Afton graptolites revisited

There are two great lost fossil sites in the Twin Cities area. (The Brickyards don't count; they're not lost, they just aren't open to collection.) One is the Johnson Street Quarry, where workers cut into a bed in the Hidden Falls Member of the Platteville Formation that had unusually abundant echinoderms. As described in Sloan et al. 1987: 200, "Sardeson mined out a spot in this unit in the old Johnson Street Quarry in Minneapolis (now filled with garbage, and covered with Interstate 35) that produced about 20 specimens of the starfish Protopalaeaster narrawayi, several specimens of the crinoid Cremacrinus arctus (Fig. 16.2), edrioasteroids, cystoids, brachiopods, bryozoans, molluscs, and graptolites." This is slightly out of date; instead of a dump, there's now a Quarry Shopping Center with a Cub Foods, Home Depot, and Target, although even with all those options you can't get an edrioasteroid there anymore. Regardless of the exact character of the overburden, it seems unlikely that anyone will be doing any paleontological follow-up there anytime soon. The other locality is the Afton graptolite locality in the St. Lawrence Formation. We already had a post on why this locality was important; what I'm curious about is where exactly it was. A locality, even if "lost", had to have been *somewhere*, and apart from the scientific and historic interest, there very well could be similar fossils in rocks nearby. Indeed, Hughes and Hesselbo (1997) reported graptolites in the lowest strata of the St. Lawrence Formation in their Afton section, where collection may have postdated the road work that destroyed the classic location. For some reason, despite its “classic” nature, nobody ever saw fit to just put a pin on the map. What clues do we have?

Well, it's not where I saw these burrows; this is the Reno Formation on the north side of town.

Ruedemann (1933) reported that Clinton R. Stauffer collected graptolite fossils from Afton in 1930, from a "light buff sandstone", "31 feet below the base of the Jordan (Norwalk) sandstone". At least some of Stauffer’s field notebooks can be viewed in the University of Minnesota archives. I checked 1928 through 1932 just in case, but didn’t come up with any collections from Afton. Since the graptolites obviously exist and we can assume it is unlikely Ruedemann received access to the specimens from a nefarious fellow impersonating Stauffer, who also arranged for the specimens to be reposited at Stauffer's institution, Stauffer evidently recorded the relevant notes in a different notebook or didn't take any notes. (Or collected them in 1924 or something and was just really bad with dates.)

Then, of course, I remembered the photos I took of the old University of Minnesota collections, and sure enough the cards with them specified "May 20th, 1930" and "31 feet below the base of the Jordan SS". All there is for geography is "Afton". The 1930 field book has nothing for May 20, and in fact doesn’t really get going until late in June.

Once again, I could have saved some time, but the rabbit holes are always fun. Mark your calendars for May 20, Afton Graptolite Day!

The University of Minnesota/Minnesota Geologic Survey appear to have added Afton as a location for stratigraphy after this. Schwartz (1936) and Stauffer and Thiel (1941) both included a section from "along the creek at the south edge" of Afton. Per the later stratigraphic description of Stauffer and Thiel (1941), graptolites were presented in a 3-foot interval of gray to buff fine glauconitic dolomite at the base of the St. Lawrence Formation, at an elevation of 780.6 feet. Thankfully this is also about 31 feet below the Jordan, so we don't have to figure out what we're missing. If we assume that this horizon is the site, or at least equivalent to the site, that elevation helps quite a bit. (The fact Stauffer initially specified 31 feet below the base of the Jordan rather than measuring from the seemingly closer basal contact suggests that the "section" wasn’t necessarily continuous from the "Franconia" all the way up, and/or he wasn't sure where to draw that line in the field because of how messed-up the stratigraphy was.)

A couple of graduate projects followed that have some bearing on the question. Nelson (1949) revisited the "creek" stratigraphic section, and his collection F-35.11 includes the graptolite taxa described by Ruedemann, so he must have thought he was looking at Stauffer's locality, or something close enough for graduate stratigraphic work. More importantly, he included an air photo of Afton with the area of the section circled. A few years later, Tyler (1956) mapped the geology of the Minnesota side of the Hudson Quadrangle, which includes Afton. Several outcrops are pointed out on a geologic map (hand-colored with colored pencils). Unfortunately, Tyler did not state that such-and-such was the "creek" section, and graptolites were not mentioned. (As a side note, anyone ever heard of Afton's "Primeval Canyon"? It’s Tyler’s Outcrop 52 and also appears in a 1973 field trip guidebook (Kain 1973); it’s easier to identify than the graptolite locality, though).

