by Carl Strang
This year on my vacation I went in search of the route followed by our lobe of the glacier. Our land surface was shaped by the most recent continental glacier, which melted away 18,000 years ago or so. Continental glaciers often are depicted as enormous masses of ice, but in fact they are a radiating group of ice rivers, flowing shoulder to shoulder out from their source area. Maps of the glacier are readily available showing general directions of flow, but I wanted to see if I could get a more detailed picture especially of the northern part of the route followed by our glacial lobe for northeastern Illinois. The maps I found were ambiguous on that point.
The last part of that glacier lobe’s route is clear enough: it followed the length of Lake Michigan. It was bounded to the west by the Green Bay lobe, the two divided by a ridge of bedrock, the Niagara dolomite of Silurian age, which also is our bedrock in DuPage County. Lake Michigan once was a river, but it was on relatively soft shale of Devonian age, which earlier glaciers progressively had gouged out on their journeys south. Now it was a convenient channel for the Lake Michigan lobe of the most recent (“Wisconsin”) glacier. When the lobe reached the south end of the lake basin it was confronted by Niagara dolomite, which wraps around the south end of the lake. The push and give of lobes on either side, along with the continued forced advance from the North, resulted in our glacial lobe cresting the dolomite and spreading out in a southwesterly direction from the lake basin, passing over Cook and Lake Counties on its way into DuPage.
When you look at chunks of rock left behind when the glacier eventually melted, you find much more than Devonian shale (nearly all of which was ground up to give us our wonderful clay soil) and Silurian dolomite (shown here outcropping near the tip of the Bruce Peninsula of Lake Huron in Ontario).
There are granites of assorted colors,
gneisses in varied patterns,
diabase, basalt, quartzite, greenstone and iron ore pieces.
I wanted to find the source of these rocks. I headed to Canada. None of these odd igneous and metamorphic rocks originated in the U.S. The bedrock of the Upper and Lower Peninsulas of Michigan, which flank the north end of Lake Michigan, is Paleozoic in age and all sedimentary, including the segments of the Niagara dolomite ring which form the Door Peninsula of Wisconsin and part of the U.P. The igneous and metamorphic foreigners are literally so, having been carried by the glacier down from the land of hawckey, loonies and twonies (this is not intended to be mocking; we should have copied Canada’s use of coins for $1 and $2 denominations long ago).
So, the goal of this trip was a very generalized inquiry, seeking out the sources of those stones, trying to match them to their bedrock sources and in the process tracing the Lake Michigan glacial lobe’s route of flow.
To be continued…