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.

About these ads

1 Comment

  1. September 23, 2009 at 6:27 am

    [...] 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 [...]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 90 other followers

%d bloggers like this: