The Glenwood Formation

Beneath the Twin Cities, sandwiched snugly between the crumbling off-white edifice of the St. Peter Sandstone and the proudly jutting gray to tan Platteville Formation, is a gray to brown layer about 3 to 5 feet thick (or about a meter to a meter and a half for our metric friends), composed primarily of shale. This is the Glenwood Formation, sometimes known as the Glenwood Shale (which is a bit of misnomer because of the sand content). Like a fawn or baby bird, it hides from the eye, although we know it must be there, because heavens knows where else we would get adult deer and birds, and the Platteville has yet to collapse onto the St. Peter. The only reliable way to find it is to look for places where both the Platteville and St. Peter are exposed, and then check the area between them. Even then, you might not be able to see it well; for example, at Minnehaha Falls, the Glenwood is so deeply recessed you can't see much at all. The most obvious exposures are at Lock & Dam 1 and Dayton's Bluff.

Lock & Dam 1 to the rescue! The Glenwood is the brownish bed above the off-white St. Peter, and the overlying light gray bed below the chunky Pecatonica Member of the Platteville.

The Glenwood has received limited respect. It seems to have been first reported in Winchell (1877), although it did not receive a name for some time. At various times, the Twin Cities version has been described as a member of either the St. Peter (Dott et al. 1986) or the Platteville formations (Stauffer and Thiel 1941). It is somewhat more impressive in neighboring states, and is even sometimes divided into members, although there's not much call for that in the Twin Cities Basin (Mossler 2008). What is happening in those other areas is that at least some of the additional thickness appears to be at the expense of the lower Platteville (i.e. the Platteville depositional conditions arrived later in those areas, so the Glenwood is thicker) (Sloan 1972).

Seems like a good time to bring this out again.

The traditional interpretation of the Glenwood is as a transition between the depositional and environmental conditions of the St. Peter and the Platteville. You go from the nearshore sand of the St. Peter to more offshore mud for the Glenwood to the carbonate bank of the Platteville (Webers 1972). This is not a terrible way to think about it; although there does appear to be some kind of depositional hiatus at both the lower and upper contacts, these pauses were apparently not lengthy enough to cause extended erosion (Mossler 2008), so the depositional conditions of one formation did more or less lead into the next. A tweaked model for the formation is one of reduced sediment input and a different sediment source: instead of the St. Peter's sand source, finer particles are being brought in, and probably at a lower rate (Schutter 1996). Better yet, you can invoke a combination of both. For whatever reasons, people seem to hate multifactorial explanations, but I've never understood why. Why, for example, can't we get behind the creatures of the Cretaceous being clobbered by the volcanic eruptions of the Deccan Traps, loss of habitat with the recession of the transcontinental seas, and the celebrated Chixulub impactor, together? Why make me choose?

The Glenwood is not noted for its fossils. Many were probably destroyed by leaching or dolomitization (Schutter 1996; Mossler 2008). On the other hand, the combination of reputation, thinness, and lack of accessibility have not helped, either. A couple of usual suspects, the Washington Avenue Bridge area and Shadow Falls, have produced a few things in the run-of-the-mill way: bryos-brachs-bivalves-snails-trilobites-ostracodes-crinoids-burrows (Johnson 1988). Where the Glenwood excels is with microfossils, which are often made of different, more dissolution-proof materials than shells. The two major types are scolecodonts (bristle worm jaws) and conodont elements (the feeding apparatuses of these eel-like things). Local Glenwood microfossils are described by Stauffer (1933, 1935) and Webers (1966). If you are blessed with extreme myopia you may be able to see these in hand sample, at least as little dark specks.

The scolecodonts got their picture, now it's the conodonts' turn. From Stauffer (1935).

One additional thought on the St. Peter: in some fairy tales, there is a parable or riddle about eternity which turns out to involve a bird scratching its beak on the peak of a mountain every hundred years, with one second of eternity being equivalent to the amount of time it takes to wear down that mountain. The St. Peter Sandstone is a second of eternity. If you've ever tried to find something in it that isn't sand, you'll experience a second of eternity.

[2017/10/27: bonus: close-up Glenwood Formation!

One of the uncommon places where a person can walk right up to it; this is south of the Platteville platform at Shadow Falls.

That chalky light gray layer in the first picture got that way through weathering; past the surface it's medium to dark gray, without obvious macrofossils.

End addition.]

References:

Dott, R. H., Jr., C. W. Byers, G. W. Fielder, S. R. Stenzel, and K. E. Winfree. 1986. Aeolian to marine transition in Cambro-Ordovician cratonic sheet sandstones of the northern Mississippi Valley, U.S.A. Sedimentology 33(3):345–367.

Johnson, J. D. 1988. A new fossil fauna from the Glenwood Shale in southeastern Minnesota. Journal of the Minnesota Academy of Science 53(3):unpaginated.

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

Schutter, S. R. 1996. The Glenwood Shale as an example of a Middle Ordovician condensed section. Pages 55–65 in B. J. Witzke, G. A. Ludvigson, and J. Day, editors. Paleozoic sequence stratigraphy: views from the North American Craton. Geological Society of America, Boulder, CO. Special Paper 306.

Sloan, R. E. 1972. Notes on the Platteville Formation, southeastern Minnesota. Pages 43–53 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Guidebook 4.

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

Stauffer, C. R. 1935. Conodonts of the Glenwood beds. Geological Society of America Bulletin 46(1):125–168.

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

Webers, G. F. 1966. The Middle and Upper Ordovician conodont faunas of Minnesota. Minnesota Geological Survey, St. Paul, MN. Special Publication 4.

Webers, G. F. 1972. Paleoecology of the Ordovician strata of southeastern Minnesota. Pages 25–41 in G. F. Webers and G. S. Austin, editors. Field trip guidebook for Paleozoic and Mesozoic rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Guidebook 4.

Winchell, N. H. 1877. The geology of Hennepin County. Minnesota Geological Survey, St. Paul, MN. Annual Report 5:131–201.