I don't usually reproduce stuff like this, but really, how many people have access to a random 1949 dissertation?

Same thing, except replace "1949" with "1956" and "dissertation" with "thesis". What isn't immediately obvious is Tyler made the regrettable decision to color the St. Lawrence brown and the Jordan orange, and they don't distinguish so well anymore. Black circles are outcrops, red circles (see near top) are wells.

In 1967 the Minnesota Geological Survey published a guide to collecting fossils in Minnesota. The discussion of the St. Lawrence Formation includes this: "Collecting sites in the St. Lawrence include: (1) a creek crossed by State 95 in the southern part of Afton, Washington County" (Hogberg et al. 1967: 32). Apparently back then you could just show up at a place and collect fossils. (Incidentally, site 2 is Fairy Falls; you can't collect St. Lawrence fossils there anymore either, because it's now managed by the National Park Service *and* because you can't find the darn St. Lawrence Formation there anyway. It's probably covered by the highway, which happens to be State Highway 95.) Aha! We need to look at State Highway 95!

Not so fast, unfortunately. Just to introduce some confusion, Highway 95 of today is not the same as it was in 1967; until 1998, it followed what is now Afton Boulevard into Afton. Yes, at the time 95 did indeed cross the southern part of Afton, if by "Afton" we mean "the large squarish area depicted on municipal maps" such as shown on the Wikipedia article. Relative to "Afton, the concentration of people and buildings", the Highway 95 of 1967 would have come in from the *west*. Furthermore, period topo maps show a definite lack of creeks being crossed by 95, it's not where Nelson (1949) was working, and geologic maps show the same route as barely nicking any St. Lawrence Formation, with no outcrops large enough to map (and definitely none at about 780 ft elevation). Tyler (1956) did not plot any, for example, just a couple of wells. Highway 95 just doesn't fit very well.

Here we are in 1949. Afton *proper* is the area surrounded by the dashed black line, Highway 95 is coming in from the west into the northwest corner, and the area circled by Nelson happens to be on a road coming into Afton from the south...

Therefore, I suspect that the road in question is a different road and someone just got their highways mixed up. My money is on County Road 21/St. Croix Scenic Byway, for several reasons: it enters Afton (the town) from the south; it crosses the St. Lawrence Formation and there are St. Lawrence outcrops mapped along it (Mossler 2005); it's the road that Nelson (1949) circled; and I can confirm observing the St. Lawrence Formation there, even if I've never poked too deeply (not sure whose property the outcrops are on, and the one thing I hate about the lower St. Croix Valley is its healthy population of ticks). Furthermore, the road clearly follows a ravine with smaller washes entering from the sides, and we can assume that even though no creek is mapped there at topo scale, this is an ideal place to get flowing water. (The road and the creek may have been more or less the same thing at the time, the creek perhaps flowing right next to the road.) I do have to confess to a twinge of self-doubt, though, as another report (Hughes 1993) also has a section "alongside west side of Hwy 95, 1 km south of Afton".* Maybe people just got confused because the two roads did merge in Afton.

*The longer I do this kind of stuff, the more I hate "[x distance] from [city]" directions when "x" distance is something short like a mile or kilometer. Where exactly in [city] did you measure from? A city is not a dimensionless point!

A clip from Mossler (2005); compare to Tyler (1956) above (I made a wider crop to show more of old State Highway 95). Tan Cf is the old Franconia, light green Csl is the St. Lawrence, yellow Cj is the Jordan, and light blue Op is the Prairie du Chien, with outcrops/bedrock shallowly buried as dark gray overlays and bedrock elevations as magenta lines. County Road 21 crosses the St. Lawrence/"Franconia" contact at about the surface, around 780 feet, whereas the St. Lawrence is well-buried by surficial deposits where 95 crosses it (in fact, the highway hits the 800-ft contour just outside of town, where it's over the buried "Franconia"). 

There you go, St. Lawrence off of County Road 21.

