Sunday, April 27, 2014

Where to see metro geology, part 3: Coldwater Spring

Picking up from where we left off last time...

Going south from Minnehaha Park on the bike trail network, specifically the Fort Snelling State Trail, we encounter Coldwater Spring. Coldwater Spring, also known as Camp Coldwater, is a small parcel situated between property of Minnehaha Park, Fort Snelling State Park, the Veterans Hospital, and other landowners (there's actually a number of small land parcels in this area, but this is a paleontology and geology blog, not a land ownership blog. If you're curious, there's a map here, with Coldwater as #8, Department of the Interior). The site is currently owned by the National Park Service as the only part of Mississippi National River and Recreation Area that is not an island. Before that, it hosted a Bureau of Mines complex. The complex was vacated during the 1990s after the bureau was closed by Congress, but the buildings still stood until 2011. In fact, if you are using the older version of Google Maps, the buildings reappear when you've zoomed in far enough to transition from flat to 45 degrees. All that's left of the complex today is a few chunks of foundation and the concrete ore storage bins. Before the Bureau of Mines, the titular spring was used by Fort Snelling, taking over what had been a squatter's camp in the 1820s and 1830s. Going back before the Fort, we get into the tribal period of use. The spring is in the Platteville Formation, which is not surprising given that the different members of the formation can promote spring outflows at the contacts (water flows through one member, then runs into another member which isn't as permeable).

Sorry, I had to borrow a photo. This is from, showing the little stream now leading from the pond, with the old stone tower in the background. Would it surprise you to learn that all of my photos of the site are geological?

Coldwater Spring is at the end of Minnehaha Park Drive South. It's not widely known at this time, perhaps because of the long Bureau of Mines history. The majority of the site is above the Platteville Formation; the bike trail, which runs along the eastern border of the property, is near the break of the bluffs. The St. Peter Sandstone is widely exposed below the bluffs. From north to south, the trail descends through the Platteville, which is convenient: you get to see the upper and lower Platteville at close range with a reduced hazard of having things fall on your head (although you need to keep an ear out for cyclists).

Coming into Coldwater Spring from the north; the upper Platteville lines the bike trail. A couple of shell beds can be seen in cross section.

The lower Platteville, Mifflin over Pecatonica (which has seen better days). The Glenwood Formation and St. Peter Sandstone are incognito beneath the dolomite/limestone.

The area west of the bike path includes a shallow depression and low ridge. Along the entire ridge is the Platteville Formation. Because this is mostly the lower Platteville, there isn't much to see in terms of fossils, although there are occasionally the usual molds and casts of brachiopods, snails, and bivalves. This is NPS land, so it's "look but don't take".

The crests of the low ridges west of the bike path have some exposed Platteville.

There are also some areas of Platteville "paving stone" exposed in the various informal walking trails. They've been worn down by weathering and walking, but you can spot some fossils in them, including some bean-like large ostracodes ("seed shrimp"). Most ostracodes are close to a millimeter in length, but the Ordovician had some much larger varieties; in older works, they usually are assigned to Leperditia. I'm not an expert on ostracode taxonomy (one of my many shortcomings), but I think they're under a different genus now.

A pavement of mostly ostracodes.

The large blocks of landscaping stone found around the spring and parking lot, and scattered throughout the park, are from the Platteville Formation. They came from excavations at the Bishop Henry Whipple Federal Building a few miles to the west. Many of them feature shell beds of brachiopod and snail molds and casts, which are often gray against the yellowish stone. If you're having trouble seeing fossils, they're a good way to get the search image in your mind.

A fairly typical shell bed in landscaping stone.

Finally, although I haven't run across any references to the site in the geologic literature, there may be a few fossils in museum collections from the vicinity. I have seen references in locality information to a stone water tower near Fort Snelling, including specimens in the collections of the Natural History Museum of Los Angeles County (LACM). How did Minnesota specimens get there? Well, our old friend Clinton R. Stauffer retired to southern California in the late 1940s, and for a while was affiliated with the California Institute of Technology (CIT). He appears to have given the CIT a small collection of Minnesotan fossils, and when the CIT collections went to the LACM, so did his fossils.

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