Friday, July 29, 2011

A Convergence of Wonders, Day 12: A Richness of Geologic Drama at the Grand Tetons (and Pelicans!)

Our continuing journey through the Pacific Northwest and the Northern Rocky Mountains, the Convergence of Wonders, reached the twelfth day. We woke up in our Colter Bay campsite and set out to explore Grand Teton National Park.

The Tetons are in the Rocky Mountains, but they are not of the Rocky Mountains. The rocks are right, but not the origin of the peaks themselves. Most of the mountains we visited since leaving Washington were the direct result of convergent tectonics, that is, the result of subduction along the west coast of the North American Continent. The forces involved are usually compressional in nature, but the Tetons are the direct opposite. They form because the earth's crust is extending; it is being ripped apart.

But first, the pelicans! We stopped at Jackson Lake Dam to have a look around (the picture above was taken from on top of the dam), and could see dozens of white things floating in the rushing waters below the dam. A zoom shot revealed the objects to be American White Pelicans. I suppose they get good fish at the outlet.
We then drove to the summit of Signal Mountain to get one of the best views of the Grand Teton region. The Tetons have an interesting glacial history. Where Yellowstone was covered by an ice cap, most of the Tetons were dominated by alpine glaciers confined to mountain canyons. Some of the Yellowstone ice cap flowed to the south around the base of Signal Mountain, so the most notable features at our feet included the Potholes, a group of kettles and kettle lakes left behind by stagnant masses of ice (below).
We had a stunning view of the Teton crest and Jackson Lake from the peak of Signal Mountain (although I think some of the students missed the view as they tried to get a signal on their cellphones).
We then headed out of the park briefly and crossed the swollen Snake River. All the rivers we saw this trip were very close to flood stage, and even now in late July they apparently are still running high and cold.
Our destination was a little east of the park in the canyon of the Gros Ventre River. Most visitors to Grand Teton National Park never go out this way, and they miss one of the remarkable geologic sights in Wyoming: the Gros Ventre Slide.

A series of Paleozoic sedimentary rocks are found in the area; the layers sloped north into the valley at about 20 degrees. Tensleep sandstone was exposed at the surface, and was underlain by much weaker shale and mudstone. Weeks of heavy rain had destabilized the rocks, and on June 23rd, 1925, a large mass nearly a mile long and 2,000 feet wide gave way and slid into the valley. It had a volume of around 50 million cubic yards.
The avalanche produced a debris dam more than 200 feet high. The Gros Ventre River began filling the new "reservoir". It was two years before the dam filled, and when the water breached the top of the dam on May 17th, 1925, the weakness of making dams out of loose debris became immediately apparent: the rushing waters tore away at the top of the dam, and much of the lake emptied into the riverbed below. Several miles downstream, the town of Kelly lay unprotected. The resulting flood killed six people and the town was washed away.
 A ghostly forest of dead trees can be seen standing in the middle of the lake.
We noticed a ranch road that crossed the debris field, so we checked it out. Huge boulders of sandstone were scattered all about. It was an eerie sight. The flooded river was pouring through the outlet of the dam, and we remarked how crazy it would be to go rafting on such a raging river. Then, of course, two kayakers made their way under the bridge and headed downstream. We sort of kept an eye on them for a few miles downstream. They apparently survived...
The road back into the park provided spectacular views of the mountain crest.
Flowers were blooming all across the Antelope Flats area. The fault line that produced the mountains has moved the mountains upwards to heights of 13,000+ feet, while the valley floor has at least 10,000 feet of sediment filling it in. The fault has moved some five miles in the vertical sense!

Our next destination was Jenny Lake, which formed behind the terminal moraines of glaciers flowing out from the mountains. It is a beautiful (and very popular) starting point for a number of hikes. I gave the students one of their very precious and rare half-days to hike or do showers/laundry (or, hopefully all of the above).

I headed back to camp, but had to stop and photograph a beautiful Western Tanager hanging out in a tree along the trail. The males are obviously very colorful!
The moraine that dammed Jenny Lake was also covered with beautiful flowers.
After dinner, we went on a moose-hunt in the twilight, and saw a moose at great distance, but more impressive was the serene sunset reflected on the Oxbow Bend of the Snake River. The next day we would be starting one of the long driving days making our way back home. There were just four days left on our journey. The story will continue in the next post.

1 comment:

Gaelyn said...

Oh, the power of water. To give life, destruction and death, and beautiful reflections.

This has been the best field trip yet.