Rockford, Part 1: The Site

A few weeks I was able to join a Geological Society of Minnesota field trip to the Fossil & Prairie Park Preserve of Floyd County, Iowa, which to some of you may be more familiar as the Rockford site. We ended up having a practically ideal day: pleasant temperatures, clear skies, dry conditions, and a light breeze. If you have an itch to collect fossils (an itch that's becoming difficult to legally scratch in Minnesota these days), I highly recommend a visit.

A view into the old quarry directly south of the parking area.

The Rockford site has been known for a long time; to put it in perspective, the big monograph on the site is a century old this year (Fenton and Fenton 1924), and the site wasn't new then. Like the Brickyards of Lilydale, this was a clay quarry, and you can visit the old kilns northeast of the quarry area. Bricks were one of the products, but the main product was drainage tiles for farms, which were produced until 1977. While wandering the park, you can find areas where chunks of broken tiles and bricks were dumped.

Looking down at the old kilns; two are standing, and one is represented by its foundation.

Several signs explain the history of the quarry.

Geologically, the Rockford quarry's clay is primarily in a unit now known as the Juniper Hill Member of the Lime Creek Formation (Hackberry Formation of older references), and the fossils are primarily in the overlying Cerro Gordo Member (a.k.a. the overburden if your interest is in clay) (Anderson and Furnish 1987). The Juniper Hill Member is mostly limy shale, whereas the Cerro Gordo begins with a marl bed (kind of a muddy limestone) and includes more marl and true limestone going up, with shale confined to the lower part. There is a subtle color change that corresponds to the lithologies: The shale is grayish and the other rocks are yellowish. Because there is shale above the first marl bed, this is not the stratigraphic contact, though.

It's not the most obvious thing, but there is a change from grayish to yellowish just below the band of vegetation.

Fossils in the Cerro Gordo pin it to the early Late Devonian (Frasnian). It's a bit younger than the youngest Paleozoic bedrock in Minnesota, the Cedar Valley Group, which tops out just at the beginning of the Frasnian and paleontologically is nothing to write home about. There are both in situ exposures and old spoil piles that can be checked. If you want to know which one you're dealing with, there are two rules of thumb: if you're on a slope that comes off the parking area or 215th Street, that's in situ; and if you can see badlands-looking beds at the top of the slope, that's in situ. The rest are piles left behind from quarrying.

Bedding near the top of the quarry.

Standing on a spoil pile, looking southwest at an in situ wall (the nose of another pile is visible on the far left margin). The contact of the Juniper Hill and overlying Cerro Gordo cannot be any higher than the lowest visible bed, which is partway down the slope.

So, what kinds of fossils can be found here? First off, the Cerro Gordo is more a "single loose fossil" unit than a "hash plate" unit, but you can certainly find examples of the latter. I came back with a reasonably broad assortment of marine invertebrates, generally small (this site has been open to collecting for a long time, so there has been selection pressure to nab larger fossils that are obviously something). Brachiopods were by far most abundant, followed by snails, horn corals, and bivalves. I'll be putting photos of examples in future posts, one for brachs and one for everything else.

A typical surface littered with shell fragments and small whole fossils.

Fossils on display at the visitor center.

If you go, don't forget: there's more to do and see than just the fossil beds. I showed a photo of the old kilns earlier. From there, if you head east, you can walk on an old railbed converted to a trail that crosses the Winnebago River. If you keep going, you end up in Rockford.

Looking south on the Winnebago River at the bridge crossing.

Arriving in Rockford.

Also, this site isn't called the Fossil & Prairie Park for nothing. West and north of the old quarry are prairie areas and ponds to visit.

Hitting the trails after fossil hunting.

Stay tuned: depending on time and barring any surprises, the next post will either continue this one with fossil photos, or will be the annual Compact Thescelosaurus update.

References

Anderson, W. I., and W. M. Furnish. 1987. The Lime Creek Formation of north-central Iowa. Pages 89–92 in Biggs, D. L., editor. North-central section of the Geological Society of America. Geological Society of America, Boulder, Colorado. Centennial Field Guide 3.

Fenton, C. L., and M. Fenton. 1924. The stratigraphy and fauna of the Hackberry Stage of the Upper Devonian. Contributions from the Museum of Geology, University of Michigan 1.