glacial deposition

A glacier picks up large boulders and rock debris from the valley and deposits them at the snout of the glacier when the ice melts. Some deposited material is carried great distances by the ice to form erratics.

Braided stream south of the Myrdalsjokull glacier, in southern Iceland. A braided stream or river is one that splits into many channels, leaving lots of bars or islands between the channels. At Myrdalsjokull the melt water river emerging from the glacier carries a huge load of sediment. It flows underneath the glacier under high-pressure conditions. As it escapes from the glacier it is suddenly able to spread out. As the height and velocity of the river is reduced, its energy drops. It begins to deposit its load, the heaviest particles first.

Laying-down (deposition) of sediment once carried by a glacier. When ice melts, it deposits the material that it has been carrying. The material deposited by a glacier is called till, or in Britain boulder clay. It comprises angular particles of all sizes from boulders to clay that are unsorted and lacking in stratification (layering).

Unstratified till can be moulded by ice to form drumlins (egg-shaped hills). At the snout of the glacier, till piles up to form a ridge called a terminal moraine. Glacial deposits occur in many different locations – beneath the ice (subglacial), inside it (englacial), on top of it (supraglacial), at the side of it (marginal), and in front of it (proglacial).

Stratified till that has been deposited by meltwater is termed fluvioglacial, because it is essentially deposited by running water. Meltwater flowing away from a glacier will carry some of the till many kilometres away. This sediment will become rounded (by the water) and, when deposited, will form a gently sloping area called an outwash plain. Several landforms owe their existence to meltwater (fluvioglacial landforms) and include the long ridges called eskers, which form parallel to the direction of the ice flow. Meltwater may fill depressions eroded by the ice to form ribbon lakes. Small depositional landforms may also result from glacial deposition, such as kames (small mounds) and kettle holes (small depressions, often filled with water).

In periglacial environments on the margins of an icesheet, freeze–thaw weathering (the alternate freezing and thawing of ice in cracks in the rock) etches the outlines of rock outcrops, exploiting joints and areas of weakness, and results in aprons of scree.