A glacier is a moving body of ice. The glacier forms when more snow and ice falls than melts and evaporates from previous years. The pressure of the increasing burden upon the snow converts the snow into ice. The glacier carries with it the dirt, gravels, rocks and boulders that it finds in its path as well as that embedded in the ice.
In Wyoming, glacial ice and permanent snow fields covered the Yellowstone and Teton area, the Bighorns and the Snowy Range (Medicine Bows) during the Pleistocene era, from 2 million years ago until the present.
The ice and rock carried by these glaciers acted as sandpaper in moving down the valley, smoothing and widening them, leaving U-shaped profiles for the valley cross-sections. At the heads of these valleys lie large steep walled bowls called glacial cirques. When glaciers carve out valleys next to each other, a sharp-sided ridge, or arete, will separate them. If several of these valleys begin near the top of a mountain, a sharp peak, or horn, such as the Matterhorn, will be left.
There were three major periods of glacial advance in the Wyoming Rockies within the last 250,000 years. the first and most widespread of these, 200,000 years ago, is called the Buffalo glaciation. Ice converged on Jackson Hole from the north (Yellowstone area), east (Wind River and Gros Ventre ranges) and west (the Teton range). Jackson Hole was under more than 2000 feet of ice, which covered many of the features seen today, such as Signal Mountain. The ice crept down the Snake River Canyon into Idaho.
The next period of glaciation, 130-200,000 years ago, is called the Bull Lake glaciation. Ice moved down the Buffalo River valley from the north and from the Tetons in the west. The glacial till from this period is seen around Jenny Lake. The name derives from the well preserved morains found in the Bull Lake vicinity near the Wind River Mountains.
The most recent glaciation, the Pinedale glaciation, began around 70,000 years ago and ended 15-20,000 years ago. This is the event that left the lakes in Jackson Hole by leaving the terminal moraines that act as dams today. The outwash gravels from this event created the large flat plain in the valley. Chunks of ice that were left stranded as the glacier retreated made the small depressions known as the Potholes. The glacier remained in the Jackson Hole area, building up a large terminal moraine which became the natural dam for Jackson Lake.
