Saturday, November 26, 2016

Cottage Grove Ravine Regional Park: the green rocks of home

I come from Cottage Grove, Minnesota, and in the southeast part of town there's Cottage Grove Ravine Regional Park. Up until a few weeks ago, I had no idea that there was actually exposed bedrock in the park. The only outcrops I knew of in town were a couple of pockets around the compost site, a couple of spots in Old Cottage Grove, and some outcrops of the St. Peter Sandstone up high on Camel's Hump overlooking Highway 61 (there's a little park up on the crest of the hill now, with a good view across the valley, by the way). Obviously, I had to have a look around.

The rocks, counterintuitively, are the green things in this photo.

Here's a geologic bedrock map of Cottage Grove, clipped from Bauer 2016 (hot off the presses!). The basic setup is the same as the map we saw a couple of weeks ago. The medium gray lines are the streets, with the thick line going northwest–southeast being Highway 61. That big area of blue with the Ops on it is the Shakopee Formation of the Prairie du Chien Group, the stern and sometimes stromatolitic Lower Ordovician dolomite lurking beneath much of southeastern Minnesota. The column is as follows, starting at the base (which happens to coincide with the south part of this map, where the Mississippi drainage system has deeply eroded): Ct = Tunnel City Group, the former Franconia Formation; Cs = St. Lawrence Formation; Cj = Jordan Sandstone; Opo = Oneota Dolomite; Ops = Shakopee Formation; Os = St. Peter Sandstone; and Opg = Glenwood Formation and overlying Platteville Formation. As before, this is a bedrock map with the surficial deposits omitted, so almost none of these rocks can be seen at the surface. The exceptions are in areas denoted by dark gray, the most obvious being the blob in Grey Cloud Island representing a major quarry in the Shakopee Formation. You'll notice there aren't a whole lot of dark gray areas on the map.

You also get Grey Cloud Island and St. Paul Park for no extra charge. The faults are pretty much decorative these days.

I would like to draw your attention to the colorful strip heading north in the eastern part of the map. This represents the "ravine" of Cottage Grove Ravine Park, where erosion from flowing water cut down through several formations. Clearly, at one time flowing water was a much bigger concern in this area to cause this much erosion (although we can guess that the presence of the Cottage Grove Fault didn't hurt for determining where the water flowed). The exposures, though, are not where the bedrock has been deeply eroded, but just to the east: see the D and U on the fault east of the deeply eroded part of the ravine and north of Highway 61? There's a little worm of outcrop represented below the D, in a secondary channel. If you're hiking in the park, these areas are just west of the unpaved path going north from the parking lot, and generally parallel the path in that area. The channel is deep-set in many places and plant growth usually does a good job of obscuring it. Today the flow in the channel is only sporadic, but there are plenty of rounded cobbles and boulders to be seen in the bed, and there are erosional features from flowing water along the banks, and even a dry waterfall.

With flowing water, this would be a human-sized waterfall.

The creek bed is a maze of eroded bedrock, loose cobbles and boulders, and branches, limbs, and logs.

At this time of year, with most of the plant growth dead or dormant, the green of the moss/algae/et cetera covering the rocks stands out brilliantly, and can look almost neon-like on a grey November day. The extent of the green invasion is remarkable; if you've ever gone snorkeling and seen rocks underwater with thick coverings of algae and other encrusters, it's kind of as if you're back in the ocean and someone has taken all the water away. It's a little hard to tell you're looking at the Prairie du Chien with so much green, but if you look close you can see some characteristic features. Broken and uncovered faces have that typical oatmeal-like color, and beds with sand grains floating in the carbonate matrix are apparent. Because this is the Prairie du Chien, I was on the lookout for stromatolites, but again the modern greenery was making it hard to see traces. There were occasional pieces with suggestive features, but not great preservation.

Not bad in terms of fine layering, but this is kind of an odd cobble, with some good-sized chert nodules like the one near the top of the photo.

The numerous rounded pocks could be from small eroded dome-like stromatolites, or just erosion. Note the greenery as well.

Scattered throughout the dry creek bed are various cobbles and boulders, most of which are also green and reasonably rounded. The population of cobbles and such is a mix, which is typical of an area with hundreds of thousands of years of glacial activity under its belt. The generous greening of everything makes it difficult to tell what is what, but some of the cobbles are clearly igneous in origin. One interesting ringer, clearly not Prairie du Chien, included weathered crinoid stem fragments and other fossils representing a more recent sea with a more favorable environment.

The objects in the lower center, right of the foreground finger, are chunks of crinoids. There is also a shell fragment near dead center, and, not visible, a piece of bryozoan.


Bauer, E. J. 2016. Geological atlas of Washington County. Minnesota Geological Survey, St. Paul, Minnesota. County Atlas 39.

No comments:

Post a Comment