Republished from the Feb. 2021 edition of the Wyoming State Geological Survey Geo-Notes
By Dr. Erin Campbell, WSGS Director – As we brave our way through a Wyoming winter, let’s talk about things cold and icy—like glaciers.
Wyoming is home to the largest glacier in the Rocky Mountains and the second largest in the continental United States. Gannett Glacier lies in the Wind River Range in central Wyoming, northeast of Gannet Peak (the highest peak in the state). The glacier is currently about 3.3 square kilometers (1.27 square miles) in area; it was 4.6 square kilometers (1.78 square miles) in 1950, making it a retreating glacier. Glaciers are constantly accumulating ice at their head and melting at their terminus (foot). When the amount of melt is higher than the amount of ice formation, the glacier shrinks and retreats.
There are 38 named glaciers across Wyoming. Most of them are in the Wind River Range, with a few in the Teton and Absaroka ranges. Evidence of past glaciation does, however, exist in many mountainous areas across the state and can be found by looking for clues left by the long-gone glaciers.
Glaciers carve out characteristic U-shaped valleys (an example is Sinks Canyon, near Lander), as opposed to the V-shaped valleys created by rivers. Rocks left behind by glaciers can be distinctive in several ways as well. As glaciers retreat, they leave behind mounds of sediment in features called moraines. This sediment, known as glacial till, can vary widely in composition and size because glaciers pick up everything the ice passes over, including very large boulders. As rocks are carried along within a glacier, they bump and slide past each other. This action can create striations (long linear groves), chatter marks (impact marks), and other evidence of their movement. The rocks in the glacier also grind against one another, creating a fine powder called glacial flour. This glacial flour is very light and stays suspended when it enters glacial lakes and streams. The light reflecting off the particles gives glacial meltwater its characteristic light-blue and green color.
The most scenic features left behind by glaciers are glacial lakes. These lakes can be high in the mountains and are often deep and clear. As the glaciers receded, they left moraines surrounded by steep mountain terrain. In many cases the moraines trapped water, creating lakes in dramatic alpine settings. A trip to the high country usually involves a visit to a glacial lake, which we can all look forward to next summer.