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.



  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

  2. Joel Steinke said,

    July 16, 2017 at 8:53 am

    The rocks left on the shore of Lake Michigan would also be biased by the glacier and wave action, since softer rocks are just ground into sand. Hence, the rocks and pebbles that remain might suggest granites near the surface were much more plentiful than they really were. Also, some rocks may not be carried by waves as readily due to their density. I know that lighter volcanic rocks (black) and black sand were plentiful when I visited the Lake Michigan beach years back, since the Lake Michigan lobe mainly followed the depression basin made by heavy lava, like the mares on the moon. I’d bet that most of the Lake Michigan glacial lobe was floating on water like sea ice or an ice shelf, so only the edges of the glacier were actually scraping up rock from below along Lake Michigan’s shores.

    The depth to bedrock map (link below), which came from data from drilling wells, suggests how much glacial till was left in Wisconsin. Illinois may not have laws like Wisconsin that say that geologists must be contacted before there’s any deep drilling for water or whatever, in case they want core samples. Hence the map stops at the state lines.

    The Illinoisian Glacial Period which covered much of Illinois, was brief, so it didn’t change the underlying topography, nor gouge out a lot of rock. It just left rounded rocks and some sand on the surface. The lobes in the later Wisconsin Glaciation lasted longer but melted 20 miles before reaching the Illinois-Wisconsin border, maybe because the ice age was not in such a deep freeze during the Wisconsin Glaciation period. Therefore, Rock and Walworth counties in Wisconsin on the border are mostly outwash plains. To see glacial lakes and moraines, one must travel just north of Janesville in Rock County.

    The Rock River must have melted the glacier lobe so west of the Rock River in Rock County there is only evidence of the order Illinoisian glaciation, e.g., rounded rocks in fields, but limestone is near the surface and is intact. The very deep ancient Rock River valley is filled in by glacial till.

    • natureinquiries said,

      July 23, 2017 at 6:15 am

      Thanks, Joel. A couple corrections. Geological research has revealed that Lake Michigan was a river valley on shale bedrock prior to the many episodes of continental glaciation (you can confirm this with any textbook on historical geology of North America). The soft rock was readily ground away, providing a least resistance route for a glacial lobe each time and dumping the resulting clay beyond the lake basin after the glacier was pushed out over the Niagaran Formation rim. The sand we see on beaches is what remains from granite and other Canadian Shield rocks after the softer minerals were ground away (of course, the process was not complete and we can find plenty of granites and other rocks in the tills and on the beaches).

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