Niagara Formation Dolomite

by Carl Strang

Bedrock is the kind of stone found closest to the surface at a particular point on the Earth. In DuPage County our bedrock is a sedimentary rock of Silurian age called dolomite, and it belongs to the Niagara formation. A bit over 400 million years ago this part of North America was a reef-dotted shallow part of the world ocean. Over a period of millions of years, precipitates and microscopic shells, along with some larger life forms, settled to the bottom of the sea and built up a layer of sediment that later solidified into limestone, or calcium carbonate. Later, some of the calcium became replaced with magnesium atoms, changing the rock enough chemically that it was less soluble, and worthy of a new name, dolomite.

Dolomite fragments b

DuPage County is part of a ring of Niagara formation bedrock that extends up the west coast of Lake Michigan, forms Wisconsin’s Door Peninsula and the southern boundary of the U.P., wraps around the Canadian side of Lake Huron to divide the main lake from Georgian Bay, continues south to form the Bruce Peninsula jutting into southern Lake Huron, and eventually wraps around northern Ohio and Indiana back into northeast Illinois. Niagara Falls pours over an erosion-resistant edge of this formation, which also has outliers in Missouri and Iowa.

Dolomite beach b

A few years ago I drove around Lake Huron. One of my stops was the tip of the Bruce Peninsula. There, the bedrock is at the surface. It’s possible to get a sense of what our landscape might look like in northeast Illinois if it were not covered by glacial deposits. Instead of being surfaced with crushed dolomite, our trails might run over the raw rock.

Dolomite trail b

A trip to the beach would look quite different.

Dolomite recreation b

We might find cliffs.

Dolomite cliff b

Though dolomite is not as subject to solution and cave formation as limestone, sea caves might occur where waves pound the shore.

Dolomite sea cave b

We might even find structures like the “flowerpots” that stand on Flowerpot Island off the Bruce Peninsula tip.

Flowerpot 2b

But as it is, there are few places in northeast Illinois where the bedrock reaches the surface.

Dolomite prairie b

For example, the dolomite prairies near the Des Plaines River offer rare habitat for the federally endangered Hines emerald, a dragonfly that can live only where Niagara formation dolomite provides the right water chemistry for its crayfish-tunnel-dwelling larvae.

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.

Canadian Shield Bedrock

by Carl Strang

This is the second installment describing my 2008 vacation to look at the route followed by the Lake Michigan lobe of the latest (“Wisconsin”) glacier. After crossing the Canadian border in early September I spent the first night in the campground at Pancake Bay Provincial Park, getting there in time to set camp and enjoy the sunset over the Lake Superior beach.

 

pancake-bay-sunset-b

The next day I headed up to Wawa, Ontario, and turned east onto Highway 101. I began stopping at road cuts to look at the bedrock. This is truly ancient stone, its age measured in billions rather than a mere few hundred millions of years like the Paleozoic bedrock over which the glacier flowed after entering the U.S. It proves to be a complex mix. Some road cuts have only one, or a few kinds of rock exposed, but most have more. One, for instance, had pink granite, gray granite, a quartzite-like metamorphosed sedimentary rock that was a beautiful gray with thin pale veins running through it. Some of these rocks had been converted to gneiss along their edges. There also was a layer of schist. Finally, a black dike of diabase cut through the whole.

 

Granite

Granite

Granites are igneous rocks, having formed deep below the ground from molten stone that, protected by the insulating earth above, had the luxury of cooling slowly and having its constituent minerals form in crystals large enough for us to see without magnification. The exact mineral composition varies, giving different colors to different granites.

 

Diabase dike

Diabase dike

Dikes are shelves of rock formed by new molten stone intruding into previously established formations. Diabase is a dark igneous rock that commonly forms dikes in the western Lake Superior region.

 

Gneiss

Gneiss

Gneiss

Gneiss

Metamorphosis of rocks involves the application of pressure and heat to previously formed stone. Sandstone can be converted to quartzite, and varied igneous and sedimentary rock can become gneiss, which is characterized by a striped or banded structure that often is quite beautiful.

Schist

Schist

Schist is a metamorphic rock with a high clay or mica mineral content that gives it a distinctive sheen.

 

Greenstone

Greenstone

Greenstones, which I encountered in places, are metamorphosed lavas or basalts. Among the rocks associated with them are layers of greywacke, formed from collapsed volcanic deposits into the sea more than 2 billion years ago.

Greywacke

Greywacke

Greywacke texture

Greywacke texture

Greywackes often are multicolored, and have a beautiful silky sheen and bumpy texture on the surfaces of their thin layers. A final rock type I want to mention here is iron-formation, which consists of alternating layers of iron-rich rock and chert.

Iron-formation

Iron-formation

One iron formation layer I examined was composed of the iron mineral magnetite, which grabs onto little refrigerator magnets.

 

Though I would not be able to make a one-to-one correspondence between the stones picked up in DuPage County with particular source sites in Canada, it was satisfying to see, in the range of bedrock I examined along the way, the same kinds of stones we see in glacial drift in northeastern Illinois.

 

The next entry in this account will compare glacial drifts in Canada with ours in Illinois.

 

Incidentally, the best explanation I have seen of how all these rocks formed is a book by Gene L. LaBerge titled Geology of the Lake Superior Region. He takes pains to provide clear explanations of complex processes and historical sequences. His examples mainly come from the U.S., but certainly describe well what I saw in Canada. The many photos were helpful, but I didn’t fully appreciate what they showed until I saw these things in person. In other words, personal inquiry is necessary to a more complete understanding.

A Nature Nerd’s Pilgrimage: Introduction

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).

bruce-peninsula-b

There are granites of assorted colors,

Granites found in DuPage County

Granites found in DuPage County

gneisses in varied patterns,

Gneisses found in DuPage County

Gneisses found in DuPage County

schist,

Schists found in DuPage (say that one carefully!)

Schists found in DuPage (say that one carefully!)

diabase, basalt, quartzite, greenstone and iron ore pieces.

Iron Ore found in DuPage County

Iron Ore found in DuPage County

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…

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