Green Bay Lobe

by Carl Strang

A second goal of my trip to Michigan’s Upper Peninsula last week was to investigate further the stones left by the most recent continental glacier. As I outlined in an earlier series of posts, my vacation trip last year was a pilgrimage into Canada to trace the route  of the Lake Michigan lobe of that glacier, which is responsible for the deposits which cover the land in the northeast corner of Illinois. The turquoise line in the map below follows the route I think that lobe followed.

Glacial lobe map b

I studied the various categories of bedrock northeast of Lake Superior, chunks of which were picked up by the glacier and now reside where that powerful river of ice left them when it melted away. I found that there appeared to be commonalities in the stones left as drift along the Lake Michigan lobe’s route in Canada, on the Upper Peninsula of Michigan, and in northeast Illinois. Stones northwest of that route in Canada seemed different, and I was curious to see if those differences might hold farther south along the route of the Green Bay lobe, which is the one immediately west of the Lake Michigan lobe. I chose to visit Muskallonge Lake State Park, on the U.P.’s north shore, approximately in the center of the Green Bay lobe’s route, so that I could compare the beach stones there to those at Whitefish Point, at the U.P.’s tip, which was on the route of the Lake Michigan lobe.

Muskallonge Lake sign b

It was a foggy day, but a few people were there. Some were gathering stones, a practice which might bias the results.

Muskallonge Lake beach 5b

For instance, it seemed to me that beach stones at Canada’s Agawa Bay, along the Green Bay route, included an unusual number of red granites and greenstones. If these are selectively removed by visitors, the remaining stones might not represent what had been there originally. I certainly found greenstones, and in the following photo two appear.

Muskallonge Lake beach 10b

However, there were very few compared to Agawa Bay. Here is a typical aggregation of Muskallonge stones, representing the Green Bay lobe.

Muskallonge Lake beach 1b

Here is a corresponding photo for Whitefish Point, along the Lake Michigan lobe’s route.

Whitefish Point 9b

While to my eye there did seem to be more reds and a few more greens at Muskallonge, and a few more grays and browns at Whitefish point, I don’t think the differences would hold up in a proper sampling procedure and statistical analysis. Furthermore, when I bring in a photo from Illinois Beach State Park (below), I am hard pressed to say that it is closer to one U.P. site or the other.

Illinois Beach 2b cropped

Nevertheless, the two years’ travel and study were enjoyable, and I learned a lot especially from studying the Canadian bedrock. The glacial drift may not provide additional support for the route map shown above, but the scratches on bedrock indicated by the little arrows in the geologists’ original map certainly are consistent with the turquoise line I added after last year’s trip.

Incidentally, there were places at Muskallonge Lake where there were deposits of black sands, I suspect composed of hematite like I found at Lake Maxinkuckee last winter.

Muskallonge Lake beach 4b

The next installation from this trip will be more biological.

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2 Comments

  1. Roy C. Wolf said,

    August 26, 2010 at 8:05 pm

    I am most interested in the flow patterns of both the Lake Michigan Lobe
    and (in particularly) the Green Bay Lobe=into the Fox River lowlands
    and south to its terminus……….however, I have a question: What was the
    farthest extent of the Green Bay Lobe during the Illinoian: Marine Oxygen
    Isotope Stage 6? I am hypothesize that it might have moved into Northern
    Illinois or at least Green County in the Monroe, WI, Area!!! rsvp!

    • natureinquiries said,

      August 30, 2010 at 5:40 am

      Hi, Roy,
      I’m not a geologist, so the best I can do is pass on my impressions from the literature I have at hand. The Illinoian glacier preceded the Wisconsin. Its deposits are older, often are overtopped by the more recent ones, but extend farther, covering almost all of Illinois.

      Descriptions of the Illinoian deposit I have seen do not refer to lobes. Whether that is because these are difficult to make out or are a more obscure topic than my references cover I cannot say. The Illinoian left a more uniform layer with fewer end moraines. It apparently originated in the same region of Canada as the Wisconsin one. While it is reasonable to think that the same topographic features may have produced a similar lobing pattern, this cannot be assumed. There may not have been analogs to the Lake Michigan Lobe and the Green Bay Lobe. On the other hand, if there were such analogs, it is reasonable to think that the Green Bay one extended well into Illinois, given the extent of the Illinoian as a whole.

      Thanks for giving me an excuse to look into this question.
      Regards, Carl


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