Geology of Voyageur’s National Park


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Published: July 15th 2018
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Arnold’s Fishing and RV Camp, International Falls, Minnesota

Yesterday was a down day. We will be taking a few of them this week as we rest up to start the trip home. It is also a little warm up here - in the icebox of the nation - so, in general, we will be trying to take it a little easy and not push our poor, old, tired bodies further than they can go. (Cue the violins!)

I made a bacon and eggs breakfast and we took an early afternoon nap. I beat Joan at Yahtzee - I never mention the times she beats me - and she got mad at me for gloating again! But it was, generally, just a do-nothing day.

Except, I actually finished a book. It is part of our traveling routine to pick up a book at every park visitor center to serve as our reading material for the stay. It is enjoyable to read about the places you are seeing - the reading and the real life experiences help reinforce each other. Related books help ground the park experiences and I think I remember more about both. The problem, though, is that I rarely finish a book before I leave one park and go on to the next one, so I have a stack of more than a dozen books laying next to the bed that I’ve started, but stopped when I get to the next park. But here I picked up a book yesterday at the visitor center about Voyageurs National Park and actually managed to finish it today, so I’m ahead of the game. The book (A Story Written in the Rocks: The Geology of Voyageurs National Park by Chris Hemstad) is short, only a hundred pages or so, which is probably why I made it through in record time.

The Park Service organizes their support services along two main lines - ‘Cultural’ and ‘Natural’. (I know this because I did some database consulting work for them back in my working days.) I believe the reason they do that is because it takes a different skill set to manage an ecological system than it does to preserve an historical building. Every site, whether it be a National Park, Monument, or Lakeshore, has different combinations of Cultural and Natural Resources. Pullman National Monument in Chicago, for example, is definitely about Cultural Resources, but it does have a few trees on the property and the park personnel are responsible for ensuring that those trees are cared for properly even though they are not what visitors to the monument are going to see. Isle Royale, on the other hand, has a couple of archeological sites where the park tries to preserve evidence of early human copper extraction activities. But people don’t clamor to go to Isle Royale to see the ‘Cultural’ side of the island.

Voyageurs National Park is a little bit more of a mix. As the name suggests, one of the most important pieces of the park is the preservation of the locations where the specialized boaters known as Voyageurs rowed across several northwestern lakes to and from Rendezvous at Grand Portage (and later Fort Williams). But while Grand Portage National Monument is overwhelmingly about the cultural phenomena involving these people, a trip to Voyageurs National Park would only tangentially involve history lessons. Most people come to this park to experience the lakes and rivers, islands and peninsulas, wildlife and forests of the north woods.

Since Joan and I already had books about the Voyageurs themselves, I decided I wanted to know more about the geology of the park and that’s why I bought and read the book I did. I’m going to try to summarize a hundred pages here in a couple of paragraphs, because the geology of this area really is pretty interesting.

Start with the well established notion that the earth is about 4.6 billion years old. The first billion years, known as the Hadean Eon(from Hades), is pretty much a big unknown. The coalescing ball we call Earth probably started as a hot ball of molten rock throttled by bombarding pieces of stellar dust, pulled together by gravity. We have no idea what things were like then, other than that it was extremely hot because of all the energy involved in planetary accretion.

After a billion years, we entered the Archeon Eon. Although we don’t know a whole lot more about that period, we do know that the glob of metals we now call earth started to cool off enough for things to harden, including some globs of minerals along the outer surface, the coldest part of the planet. But, as nature does not do things with perfect consistency, there were blobs of molten material bubbling to the surface at different points around the planet. At around 2.7 billion years ago stuff bubbled up and began to harden into what we now call the Canadian Shield, the original craton (or continental core) of North America. These original rocks, although not the oldest in the planet, are the oldest on our continent and Voyageurs National Park is at the southern edge of that craton which extends north and east from here. (I’m hoping to see and pick up some of this original stuff, called greenstone, that populates a lot of the islands and land surfaces in the park.

Plate tectonics, working the way it does, resulted in this craton of original rock moving around the surface of the earth. As it moved, it collided with other smaller blobs of stuff, that had formed in other, later bubbling systems on the planets surface. If the other bubbles were small enough, then the craton simply accreted them onto its edge, eventually adding on the landmasses we now know as the rest of the United States. If the bubbles were large enough, the collision could produce warping in one or both of the cratons. Such warping is what we call mountain building. (One set of collisions 300 - 200 million years ago between the North American craton and the European land mass, produced the Appalachian mountains. Another collision about 60 million years ago resulted in the Rocky Mountains, but neither of those stories had much impact on the Voyageurs area.). There are fault zones and metamorphic rock layering that suggested that these plate collisions started and continued shortly after the original rocks were formed, producing layers of rocks within the park over the next couple hundred million years.

There is also evidence of dramatic volcanic activity in the area a half billion years after the original land masses cooled, producing the youngest rocks in the area (just 2.1 billion years old). This younger rock appears as dikes in the older rock, occasionally coming to the surface as lava flows and eruptions, over the older original rock.

Then the record goes curiously kind of silent. There are no additional rock layers over the next 2 billion years. Whatever was going on with all the plate activity was happening at the edges of the continent, not in its middle. There is evidence of a lot of erosion as wind and water tore away at whatever mountains had been built. There may have been times when the area was under water, but never long enough to accrue significant sedimentary deposits. The term geologists use is an ‘unconformity’, or a set of missing layers. In this case it covers a massive time period of nearly half the life of the planet.

Two million years ago, for reasons that aren’t real clear yet, the Earth cooled more than usual and entered a series of ice ages. Starting pretty much in the Hudson Bay Area, ice fields up to two miles thick,grew to cover a great deal of the continent, receding and advancing over time. By looking at deposits at the edge of each of the advanced stages, geologists are able to piece together a history of at least four and possibly ten glacial stages over the last two million years. Given the way glaciers work, scraping away everything on top of the bedrock, the only evidence left here is of the last one which started around 40,000 years ago and ended around 12,000 years ago.

Scraping away softer stuff, leaving deposits and water when they melt, the glaciers carved and created the network of lakes and streams that we call the Land of 10,000 lakes. After that, natural processes of vegetation take over to create the boreal forests, with heavy doses of pine, that populate the park and created the environment necessary to support the Voyageurs.

So, there you have it - a short geological history of the Park. I’m hoping I can actually see real evidence of all this in today’s boat ride.

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