Depth to Bedrock

Michigan doesn’t make it easy to find the generalized bedrock topography below State the way our neighbors in Ohio and Indiana do. There’s no one-stop map that’s been published (that I can find) that shows this information so it has to be inferred from published maps showing the glacial drift thickness above the bedrock (as is shown below).

If you know the elevation of the ground you’re standing on, and the thickness of the drift below you, you can determine the depth to bedrock.

Washtenaw County is shown to have glacial deposits between 50 to 400 ft thick in the below figure. Since there is a bit of relief (elevation change) across the county, this isn’t the best measure of Bedrock Topography but does provide some indication of how deep bedrock is. This Figure was taken from “The Groundwater Atlas of the United States”. J030.jpeg

The below drift thickness map was taken from the November 2012 Michigan Basin Geological Society Newsletter from a paper to be presented by Mr. John Esch (newsletter found here). The paper itself that this came from is still behind a paywall as far as I can tell.

drift-thickness

The newsletter also provides the below bedrock topography map which correlates with drift thickness and shows that Washtenaw County is roughly split into regions with (generally) a shallower depth to bedrock in the Northwest portion of the County, a middle band of deeper bedrock and then shallower bedrock in the Southeast corner again.

Bedrock Topography Map.PNG

Mr. Esch describes the relatively higher elevation band of bedrock in the Southwest portion of the county : “A combined Marshall Sandstone-Coldwater Shale Cuesta is evident as a pronounced bedrock high extending from the western Branch County northeasterly to the tip of the thumb.” A “cuesta” is “a ridge with a gentle slope (dip) on one side and a steep slope (scarp) on the other.”

So that is a general look at the bedrock below Washtenaw County. No rock outcrops and it’s all buried by approximately 50 to 400 feet of glacial deposits. Next i’d like to provide a general overview of the glacial features and deposits that cover the county and also how they begin to define what we see today when we drive around Washtenaw.

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