How Glaciers Carve and Build Landscapes

Glaciers form where more snow accumulates than melts over many years. Layers of snow compress under their own weight, becoming dense grains and eventually glacial ice. Once the ice is thick enough, gravity causes it to deform and move.

Some movement happens within the ice as crystals slowly change shape. A glacier may also slide over its bed when meltwater reduces friction beneath it. The centre and upper parts often move faster than the edges, where rock and valley walls create resistance. Gravity supplies the overall pull, but temperature, ice thickness, slope and the amount of meltwater all influence how the glacier responds.

Glacier speed varies widely. Some move only a short distance in a year, while others can move much faster during brief surges. Even slow ice can transform land because it keeps pressing, scraping and transporting material over long periods.

Water can seep into cracks in rock beneath or beside a glacier and freeze. As the glacier moves, it can pull loosened blocks away. This process is called plucking. The captured rocks become part of the moving ice.

Those rocks scrape across the ground like rough tools, grinding and polishing the surface. This abrasion can leave scratches called striations that show the direction of past ice movement. Fine ground rock may be carried away as sediment.

River valleys are often narrow and V-shaped because flowing water cuts most strongly into the bottom. Glaciers widen and deepen the whole valley, producing a broad U-shaped profile. They can also carve bowl-shaped hollows, steepen mountain peaks and deepen basins that later fill with lakes. Along coastlines, deep glacial valleys can later fill with seawater and become fjords. Smaller side glaciers may leave hanging valleys above the deeper main valley.

A glacier carries sediment ranging from fine clay to enormous boulders. When the ice melts, it drops this material. Unsorted deposits are called till, while meltwater streams sort sediment by size as they carry it away. Moraines can form along the sides, centre or front of a glacier, recording former ice positions. Meltwater may also build broad plains of sand and gravel beyond the ice.

Common clues of former glaciation include several features. No single feature proves that a glacier was present, but several clues together can provide a strong explanation.

• •U-shaped valleys with steep sides.
• •Striations scratched into exposed rock.
• •Moraines made from ridges of deposited debris.
• •Erratics carried far from their original rock source.
• •Lakes occupying glacially carved basins.

Glaciers shape land by moving, eroding and depositing material. Plucking and abrasion carve valleys and basins, while melting ice leaves ridges, boulders and sediment behind. Read those landforms together, and you can reconstruct the slow journey of ice across a landscape.