Looking at maps of the geological layers in Michigan it looks like an onion starting in the center of the state. Why?

Thinking of the recent glacial history of the state that would seem to have created streaks from glacial advance and retreat that would carve valleys or create Moraines. Could it have been a much older meteor strike like with the Sudbury Basin? What could have pushed the layers down so deep for such a wide area?

enter image description here This ring pattern continues into the Upper Peninsula (see full version)


2 Answers 2


A bedrock geology map will not show surficial deposits such as glacial tills, river sediments, and lake deposits. Bedrock maps have to be constructed using exposed bedrock, borehole data, and roadcuts. My guess is that given the areal extent and depth of glacial deposits that much of the bedrock data is gathered during exploratory drilling for groundwater and mineral exploration and compiled by the state bureau of natural resources (as is done in all US states).

Here is a map of the surface geology of Michigan, which, incidentally looks a lot like a topographic map of Michigan, below. This indicates that bedrock is actually hard to find in Michigan.: enter image description here enter image description here

As for why the bedrock geology looks like an "onion", it is a basic depositional basin in which the center drops slowly and sediments, over time, accumulate with the oldest materials on the outer edge and the youngest in the center. There's nothing special about the Michigan Basin other than the fact that it coincides neatly within the state's southern outline. Below are the other depositional basins of the US: enter image description here

The only other state with a basin within its borders is Illinois, whose onion-like bedrock structure looks a lot like Michigan's: enter image description here

Hope that helps!


Not a meteor strike. There are many ways to form circular features on Earth that does not include the Armageddon.

As for the Michigan basin, during the Paleozoic is was a somewhat oval sea:

enter image description here

Getting an onion-structure of sedimentary rocks is then pretty straightforward. The basin was slowly getting filled by successively younger sediments. In this example, red, then orange, the purple:

enter image description here

  • $\begingroup$ I like the simple diagram in your explanation. $\endgroup$ Feb 13, 2022 at 19:25

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