at two places along the Mississippi. The best find was a bed "densely crowded" with snail casts, about 5 ft (~1.5 m) above the water level, a mile or so above Grey Cloud Island. The other was a site with more snail fossils about 2 miles (3 km) above the confluence with the St. Croix (Shumard 1852). Although the exact stratigraphic positions are not known, by geography the first site was probably in the Shakopee Formation, the upper formation of the group, and the second site was more likely in the Oneota Dolomite, the lower formation of the group.

Today, Grey Cloud Island is noted for the quarrying operation that occupies much of the western end of the island (you can identify it easily both on Mossler 2013 and with Google Maps), as well as for its archeology and eagles. The geology is not especially visible except at the quarry unless you get off of the roads, and it should be noted that outsiders wandering around is a touchy issue on the island (the residents are fed up with people looking for ghosts). The rocks can be seen sporadically along Grey Cloud Island Drive South, although if you're driving around the island you'll probably be more taken with the scenery than a couple of spots with outcrops.

Spring Lake Park Reserve

Spring Lake Park Reserve is a great place for the view.

Directly seeing the rocks is a bit of a problem, because you are on top of the Prairie du Chien platform, and the bluffs are vertical.

This combination of circumstances means that you are deprived the opportunity to look for cuts through stromatolites, like at Route 10, but on the other hand you've still got the view. [Update, 2020/12/13: The situation has changed since then: follow the paved bike path south from the Schaar's Bluff parking area for 10-20 minutes, depending on your walking speed, and receive a nice surprise!] There's a great deal of human history near here as well, just like at Grey Cloud Island: archeological investigations in the 1950s and 2010 found a number of sites going back thousands of years around Spring Lake.

Hastings

Hastings turns up in the geological literature off and on, usually either in connection to the outcrops along the north side of the river across from town, or in connection to one of the local quarrying operations in the Prairie du Chien Group. The north shore is occupied by train tracks, and you'd be too close to get the full effect anyway, which is why it's handy that there are many good places to see the rocks from the south side. The most notable geologic attractions are the Jordan Sandstone (the only place to see it on the Mississippi above the St. Croix) and the faults. The Jordan shows up as the rusty colored strip of rocks beneath the off-white Oneota Dolomite. It's a Cambrian formation, and like the St. Peter Sandstone is not noted for its fossils. The faults cannot be seen directly, but you can certainly see their effects, such as the Jordan Sandstone being visible. The most reliable feature to look for with faults is the vertical offset of the Oneota Dolomite. The observation area at Lock & Dam 2 has the nearest view, but you're not going to go wrong closer to town, either.

There is also an unexpected bonus lurking in the riprap used along the shore near the boat launch. Scattered in the stones are occasional fossils, including brachiopods and bivalves. When I first noticed them, I thought that the stones were from the Prairie du Chien, because it is quarried locally, but the types of fossils bothered me; the Prairie du Chien is not noted for its bivalves, and if in fact it can be said to be noted for any fossils at all besides stromatolites, it would probably be snails. I checked with people from Hastings Parks & Recreation and the company that brought in the stones, and it turned out that although some of the local riprap is indeed Prairie du Chien, this stuff came from a quarry in Cannon Falls, which is in the Prosser Formation. The Prosser Formation is the unit above the Cummingsville Formation (Mossler 2008), which you may remember just pokes out above the Decorah at the Brickyard and nearby. Thus, we have here a totally unexpected "outcrop" of a formation not present in the Twin Cities. Building stone is great for this, and there is in fact a guidebook for walking tours of St. Paul (Kain 1978), which is another great thing to do in the fall.

References:

Kain, J. 1978. Rocky roots: three geology walking tours of downtown St. Paul. Ramsey County Historical Society, St. Paul, Minnesota.

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, MN. Report of Investigations 65.

Mossler, J. H. 2013. Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Miscellaneous Map Series 194. Scale 1:125,000.

Shumard, B. F. 1852. Geological report of local, detailed observations, in the valleys of the Minnesota, Mississippi, and Wisconsin rivers, made in the years 1848 and 1849, under the direction of David Dale Owen, United States Geologist, by B. F. Shumard, head of subcorps. Pages 481–531 in Owen, D. D. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, Pennsylvania. Available at http://archive.org/details/mobot31753000174885 (plates not included), https://archive.org/details/reportofgeologi00owen (full plates) or http://books.google.com/books?id=Y_ZYAAAAYAAJ.