Practical guide to MNRRA/metro-area bedrock geology

I can't believe I didn't put up a post like this earlier. Here is a thumbnail guide to the various bedrock formations exposed within MNRRA and, by extension, most of the Twin Cities metro. Further information can be found in Ojakangas and Matsch (1982) or Ojakangas (2009), if you'd prefer a nontechnical level of discourse, or Mossler (2008) if you want a technical overview. The maps published by Mossler and Tipping (2000) and Mossler (2013) (see previous post) are also useful.

The formations of interest are the Jordan Sandstone, Prairie du Chien Group, St. Peter Sandstone, Glenwood Formation, Platteville Formation, Decorah Shale, and Cummingsville Formation. We'll take it from the top, or rather, the bottom, going in ascending order from the oldest rocks that are exposed (the Jordan Sandstone; there are older rocks below it, but they aren't exposed within MNRRA or in the central Metro. You can find most of them along the St. Croix, though).

A brief meditation on the joys of packrat middens

One more trip to the Quaternary, for a personal favorite...

Imagine someone has asked you to describe what an area was like hundreds, thousands, perhaps tens of thousands of years ago. They're interested in the ecology, the climate, and so forth. What could you use to accomplish this? If there are lakes and marshes, you might start by taking sediment cores and looking for pollen, which tell you what kinds of plants were present, and how the flora changed over time. If you're on the coast, you might look for deposits of shells, which are useful for describing the conditions of the water (salinity, temperature, how energetic the setting was) and for stable isotope analyses. If there's a lot of ancient wood, you might get into tree-ring data. In dry protected places of western North America, there is another type of paleoecological indicator: middens constructed by packrats.

Metro mammoths

Many states have had inventories published of Pleistocene mammal finds; I'm putting together a file for a future post. The most recent such report for Minnesota was complied by Clinton R. Stauffer and published sometime in the late 1940s. It is often cited as Stauffer 1945, but he cites several finds from 1946 and 1948. The journal is a proceedings volume for 1945, which is where the usual date comes from, but the Minnesota Academy of Sciences must have had some trouble getting it out that year. Anyway, 1945, 1948, the immediate point is that it's been a while.

Little island mammoths of California

Doing my bit to encourage warm thoughts for those of us in the frozen north...

I present to you an island off the coast of what is now California, approximately 15,000 years ago. This island is called Santarosae (or Santa Rosae); as sea level rises following the melting of the great continental ice sheets, it will be dissected into the four northern Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa), which are eventually reunited as Channel Islands National Park (with Santa Barbara Island). These events, though, are in the future. For "now", we have one large island a handful of miles from the coast:

A. The modern northern Channel Islands. B. The geography of the islands at various times in the recent past. From Collins (2009).

Platteville follies: a crushed giant rodent from Hidden Falls

I'd like to start off with a couple of photographs, to illustrate some facts of life concerning our old friend, the Platteville Formation:

An ever-so-slightly hazardous ledge along the road into Crosby Farm Regional Park.

Hidden Falls Regional Park: when good rocks go bad.

The Platteville Formation just loves to ledge out, particularly the lower part, which rests on the recessive Glenwood Formation, and once you get heavy rocks ledged out far enough... About ten thousand years ago, not very far from the second photo, there was a ledge of the Platteville Formation left jutting out in the figurative "wake" of the erosion of the Mississippi River system. At some point, a dog-sized rodent opted to go underneath it. Maybe this was its usual shelter, or maybe it was driven there by some circumstance; we'll never know, short of time travel. Unfortunately for the rodent, the slab failed, with predictable results. It would be covered by the debris until July of 1938, when a Works Progress Administration crew uncovered the site while widening what is now the north entrance road into Hidden Falls Park. Its skeleton, crushed and coated with calcite leached out of the slab, was recovered and eventually prepared, reconstructed, put on display at the Science Museum of Minnesota, and described scientifically as an example of the extinct mammal known as Castoroides ohioensis, a.k.a. the giant beaver.

From Powell (1948): the WPA project that led to the discovery (site with the superimposed white circled cross).

"Well," you say, "that was refreshingly concise." Ah, but there's always more to say.

Anchisaurus is older than you might think

Anchisaurus is kind of a second-tier dinosaur, not up there with Tyrannosaurus, Triceratops, Velociraptor, and their ilk, but it gets a fair amount of exposure, especially if you pull in the old synonym Yaleosaurus. It gets points for having a long history, having a few interesting anecdotes associated with it, being one of the first early dinosaurs known from good remains, and beginning with the letter "A"; this latter bit is important, because it means Anchisaurus ends up near the beginning of children's A-Z dinosaur books. (Is this why so many dinosaur names begin with "A"?)

Divisions of the Platteville Formation

There comes a time when we must speak of the Platteville Formation in detail. If there wasn't a Platteville Formation, there would be no narrow gorge in the Twin Cities, no Minnehaha Falls or Hidden Falls, and most importantly no Saint Anthony Falls: if there's no Saint Anthony Falls, Minneapolis develops in a radically different way. No falls, no easy water power, no mills. The Twin Cities owe their existence to a 455 million year old carbonate bank and the vagaries of glacial and river erosion.

And today the Falls of St. Anthony are mostly artificial. Turns out that drilling holes through the Platteville into the St. Peter Sandstone near the edge of a cataract can be unsafe.