Did Sloan have anything else to say? In his autobiography (Sloan 1996), he mentioned that the "classic graptolite locality" of Afton was destroyed by highway relocation. (Note that the reassignment of Highway 95 did not take place until 1998.) That's about it. Inexplicably, he mentioned the graptolite locality nowhere in his magnum opus on the fossils of Minnesota (Sloan 2005), which is disappointing.

There is one more piece of information I've been able to find. In 1980 the Minnesota Natural Heritage Program produced an inventory of features that might be affected by proposed power transmission corridors. One of two geologic features highlighted was the "Graptolite Fossil Site", including this in the description: "This site, located just south of the town of Afton is believed to be the best occurrence of this index fossil [graptolites] in the state. The fossil bed of graptolites is in a soft siltstone of the St. Lawrence Formation, exposed at water level in a small creek" (MNHP 1980: 16). It's supposed to be Occurrence #43 on an included map, which, of course, is not included in the pdf. (Drat.)

Now that we've got a file full of information about the site, it's time to make some conclusions. I’m going to go with the County Road 21 hypothesis, based on the reasoning above. The main weakness is I don't know a darn thing about highway work in Afton that would have taken place between about 1980 (the MNHP report) and 1996 (Sloan's autobiography). The best I can do there is to back up any roadwork to pre-1992 based on satellite images available on Google Earth. The road does appear to be less curvy compared to the old topo map, though, and as recently as 2017 it was obvious that growth was being encouraged on bare faces to prevent mass wasting, suggesting road work. One of the places where the curves seem tamer is actually just about where I'd guesstimate the locality to have been: where the road made two "steps" to the southwest.

They don't generally look like that naturally.

My guess is indicated by the arrow on the close crop of the Mossler (2005) map. It works out stratigraphically (near the lower contact of the St. Lawrence); it works out elevation-wise; there are outcrops in the area; and it's right by the road.

The arrow is cheated a bit, as I didn't want to overlap the geologic features, but the dark blob it's pointing at and the immediate vicinity are what I think.

Now that we've gone through all that, is there anyone reading this who visited the site and knows where it was? I don't care if I'm wrong, I just want some resolution!

References

Hogberg, R. K., R. E. Sloan, and S. Tufford. 1967. Guide to fossil collecting in Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Educational Series 1.

Hughes, N. C. 1993. Distribution, taphonomy, and functional morphology of the Upper Cambrian trilobite Dikelocephalus. Milwaukee Public Museum Contributions in Biology and Geology 84.

Hughes, N. C., and S. P. Hesselbo. 1997. Stratigraphy and sedimentology of the St. Lawrence Formation, Upper Cambrian of the northern Mississippi Valley. Milwaukee Public Museum Contributions in Biology and Geology 91.

Kain, J. 1973. Along two rivers. Unpublished field trip guidebook, independent study project for Geology 697 course at the University of Northern Arizona, Flagstaff, Arizona.

Minnesota Natural Heritage Program. 1980. Report on the occurrence of rare, threatened and/or exemplary elements of natural diversity in the Project NSP-TR-2 study area. A report submitted to the Power Plant Siting Program of the Minnesota Environmental Quality Board. Department of Natural Resources, St. Paul, Minnesota.

Nelson, C. A. 1949. Cambrian stratigraphy of the St. Croix Valley. Dissertation. University of Minnesota, Minneapolis, Minnesota.

Mossler, J. H. 2005. Bedrock geology of the Hudson quadrangle, Washington County, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Miscellaneous Map 154. Scale 1:24,000.

Ruedemann, R. 1933. The Cambrian of the upper Mississippi Valley. Part III, Graptolitoidea. Bulletin of the Public Museum of the City of Milwaukee 12(3).

Schwartz, G. M. 1936. The geology of the Minneapolis–St. Paul metropolitan area. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 27.

Sloan, R. E. 1996. Autobiography of Robert Evan Sloan.

Sloan, R. E. 2005. Minnesota fossils and fossiliferous rocks. Privately published, Winona, Minnesota. Available from the Minnesota Geological Survey.

Sloan, R. E., D. R. Kolata, B. J. Witzke, and G. A. Ludvigson. 1987. Description of major outcrops in Minnesota and Iowa. Pages 197–231 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

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

Tyler, S. R. 1956. Geology of the Hudson Quadrangle. Thesis. University of Minnesota, Minneapolis, Minnesota.