Sunday, October 31, 2021

Barn Bluff

One of the outstanding geological sites in southeastern Minnesota is Barn Bluff (He Mni Can to the Dakota, La Grange to the early French explorers*) in Red Wing. The bluff is a bedrock island oriented roughly east–west, adjacent to the Mississippi River just above the Lake Pepin section. If you were to have gone back to the early postglacial period, it would have been a literal island thanks to meltwater filling the Mississippi River valley.

*All of the names are kind of prosaic, actually. "He Mni Can" is "hill, water, wood", "La Grange" or "Lagrange" is "the barn" for its general shape, and the English name is just a translation of the French.

The view of the south side of the west end. Observant eyes may notice that the color of the lower outcrops change near the left side of the picture, from orangish on the left to banded green on the right. Read on for why!

Sunday, October 24, 2021

The Matagamon Sandstone

I recently had occasion to go to north-central Maine for work. If you've never been there, the geology is the tectonic equivalent of taking a bunch of little leftover bits of colorful modeling clay and smooshing them together: the area was on the margin of the North American craton during the Paleozoic and thus was the recipient of a conveyor belt of crustal fragments. Of course, when this happens, you get all kinds of interesting structural features and metamorphism, which does unfortunately tend to obscure the original geology. Tack on the Pleistocene glaciations and subsequent dumping of drift, followed by the growth of forests, and you can see how things can get complicated and confusing to follow.

One of the geologic units I observed in this region is the Lower Devonian Matagamon Sandstone. The Matagamon has been interpreted as part of a deltaic system that advanced to the northwest during the Acadian Orogeny (Hall et al. 1976; Pollock et al. 1988). We've got a pretty good idea of when its deposition ended because it transitions upward into the Traveler Rhyolite (Rankin 1965), the explosive component of a supervolcano that erupted approximately 407 million years ago (Seaman et al. 2019). Curious about the guts of that volcano? Look no further than Katahdin.

Maine's tallest mountain? The crystallized heart of one of North America's largest volcanoes? Why not both?
(Incidentally, they told us this was the best time of year to do geologic work in north-central Maine. They were right!)

Anyway, the Matagamon is a fossiliferous unit, with an assemblage dominated by brachiopods. Clarke (1909) described a few assemblages from this formation, which was then identified as the Moose River Sandstone (it did not receive its present name until Rankin 1965). The fauna includes plant fragments, corals, brachiopods, monoplacophorans, bivalves, nautiloids, gastropods, tentaculitids, trilobites, crinoids, and invertebrate trace fossils. Brachiopods certainly seemed to be the most abundant fossils in the outcrops I saw.

A fairly large brachiopod with Leptaena-type ridges.

A bulbous shell on the left and a cylindrical object of unknown origin on the right.

A shell bed.

Fossils tended to be abundant in localized areas, mostly preserved as molds, external casts, and steinkerns, with occasional shell material in the brachiopods. (Overall, the rocks, the fossils, and their preservation rather reminded me of the somewhat younger Mahantango Formation from the Delaware River valley.) In some cases, the fossils had been stained bright orange, very appropriate for autumn and Halloween.

Slightly orange small flat ribbed brachiopods, resembling potato chips.

Two strongly orange brachiopods: a small shell with few but heavy ribs on the left, and a much larger brach with many fine ribs in the center.

Some calcitic material remains with these shells.


Hall, B. A., S. G. Pollock, and K. M. Dolan. 1976. Lower Devonian Seboomook Formation and Matagamon Sandstone, northern Maine: a flysch basin-margin delta complex. Pages 57–63 in L. R. Page, editor. Contributions to the stratigraphy of New England. Geological Society of America, Boulder, Colorado. Memoir 148.

Pollock, S. G., A. J. Boucot, and B. A. Hall. 1988. Lower Devonian deltaic sedimentary environments and ecology: examples from the Matagamon Sandstone, northern Maine. Pages 81–99 in R. D. Tucker and R. G. Marvinney, editors. Structure and stratigraphy. Maine Geological Survey, Augusta, Maine. Studies in Maine geology: papers to commemorate the 150th anniversary of C. T. Jackson’s reports on the geology of Maine. Volume 1.

Rankin, D. W. 1965. The Matagamon Sandstone–a new Devonian formation in north-central Maine. U.S. Geological Survey, Washington, D.C. Bulletin 1194-F.

Seaman, S. J., R. Hon, M. Whitman, R. A. Wobus, J. P. Hogan, M. Chapman, G. C. Koteas, D. Rankin, A. Piñán-Llamas, and J. C. Hepburn. 2019. Late Paleozoic supervolcano-scale eruptions in Maine, USA. GSA Bulletin 131(11–12):1995–2010.

Sunday, October 10, 2021

Compact Thescelosaurus Year Six

The time has come again for the annual review of The Compact Thescelosaurus. This year's new page is on aetosaurs and their close relatives, with the classification diagram page updated. (Don't forget, Wednesday the 13th is also National Fossil Day!)

Aetosaur Desmatosuchus spurensis is one of the subjects of the NPS Prehistoric Life Coloring Book. Coincidentally, an aetosaur also featured in the 2014 National Fossil Day artwork.