Glaciers

Glaciers are masses of ice moving as sheets over land (continental glaciers, or piedmont glaciers spread over plains at a mountain's foot) or as linear flows down mountain valleys (mountain and valley glaciers). They move slowly, from a few centimetres to a few metres a day, under gravity.

• India has many glaciers in the Himalayas. The Bhagirathi is fed by meltwater from Gaumukh, the snout of the Gangotri glacier; the Alakananda is fed by the Alkapuri glacier; the two join at Devprayag to form the Ganga.
• The great weight of ice causes huge friction. Angular blocks plucked from the land are dragged along the floor and sides, eroding by abrasion and plucking. Glaciers can erode even unweathered rock and reduce high mountains to low hills and outwash plains.

Erosional landforms:

• Cirques: the most common landform in glaciated mountains: deep, wide basins at the heads of glacial valleys with very steep, concave walls. A lake left in one after the ice melts is a cirque (tarn) lake.
• Horns and arêtes: when three or more glaciers cut headward until their cirques meet, a high, sharp, pointed peak (a horn) is left; the narrowed walls between cirques become serrated (saw-toothed) ridges called arêtes. The Matterhorn in the Alps is the classic horn.
• Glacial valleys (troughs): U-shaped, with broad floors and steep, smooth sides; they may hold moraines and lakes. Side valleys left high above the main valley are hanging valleys, their spurs cut into triangular facets.
• Fjords: very deep glacial troughs drowned by the sea, on high-latitude coasts.

Glacial deposits:

• Till: unsorted coarse and fine debris dropped by melting ice; fragments are angular.
• Outwash: debris carried by meltwater streams; roughly stratified, sorted and somewhat rounded.

Depositional landforms:

• Moraines: long ridges of till. Terminal moraines at the glacier's end; lateral moraines along its sides (they may join the terminal moraine in a horseshoe); medial moraines in the middle of the valley; ground moraines as an irregular sheet of till left by a retreating glacier.
• Eskers: meltwater flows in a channel under the ice, with ice for banks; coarse debris settles in it and, when the ice melts, remains as a sinuous ridge.
• Outwash plains: broad, flat fans of glacio-fluvial gravel, sand, silt and clay beyond the ice.
• Drumlins: smooth, oval ridges of till, up to about 1 km long and 30 m high, with their long axis parallel to the ice movement. The blunt, steep stoss end faces the glacier; the gentler end is the tail. They show which way the ice moved.

Example (new): a trekker who climbs out of a narrow V-shaped river gorge into a wide, flat-floored valley with steep walls and heaps of jumbled boulders has crossed from river country into old glacier country.