Stop 7

July 12, 2004

Ray Collins & Josh Sheldon

Our bus went through a road cut where the land rose about 5 meters above the road. The exposed sediment included many different sizes of stones: sand, gravel, and rocks up to the size of a kitchen table. Many of the rocks looked jagged and broken.

The bus stopped about one quarter mile farther along, and we were able to look back at a very broad rise in the landscape behind us. For many miles, the land rose gradually to the same height (about 5 m) as the gap that we drove through.


Many of the features of this area were formed about 12,000 years ago by a glacier. A glacier is a thick river of ice that acts like a bulldozer. Glaciers convey and push any rocks or sediment that they move across. When the climate warms to the point that the glaciers begin to melt, they drop the sediment they are carrying in a curved hill, much like the one we saw. These formations are called moraines.

If the glacier pauses in its retreat, it can form multiple moraines, one behind the other. The moraine at the glacierís farthest extent is called the terminal moraine. The rise we saw at Polson was the terminal moraine for the Flathead glacier.

Polson Moraine (gray linear feature).

Extension of the Flathead lobe created the terminal moraine at Polson during the last glaciation about 12,000 years ago.