There are a ton of great resources out there in print and on the internet that describe the glacial epoch in Michigan and its effect on the State. I’m partial to the classic textbook “The Geology of Michigan” by John Dorr and Donald Eschman. Although it’s 46 years old now it’s still a great book with great insights regarding many of the features we see today. I would love to nerd it out and start going on about all the different terms for the glacially derived features of Michigan but will try to keep this short and stick to Washtenaw County as much as possible with the intent to use this post not as an end to itself but to set the stage for our further discussion about the infrastructure and other civil engineering works in the County.
Quaternary Geology is geological stuff that’s happened in the last 2.6 million years. Since the last glacier retreated from Michigan around 11,000 years ago, all the glacially derived features and overburden that cover the county today are from the Quaternary Period (or more precisely the Pleistocene Epoch). The image below of Washtenaw is taken from the USGS’ 1998 Quaternary Geology Map (I-1970-B) of the Midwest. These are the sediments you see if you do any digging at all in the county and cover all of the bedrock that was previously described.
Generally, the blue colors are lake-laid (lacustrine) fine-grained sediments consisting of clays, silts, fine sands and the like. Green is glacial till (unsorted material in a typically clay matrix dumped by melting glaciers). Red-Yellow are river (alluvial) deposits of sand and gravels. The darker the color, the thicker the deposit. MDEQ’s GeoWebFace shows these Quaternary geologic features in a little more detail with more nuanced geological descriptions shown below.
It’s easier just to copy the color descriptions than describe them:
The brown lines that run NE-SW just east of Ypsi are former shorelines from glacial Lake Erie (aka Lake Maumee) and describe its regression to its present stage
My takeaway from all this is that we have a ton of different types of glacially soils here in Washtenaw which is really cool!