Glaciers are large bodies of ice formed where accumulated snow persists and is gradually transformed under pressure. Unlike a patch of seasonal snow, glacier ice moves under its own weight. This movement may be slow and difficult to notice during a short visit, but it helps shape the surrounding landscape over time. Glaciers occur in settings that allow snow to survive across successive seasons.
Different parts of a glacier gain and lose ice at different rates. Snowfall can add material to an upper area, while melting and other forms of loss affect lower or more exposed parts. The balance between these processes influences whether the glacier grows, remains relatively stable, or becomes smaller. A glacier can continue moving forward even while its outer edge retreats, because the position of that edge reflects more than the direction of ice flow.
Moving ice interacts with the ground beneath it. Rock fragments carried in or under the glacier can scrape surfaces and help remove material. Valleys may be widened and deepened over long periods. When the ice melts, it leaves behind sediment of different sizes. Some material forms ridges, while finer particles can be carried away by flowing water. These features remain as evidence after a glacier has disappeared from an area.
The surface of a glacier is not always smooth. Differences in movement and the shape of the underlying ground can create cracks. Snow, debris, and meltwater introduce further variety. Streams may flow along the surface, disappear into openings, or emerge from beneath the ice. The glacier and its drainage system are closely connected, although much of this system may be hidden from direct observation.
Understanding glaciers requires attention to both seasonal conditions and longer records. A single snowy winter does not describe the history of an entire ice body. Repeated photographs, measurements, and observations of the surrounding land provide a more useful picture. Glaciers illustrate how a substance that appears solid and motionless can participate in the continuing movement of water and in the gradual transformation of a landscape.