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).

Watching the river flow

Nothing in particular, just a few observations:

I had some errands to do in St. Paul on Saturday and ended up showing my mother around some of the localities I'd been to last year for a project. There was the overlook north of the Ford Bridge, across from the Temple of Aaron, where I'd seen a shell bed in the Magnolia Member of the Platteville, but the site turned out to be a lot more interesting with a year of experience under my belt. This time, we could follow the shell bed around the outcrop, and I could pick out the overlying Carimona Member of the Decorah Shale.

The shell bed, highlighted by a helpful graffiti artist.

Where to see metro geology, part 2: Minnehaha Regional Park

When we left off in Part 1, we were hanging around the observation area of Lock & Dam 1. Directly west of the observation area is Minnehaha Regional Park, famous as the home of Minnehaha Falls. Minnehaha Falls is one of a series of cataracts that have formed along the Mississippi in the Twin Cities. The basic processes went like this: each time the glaciers rolled through, they filled up the preexisting river system with glacial debris, and each time they retreated, the river system reestablished drainage. The last time the system was reset, the Minnesota–Mississippi network was grinding its way through the Platteville until it scoured into an old channel in the vicinity of Dayton's Bluff. It's a lot easier to wash out loose debris than it is to erode the Platteville, and a waterfall (proto-St. Anthony Falls) formed where the two met. Over the years, the waterfall eroded upriver, and every time the falls passed a tributary, a smaller waterfall ended up forming on that creek or river, too. This is how we got Hidden Falls, Shadow Falls, and a host of falls that are now mostly to well and truly extinct (human intervention in the drainage systems), as well as Minnehaha Falls. There is something of a cottage industry among Minnesota's geologists of reckoning the elapsed time since deglaciation by calculating the rate of retreat of St. Anthony Falls, using historical observations of its locations before the 1870s, the properties of the underlying formations, and the starting point, but that is for another time.

Practical considerations and hazards

If you do choose to explore Twin Cities geology beyond the most convenient couple of places, it is worth knowing something about the terrain. Most practicalities are the same as you would use for any hiking trip: sturdy boots, sunscreen, insect repellent in the buggy times of year, a hat, and so forth. I recommend long pants and sleeves, to protect your arms and legs from banging against things and to give you less to worry about in terms of bloodsucking multi-legged friends, but I realize that this can get extremely uncomfortable in the summer, when the atmosphere transitions from hot and humid to hot and gelatinous. At any rate, choose clothes that you wouldn't mind getting ripped, roughed-up, or coated in mud. Have water on hand, too.