Let’s start this first post in the beginning. With the bedrock beneath Washtenaw County. The Michigan DEQ has a great web tool to quickly map and show the underlying bedrock formations on a GIS type system called Geowebface. The results are shown below (from Geowebface). I have also added the formation names to the image.

Geology w Formations.png

This is actually pretty cool – there are a bunch of different formations in the county that represent many of the Michigan Basin rock formations. Ypsi and Ann Arbor are separted by three different formations. The rocks go from younger to older heading east and are all of the Mississippian and Devonian periods. The United States Geological Survey (USGS) provides the following description for the Marshall FormationColdwater ShaleSunbury ShaleBerea Sandstone and Bedford ShaleAntrim Shale, and the Traverse Group. I will explore these formations individually in future posts.

The generalized section with Formation Descriptions is shown below from the “Description of the Ann Arbor Quadrangle” by I.C. Russell and Frank Leverett (found here) published in April, 1907 and hosted by the USGS. It’s not up to date but provides a good historical reference and the sequence still looks correct (the Carboniferous Period is now broken into the Pennsylvanian and the Mississippian in the U.S.). I also find the description of the “brine layers” fascinating as well.


I have no knowledge of any naturally occurring outcrops of these rock formations in Washtenaw County. All of it is covered by glacially derived soils and other material. The non-soil material above the rock formations is called overburden or drift. In my next post I will explore the overburden thickness across the county and with it the general depth to bedrock.


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