Life on Mill Street

We held another Cambrian hike at Interstate State Park on Saturday, and I thought it would be a good time to revisit some ground we covered in June of 2016. Specifically, at that time I glossed over the fossils of the Mill Street Conglomerate, but they're worth another look.

I'd like to start off by introducing you to another geologist, C. P. (Charles Peter) Berkey (1867–1955). (If you're curious, I'm paraphrasing from a short biography available online, Kerr 1957.) Berkey was a student at the University of Minnesota in the 1890s; in fact, he got his bachelor's degree (1892), master's degree (1893), and doctorate (1897) from the university. For his doctoral work, he described the geology in and around Taylors Falls and St. Croix Falls; his findings were published in several issues of The American Geologist (Berkey 1897, 1898a, 1898b), which, as you may remember, was run by Newton Horace Winchell's family. Among his contributions was the naming of the Franconia Formation, from outcrops at Franconia. A significant part of his doctoral work was devoted to an unusual fossil assemblage he found in a conglomerate at Taylors Falls. Paleontology, however, was not his primary interest. As things turned out, Berkey was really an engineering geologist. He cut his teeth on the Catskill Aqueduct project and worked his way up to dams, being involved in the construction of Grand Coulee Dam, Hoover Dam, Hungry Horse Dam, and Shasta Dam. He also eventually became the president of the Geological Society of America and was part of Roy Chapman Andrews's Central Asiatic Expedition to the Gobi. All in all, a full career, and quite a contrast from the tumult of his fellow graduate student, friend (Weiss 2000), and frequent "guest" of this blog, Frederick William Sardeson.

A geologic map of the Taylors Falls/St. Croix Falls area, from Berkey (1897); some of the colors for the geologic units are difficult to distinguish.

Dams, aqueducts, and the GSA were all far in the future for Berkey in the 1890s when he went to work in Taylors Falls. Berkey found several areas with outcrops of the basalt conglomerate, although only the Mill Street location and another along the river were of any size. At the time, the Mill Street Conglomerate was located very near a railroad; the railroad itself is long gone, although Mill Street remains, but Interstate State Park uses the rail bed for a nice level trail. It was in the sedimentary matrix of the conglomerate that Berkey found fossils of various tiny animals living on and around the basalt boulders of the ancient sea cliffs. These fossils went beyond the typical brachiopod/trilobite-dominated assemblages of the other Upper Cambrian rocks in the area; instead, the fauna included abundant small mollusks, generally limpet-like or conical-shelled snaily things. Most of these animals were completely new to science, and he ended up naming about a dozen species and varieties of mollusks and two species of trilobites from the fauna. As happens sometimes, Berkey's collections have been divided and whittled down over the years; a lot of the material went with Berkey to Columbia University and eventually to the American Museum of Natural History, while a small group of fossils at the University of Minnesota which he might have collected have been donated to the National Museum of Natural History (Yochelson and Webers 2006).

Two fossil taxa described by Berkey (1898b) (plus an early example of photos being used in a paleontological publication). Cheilocephalus st. croixensis is a trilobite (you're looking at a glabella, kind of like a forehead if you want to anthropomorphize), now known as Cheilocephalus stcroixensis because punctuation is not permitted in taxonomic names. Hypseloconus recurvus var. elongatus is a monoplacophoran, now known as Hypseloconus recurvus (Yochelson and Webers 2006).

Yochelson and Webers (2006) revisited and reevaluated Berkey's species. They found that overall he had done well with his naming (not always a given for 19th century taxonomists!), although some of the species are now placed in different genera. Most of what Berkey considered gastropods are now considered monoplacophorans (or tergomyans, if you prefer), with a few true early gastropods thrown in. Yochelson and Webers (2006) compared the mollusks to a broadly similar association of comparable age in the Ozarks. The Ozarks mollusks were found with stromatolites, and presumably were grazing on them; there is no evidence for stromatolites at the Mill Street Conglomerate, but microbial films might have played a similar role. Berkey's trilobites proved to be a bit more problematic, due to some misidentifications. Berkey also noted the presence of inarticulate brachiopods, but apparently wasn't much interested in them, and his existing collections do not include any (Yochelson and Webers 2006).

Today, the outcrop no longer produces mollusks, suggesting Berkey's timing was just right to exploit what had just been affected by railroad construction (Yochelson and Webers 2006). The brachiopods proved more enduring, and at one time the site was extensively collected by commercial outfits for the educational trade, allegedly to the point of undercutting the outcrop (Yochelson and Webers 2006). I can't speak to the history, having not been there, but the outcrop does indeed have an undercut profile, and there hasn't been much for fossils either time I've visited. We did find one piece of sedimentary matrix with small brachiopod fragments, which has gone to the park for their use.

Click to embiggen and see the brachiopod fragments, highlighted by white arrows.

References

Berkey, C. P. 1897. Geology of the Saint Croix Dalles. Part I.—Geology. American Geologist 20:345–383.

Berkey, C. P. 1898a. Geology of the Saint Croix Dalles. Part II.—Mineralogy. American Geologist 21:139–155.

Berkey, C. P. 1898b. Geology of the Saint Croix Dalles. Part III.—Paleontology. American Geologist 21:270–294.

Kerr, P. F. 1957. Charles Peter Berkey 1867—1955. A biographical memoir. National Academy of Sciences.

Weiss, M. P. 2000. Frederick William Sardeson, geologist 1866–1958. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 48.

Yochelson, E. L., and G. F. Webers. 2006. A restudy of the Late Cambrian molluscan fauna of Berkey (1898) from Taylors Falls, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 64.