A common feature of glaciated mountain ranges, a cirque is a semicircular rock basin or bowl, as the skiing term goes, framed by steep headwalls varying in height from a few hundred to several thousand feet. Glaciers are instrumental in hollowing out these rock amphitheaters. As they move, cirque glaciers tend to rotate on the bedrock, grinding out the cirque’s ﬂoor and creating a characteristic rock threshold on the down-valley edge. The cirque’s headwall is steepened by eroding freeze-and-thaw cycles both between the glacier and the rock and within the glacier’s bergschrund. Some cirques contain active glaciers, while others are relicts of past glaciation and may or may not be ﬁlled with ice and snow. Frequently they cradle small lakes called tarns. Back-to-back compound cirques can create knife-edge arêtes or sharp spire-shaped horns like the Matterhorn. Wyoming’s Wind River Mountains feature a complex group of cirques and tarns carved by Pleistocene glaciers.