Geology shapes the park’s natural communities from valley wetlands to the mountain alpine. Ice-age glaciers periodically blanketed this landscape, last retreating 14,000 years ago and leaving behind river channels, outwash plains, glacial moraines, potholes, deep canyons, and jagged peaks. As glaciers moved, they deposited rocky debris in ridges called moraines that contain rocks ground to the consistency of flour. This rock flour retains moisture, allowing lodgepole pine forests to flourish on these moraines. As glaciers retreat, blocks of ice drop to the valley floor and become buried in outwash gravel. When the ice melts, the resulting depression, or pothole, forms a pocket of forest or wetland surrounded by a sea of sagebrush in the outwash plain.