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.

Beach Stripe

by Carl Strang

 

Yesterday I described my return to check on a seedling growing near the town park at Culver, Indiana. On the way to the seedling I noticed a black line of material that had been sorted out from the beach sand at the edge of Lake Maxinkuckee.

 

beach-black-line-1b

 

I don’t remember seeing this at Culver, before, but it reminds me of similar deposits at Illinois Beach State Park that are of finely ground iron minerals. Here’s a close-up of the Culver stripe.

 

beach-black-line-2b

 

I had no trouble finding a bottle cap in the litter along the lake edge. Sad, that, but it provided the tool I needed. I used it to scoop up some of the black material and carry it back home.

 

beach-black-line-3b

 

Notice how the mainly quartz sand is revealed beneath the thin layer of black particles. The next stage was to dry out my sample, then see if it would respond to a magnet. My hypothesis was that it would prove to be magnetite or hematite, both black iron minerals, possibly from chunks of banded iron formation in Canada that were picked up by the glacier, deposited in Culver, broken into tiny pieces by glacial and/or wave action, and finally distributed along the beach.

 

The black powder was strongly attracted to a magnet. Here is the sample after I poured it onto a white card and dragged a magnet beneath it. The sand was left behind.

 

beach-black-line-4b

 

A close-up shows the particles aligning themselves with the magnetic field.

 

beach-black-line-5b

 

The particles were not themselves magnetic; they didn’t stick to iron. So, I conclude that they are not magnetite but rather bits of hematite, an iron mineral very common in banded iron formations, and deposited billions of years ago in Canada (in this case; Culver is on the route of the glacier’s Saginaw lobe). During the early days of life on Earth, oxygen was increasing in the atmosphere through the work of stromatolite-forming, photosynthetic cyanobacteria in shallow seas. Iron was abundant, dissolved in the water, but it reacted with the oxygen to form the hematite, which precipitated out to produce iron formation deposits that in recent times have been our important sources of iron ore.

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