Alpine glaciers form on mountain slopes and are also known as mountain, niche, or cirque glaciers. An alpine glacier that fills a valley is referred to as a valley glacier. Larger glaciers that cover an entire mountain, mountain chain, or volcano are known as an ice cap or ice field, such as the Juneau Icefield. Ice caps feed outlet glaciers, tongues of ice that extend into valleys below, far from the margins of the larger ice masses. Ice sheets are the largest glaciers. These enormous masses of ice are not visibly affected by the landscape as they cover the entire surface beneath them, with possible exception near the glacier margins where they are thinnest. Antarctica and Greenland are the only places where continental ice sheets currently exist.