When a glacier moves down a drainage or a valley, it pushes ahead of itself a large burden of debris, known as glacial till. This debris is completely unsorted by the long term geological processes that usually separate materials and is composed of gravels, dirt and clay, and large and small boulders call glacial erratics.
The maximun advance of a glacier is marked by a terminal moraine. Later stages in the recession of the glacier are marked by end moraines. Lateral moraines may mark the height reached by the glacier up the sides of the glacier's path. These moraines hold more water than the surrounding glacial gravels in their soil and are therefore more heavily timbered than the surrounding area.
In Wyoming there are many lakes formed by the impoundment of water by glacial moraines. In Grand Teton National Park we have Taggart, Jenny and Jackson Lake (enlarged by Jackson Lake Dam. In the Wind River Range lie New Fork, Willow, Fremont, Half Moon and Boulder Lakes, all formed by glacial moraines.
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© 1995 Karen M. Strom