Fossils from Loring Park

For the first post on something specific, I'm going a bit outside of "equatorial" and heading, instead, for approximately the end of the Pleistocene. The geologic time scale is worth a post on it own, but for now, if you're not familiar with the geologic time scale, a brief version is available through the University of California Museum of Paleontology, or you can get all the nitty-gritty from the International Committee on Stratigraphy; the ICS version is slightly more recent, so some of the dates are slightly different because they're a bit more refined. For our purposes, the Pleistocene was a time when glaciers advanced and retreated through Minnesota.

In 1923, the Northwestern National Life Insurance Company was building a new home office across from Loring Park. This building still exists, by the way (see this for its qualifications as a historic site), although the insurance company has long since moved to larger digs. During excavation, workmen cut through old moss beds and other deposits with fossils, eventually described by William S. Cooper and Helen Foot in 1932. It isn't stated, but because both authors were affiliated with the University of Minnesota, I can only guess that this is where the fossils ended up (with the exception of the snails and clams, which went to the Museum of Natural History of the University of Illinois). The following information comes from their publication, except where otherwise indicated.

The beds of moss and other organic remains were found among lenses or pods of sand and silt and the occasional buried soil horizon in a narrow trench (5 ft/1.5 m wide) dug for the building's foundation, to a maximum depth of 12 ft (3.7 m). These beds were deposited by flowing water shortly after the retreat of the most recent glaciers. The area is associated with the valley of the former Bassett Creek (or Bassett's Creek), which again is something deserving of its own post. Fortunately, someone else agrees: see "The Myth of Bassett's Creek" at Minneapolis Park History (the myth is not its existence, but about its potential as a city park). Before St. Anthony Falls advanced up the Mississippi and formed the current narrow gorge (another great topic), Bassett Creek was a sort of backwater into the river, and Cooper and Foot suggest that Loring Pond is a remnant of this period of its existence.

A total of 83 samples of fossil material were collected from the trench, and a number of specialists contributed identifications. So, what was living in the Loring Park neighborhood just after glaciation? The bulk of the remains come from four species of moss, two of which are living aquatic species and two of which are extinct. They grew in the water during times when sedimentation was light. Also abundant were a number of species of freshwater snails and clams, several of which are extinct. Together, they suggest quiet water of perhaps 2 to 10 ft (0.6 to 3 m) in depth. Fragments of green algae and pondweed, and a few ostracodes (tiny shelled crustaceans, also known as "seed shrimp") are also present.

There are also fossils of terrestrial plants and animals, with tree fossils representing bog forest and climax forest settings. Bog forest plants are represented by tamarack cones and cones and other fragments of black spruce. Climax forests are represented by white spruce cones, pollen of fir and white pine, and birch bark. A few of the plant remains are charred by fires. Finally, there are rare bits of grass and ground beetle wing covers. Together, the aquatic and terrestrial fossils suggest a pond surrounded by bogs, with upland forests on the ridge of glacial debris just south of the depression. The Mississippi River was nearby and looked significantly different, with no gorge, a much higher elevation than present, a heavy glacial load, and numerous tributaries. The glacial ice sheet was probably still within 250 miles (400 km) of the area, but was in full retreat. Similar settings can be seen today in recently deglaciated areas of Alaska.

There are a couple of radiocarbon dates for the fossils. Wright and Rubin (1956) dated wood from the site to 11,790 ± 200 radiocarbon years before present, and dated peat, probably less accurately, to 10,200 ± 300 radiocarbon years before present. Converting these to calendar years (radiocarbon years being not quite the same thing), we get 14,060 to 13,250 calendar years before present and 12,660 to 11,080 calendar years before present, respectively. These dates put us smack at the end of the Pleistocene.

The Loring Park fossil site is just one example of the many fossil sites in the metro. As we will see, there are plenty of surprises waiting under the surface!

References:

Cooper, W. S., and H. Foot. 1932. Reconstruction of a Late-Pleistocene biotic community in Minneapolis, Minn. Ecology 13(1):63–72.

Wright, H. E., and M. Rubin. 1956. Radiocarbon dates of Mankato drift in Minnesota. Science 124(3222):625–626.