Glacial landform

Glaciers shape the land in two main ways: by eroding it and by depositing material. Most glacial landforms seen today were carved by vast ice sheets during the Quaternary glaciations. Regions such as Fennoscandia and the southern Andes are rich in these features, while places like the Sahara contain rare, ancient fossil examples.

**Erosional landforms** As glaciers grow heavier with snow and ice, they grind, scrape, and scour the bedrock beneath them. This erosion produces features like striations—grooves in rock that show the glacier’s direction of movement. Cirques are bowl-shaped hollows left where a mountain glacier once started; a series of these is called a cirque stairway. U-shaped valleys, also known as trough valleys, form when mountain glaciers widen and deepen a valley; when seawater fills one, it becomes a fjord. Sharp ridges between two glaciers are arêtes, and if erosion cuts through an arête, a col or spillway appears. A horn is a pointed peak where several arêtes meet. Valley steps are abrupt changes in a glacial valley’s slope. Hanging valleys, often marked by waterfalls, are carved by glacial meltwater. Other erosional forms include roches moutonnées, nunataks, trim lines, and overdeepenings.

**Depositional landforms** When glaciers retreat, they leave behind a mix of crushed rock and sand called glacial drift. Much of this material is unsorted till—sediments of various sizes carried from far away. Depositional landforms include moraines (mounds of till at a glacier’s end, sides, or where tributaries merge), eskers (long, winding ridges from subglacial streams), and kames (irregular mounds from sediment falling into ice openings). Drumlins and ribbed moraines also form during retreat. Meltwater reshapes some deposits into fluvioglacial landforms, which are more sorted than till. Outwash fans are braided streams spreading sediment onto flat plains. Glacial erratics—boulders dragged far from their source—are common in places like New England’s stone walls.

**Glacial lakes and ponds** Lakes can form from glacial activity. Kettle lakes appear when a buried or surface ice block melts, leaving a depression. Moraine-dammed lakes occur when glacial debris blocks a stream; Jackson Lake and Jenny Lake in Grand Teton National Park are examples (though Jackson Lake is partly dammed by humans). Tarns are lakes in cirques formed by overdeepening. Paternoster lakes are a chain of lakes in a valley, dammed by successive recessional moraines. A glacial lake sits between a glacier’s front and the last recessional moraine.

**Ice features** Glaciers themselves are dramatic landscape features, especially in polar regions. Valley glaciers are confined by valley walls. Crevasses crack the upper ice, and icefalls are the icy equivalent of waterfalls.

**Disputed origin** Some landforms’ glacial origins are debated. Erling Lindström argued that roches moutonnées may not be entirely glacial; their shape might predate glaciation, as jointing often does, and similar forms appear in tropical areas like East Africa and Australia. At Ivö Lake in Sweden, weathered rock surfaces from kaolin mining resemble roche moutonnée. The idea that elevated flat surfaces were leveled by a “glacial buzzsaw” effect has been rejected. In Norway, the paleic surface consists of steps at different levels, not matching any single glacial equilibrium line altitude, and cirques don’t align with those levels. West Greenland’s elevated plains also show no buzzsaw influence. The origins of the Gulf of Bothnia and Hudson Bay remain similarly contested.