The geological view at Fort Snelling (or, more properly, below it) is not particularly inspiring today, if you are looking for bedrock and not the effects of post-glacial river action. (Fort Snelling is a great place if you're pondering post-glacial river action, the failure of post-glacial lakes, outburst floods, and so forth, but that is for another day.) The people responsible for the fort had to make a choice: keep the bluffs uncovered and risk parts of the fort eventually falling off, not to mention the inevitability of the Platteville Formation shedding chunks onto a path in a popular state park from a significant height; or put up something to forestall such events. Given that it's called "Fort Snelling State Park" and not "Crumbling Fort Over Hazardous Bluffs State Park", you can guess which way things went.
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| Oh, well... |
The stretch of bike path between Fort Snelling and Coldwater Spring, though, has some picturesque outcrops, mostly of the St. Peter, which nicely complements the Platteville outcrops of Coldwater. (Or, I suppose, if you started at Fort Snelling State Park, Coldwater complements the lower part of the trail.) The St. Peter shows up beginning just south of where the bridge for Fort Road (aka West 7th or Highway 5) crosses over the path and continues along most of it.
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| When I say "just south", I mean "just south" |
The west part of the park doesn't show much for outcrops, being generally removed from the bluffs. There are a couple of very large Platteville blocks that separated from the now-retreated bluffs at some point, but otherwise you're looking at the effects of processes from the past few thousand years.
Now, Fort Snelling's place in the history of Minnesota geology:
We've already taken a look at Major Stephen Long's 1823 expedition that visited then-Fort St. Anthony, as documented by William Keating (Keating 1824). The fort offered a commanding view of the Mississippi and "St. Peter" (=Minnesota) rivers, perched on the northeast side of the confluence. Beneath it was the classic Minneapolis–St. Paul river bluffs of limestone/dolomite over sandstone, which we would now call the Platteville Formation and St. Peter Sandstone. The Glenwood Formation would be in there too, but it's not especially obvious. As noted in the previous post, Keating was of the mistaken impression that there was another limestone unit below the sandstone, which later observers recognized as fallen blocks of Platteville (Featherstonhaugh 1836; Nicollet 1843). Featherstonhaugh (1836), Nicollet (1843), and Shumard (1852) all visited the fort and commented on the geology.
Fort Snelling's greatest contribution to our knowledge of bedrock geology is serving as the "type locality" for the St. Peter Sandstone. A type locality for a formation is intended as the reference section for that geological unit. Of course, seeing the sandstone directly beneath the fort is a little difficult today (unless you have x-ray vision, in which case you probably have better things to do with your time), but it's still well-exposed nearby. The sandstone was named when the Minnesota River was still known as the St. Peter River, preserving for all time that bit of history. The citation is apparently Owen (1847), but I haven't seen it; I can usually track down pre-1923 documents relatively quickly, but not this one. Owen (1852) did include two figures of a snail shell from a few miles upstream, though. These illustrations and a depiction of a slab of brachiopods from St. Anthony Falls are the oldest illustrations of Twin Cities fossils I've been able to find.
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| Sorry about the size; this is a "Bucania, three miles above Fort Snelling", as figured in Owen (1852) |
More recently, Fort Snelling has been a favorite of field trip guidebooks and books on state geology, rating appearances in Bray (1985), Campbell and Kirkby (2005), Ojakangas (2009), and Pound et al. (2011). Stauffer (1934) and Stauffer and Thiel (1941) provided descriptions of the Fort Snelling stratigraphy. Per the latter, if you go down far enough in the subsurface, you do eventually get to carbonate rock, but it wouldn't have been seen by Keating; there's 155 ft (47 m) of St. Peter before you hit the Shakopee Formation. Some fossils were noted in the Platteville, but apparently they were not interesting enough to elaborate upon. Stauffer and Thiel also put 11 ft (3 m) in the Glenwood, which seems rather thick; probably some of the lower part is actually upper St. Peter.
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| "Yes, yes, that's too thick. I don't care what he says. Just get me a boat and a Jacob's staff!" (from Wikimedia Commons) |
References:
Bray, E. C. 1985. Billions of years in Minnesota: the geological story of the state (2nd edition). The Science Museum of Minnesota, St. Paul, Minnesota.
Campbell, K., and K. Kirkby. 2005. The geology of the Mississippi River Valley, Twin Cities region: using an urban river for inquiry-based earth science education. Pages 1–7 in L. Robinson, editor. Field trip guidebook for selected geology in Minnesota and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook Series 21.
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.
Nicollet, J. N. 1843. Report intended to illustrate a map of the hydrographical basin of the upper Mississippi river. Blair and Rives, Washington, D.C.
Owen, D. D. 1847. Preliminary report of the geological survey of Wisconsin and Iowa. Pages 160–174 in U.S. General Land Office Report 1847. Washington, D.C. 30th Congress, 1st session, Senate Executive Document 2, Congressional Serial Set Volume 504.
Ojakangas, R. W. 2009. Roadside geology of Minnesota. Mountain Press Publishing Company, Missoula, Montana
Owen, D. D. 1852. 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.
Pound, K., K. Campbell, and L. Schmitt. 2011. An examination of the bedrock geology and the Mississippi River valley in the Twin Cities: pedagogical strategies for introductory geology field trips. Pages 505–523 in J. D. Miller, G. J. Hudak, C. Wittkop, and P. I. McLaughlin, editors. Archean to Anthropocene: field guides to the geology of the Mid-Continent of North America. Geological Society of America, Boulder, Colorado. Field Guide 24
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.
Stauffer, C. R. 1934. Type Paleozoic sections in the Minnesota Valley. Journal of Geology 42(4):337–357.
Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Bulletin 29.
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