Saturday, July 9, 2011

A Convergence of Wonders, Day 7: Of Time and Pressure in Glacier National Park

There is a movie quote that I've always appreciated, from a movie with a several surprising references to geology, The Shawshank Redemption:
All they found of him was a muddy set of prison clothes, a bar of soap, and an old rock hammer, damn near worn down to the nub. I remember thinking it would take a man six hundred years to tunnel through the wall with it. Old Andy did it in less than twenty. Oh, Andy loved geology. I imagine it appealed to his meticulous nature. An ice age here, million years of mountain building there. Geology is the study of pressure and time. That's all it takes really, pressure, and time.
Time and pressure is the story of Glacier National Park. There are the glaciers, of course, for a while more, maybe twenty years, but there are also the rocks, and there are the mountains too. Glacier National Park in northern Montana has some of the most incredible scenery of any national park but it has a fascinating geological story as well.
I mentioned at the beginning of this mini-blog series that an overall theme was convergence, due to the influence of the subduction zone that has existed off the west coast of the U.S. for several hundred million years. It is not unusual to see the effects of subduction for eighty miles or so inland where stratovolcanoes like those of the Cascades develop. But we had been traveling east now for more than five hundred miles. How could a subduction zone influence the crust so far inland?
Before we could find out, we needed to decide whether or not to make a run for the border. Glacier National Park is actually properly called Waterton-Glacier International Peace Park, as it shares a boundary with Waterton National Park in Alberta, Canada. You can drive to the border as we did here, or you can backpack through the park (and still go through customs, apparently).

We bravely set foot into the wild frontier of Canada, and also wondered who has the job of keeping that line clear through the miles of forest. It was one of the busiest border crossings I've ever seen, as there was at least one motorcycle that came through while we hung around.
Lots of flowers were out and about. Since we missed Logan Pass and the Going to the Sun Highway and the fields of glacier lilies often found there, I was glad to find a few along the highway near the border.
We reached a vista point for looking at Chief Mountain (9,080 ft; 2,768 m), one of the truly unique peaks in the region. Click on the panorama shot below to see just how isolated the mountain is. It is an eastern outlier of the Rocky Mountains, standing some 5,000 feet above the Great Plains. It is visible for miles, and is a sacred place to the local Native Americans. Half of the mountain lies within the boundaries of the Blackfeet Reservation, and the Blackfeet people claim jurisdiction over the entire region (a fact that I learned, admirably, from the visitor center for Glacier National Park).
I was briefly distracted by some beautiful Shooting Stars...
The unique shape of the mountain derives from its origin as a fault klippe, an erosional remnant of a thrust fault . It is one of the world's best examples of this type of feature. The rocks forming the plateau and summit are actually older than the softer Cretaceous sedimentary rocks below. The older rocks were pushed upwards from deep in the crust and then pushed over the younger rocks by intense compressional forces.

I wish I had a chalkboard to illustrate, but the wikipedia diagram will have to suffice in this instance!
From Wikipedia (http://en.wikipedia.org/wiki/File:Thrust_system_en.jpg)
Where did these compressional forces come from? Apparently, the subducting slab from way out west in Washington got trapped along the base of the crust and never plunged into the mantle until it was far inland. This caused a crumpling of the crust in late Cretaceous and early Cenozoic time (between about 100 and 50 million years), a series of related mountain-building events called the Sevier and Laramide Orogenies. At Glacier National Park, the fault zone is called the Lewis Thrust.
We then drove to Upper St. Mary Lake, for a look at the unique rocks of Glacier National Park (and one of the most iconic views in the national park system at Wild Goose Island Overlook, below). All of the rocks visible in the picture below are more than a billion years old, and sit on top of the much younger Cretaceous sedimentary rocks. This is another manifestation of the Lewis Overthrust that we first saw at Chief Mountain.
The glacial features are outstanding: U-shaped valleys, hanging valleys, truncated spurs, aretes, horns, cirques, moraine lakes, and the occasional surviving active glacier. The park once had around 150 glaciers. Today there are no more than 25. They are expected to be gone within twenty years. Ignorant politicians should be forced to stand at the base of one of these disappearing glaciers when they pontificate about how global warming is a hoax. They shouldn't hide in places like drought-stricken Oklahoma (where coincidentally they shouldn't be allowed to fly airplanes).
Picture by Susan Hayes
The slightly dipping layers making up much of the park are a series of sedimentary rocks called the Belt Supergroup. The rocks were deposited in fault-controlled basins at the edge of the ancestral North American continent over a billion years ago. It was a strange time in Earth history...no plants, no animals, just barren rock on land, and only bacteria and other single-celled organisms in the lakes and oceans. The only real fossils are layered mounds called stromatolites.

One of the most vivid layers was the dark red Grinnell Formation. There was no oxygen in the Earth's original atmosphere, but when photosynthesis evolved, oxygen was released. It immediately reacted with iron in the sediments, and the world turned rusty red. Although the rocks are from an alien time in our own history, they still contain recognizable features like the exquisitely preserved mudcracks seen below.
The group was fascinated by the outcrop (always a pleasing moment for a teacher, especially seven days into a trip!).
The group had the afternoon off, and most took off on hikes (and saw grizzlies and bighorn sheep). Others found a wi-fi signal in the wilderness, and did some homework (along with some laundry)...times have changed in the world of field-tripping!
I had a few moments in the late afternoon, so I went moose and grizzly hunting around the outlet of Upper St. Mary Lake. The rivers were swollen with snowmelt (flooding is still a problem across the northern tier of states). I didn't find any animals, but they no doubt noticed me crashing through the brush.
It was a beautiful evening, and I had a fine time photographing the lovely clouds that swirled above us. I realized with a start that we had reached the half-way point of our trip, and that the moment we turned our backs on the Canadian border, we were turning towards home. There were many wonders yet to come, though. Tomorrow we would cross part of the Great Plains, and make our way to Bozeman through the lands once trod by the dinosaurs.

Time and pressure....