Bedrock Geology of the Twin Cities Ten-County Metropolitan Area, Minnesota

Just a quick midweek post...

I'd like to plug the Minnesota Geological Survey, in particular their fantastic collection of online publications. Everything published by the modern Survey or its 19th century incarnation is available for download as pdfs. Looking for, say, the fossil volume of The Geology of Minnesota monograph series? Parts One and Two. The geology of Anoka County? Right here. A publication on Minnesota brick and tile manufacturing? Here (and it's by a man named Grout, which just seems like some strange kind of destiny). Anyway, if you poke around the main list, found at the second link above, you'll find just about anything that can be written about Minnesota rocks. If you're not especially familiar with geology, I'd recommend starting with the "Minnesota At A Glance" documents under "Miscellaneous Publications", or the publications under "Educational Series".

I'd like to draw your attention to one of the newest publications, "Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota" by John H. Mossler. This is the newest in a line of geologic maps of the Twin Cities going back to the 19th century. The link I provided has three links at the bottom; unless you have GIS software, I'd recommend just going for the first link, "10 county map.pdf". To briefly explain if you haven't had much contact with geologic maps, this map shows the locations of various rock formations, as well as folds and faults (not that we're swarming with them in east-central Minnesota, but there are some). The rock formations are denoted by color and by a short letter code; for example, the Decorah Shale is represented by a teal color with Od, the "O" for the Ordovician Period and the "d" for the Decorah. (the map is a bit inconvenient in that several of the formations have been given similar shades of blue; fortunately, because the rocks in question are flat-lying with few folds and faults to break up the succession, it's usually not much of a problem to tell what is what once you are familiar with the order.) Older rocks are found where the younger rocks have been eroded, such as in the various modern and ancient river channels.

An important caveat is illustrated by the cross-section, which shows an otherwise un-mapped brown layer covering almost everything to the tune of a couple hundred feet (60+ m); this brown layer represents Quaternary deposits, mostly the stuff so generously left behind by glaciers. In other words, the main map is what you'd get if you vacuumed up all of the loose stuff on top. The terrain left behind would be kind of strange, because the glacial deposits cover a lot of old river channels (every time there was a glacial advance, the previous channels would be buried, and new channels would be excavated after the glaciers retreated; some of the older channels cut down significantly deeper than the modern channels – the modern Mississippi above Fort Snelling doesn't cut into "yellow" rocks [Jordan Sandstone] until it gets out of the central metro, for example). The majority of the map was made from subsurface data, such as well logs; the ubiquitous circles are water wells with logs. Areas where the bedrock is exposed at the surface (and which are large enough to be mapped at a scale of 1:125,000) are denoted by a dark gray color. Zooming down to 300x or 400x, where the street grids become comfortable to read, shows that most of these outcrops are near the modern river system. This is because the river have cut through the glacial deposits into the bedrock. Some of the large areas of dark gray pick out quarries, like the one on Grey Cloud Island. There are also thumbnail descriptions of the formations at the bottom, describing the thicknesses, what the formations are made of, and so forth.

References

Mossler, J. H. 2013. Bedrock geology of the Twin Cities ten-county metropolitan area, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Miscellaneous Map Series 194. Scale 1:125,000.