How does a glacier transport material?
As well as eroding the rock over which it is flowing, a valley glacier is also capable of transporting large amounts of debris. Some of this may be drived from rockfalls on the valley side. It is then transported on the surface of the glacier (supraglacial debris) or buried within the ice (englacial). Material found at the base of the glacier is known as subglacial and may include rock fragments that have fallen down crevasses and material eroded at the base.
The huge amounts of material carried down by a glacier will eventually be deposited. the bulk of this will be debris released by the melting of the ice at the snout. It is also possible for the ice to become overloaded with material, reducing its capacity. This may occur near to the snout, as the glacier melts, or in areas where the glacier changes between compressing and extending flow. Material that is deposited directly by the ice is known as till or boulder clay, although the latter terms tends not to be used today.
Till is used to describe an unsorted mixture of rocks, clay and sand that was mainly transported as supraglacial or englacial debris and deposited when the ice melted. Individual stones tned to be angualr to sub-angular, unlike river and beach material which is rounded. Till reflects the character of the rocks overs which the ice has passed.
Sometimes its is possible to find a large block of rock that has been moved from one area and deposited in another which has a very different geology. this is known as an erratic.
Two types of glacial deposit are recognised:
Lodgement till – subglacial material that was deposited by the actively moving glacier. A drumlin is a typical feature formed from this material
Ablation till – produced at the snout when the ice melts. Terminal, push and recessional morianes are typical features produced from ablation till
Drumlins are smooth egg-shaped hills which may be 10’s of metres in height and several hundred metres long. They are usually found in ‘clusters’ or ‘fields’ on the floor of glacial troughs. They are made of morainic material which has been shaped and moulded rather than dumped. Drumlins usually have a blunt end, which faces up-valley, and a more pointed end, which faces down-valley. This makes them useful indicators of the direction of glacial movement. They are formed from unsorted till.
Formation: the ice at the bottom of a glaicer becomes overloaded with debris. This reduces its capacity to carry debris and deposition then occurs ate the base of the ice. Once this material has been deposited, it is streamlined by further ice advance. There could also be pre-existing sediment (older till from a previous glacial advance, for example) that is caught up in the streamlining process.
Moraine is the term given to the angular material transported and then deposited by the ice. Moraine can also be called till or boulder clay due to the range of sizes of sediment present.
Ground moraine – this is the material which was dragged underneath the glacier and is simply left behind when the ice melts. It often forms hummocky or uneven ground.
Lateral moraine – this forms at the edges of the glacier. It is mostly scree (broken pieces of rock from freeze-thaw weathering) that has fallen off
of the valley sides. When the ice melts it leaves a slight ridge on the side of the valley.
Medial moraine – When a tributary glacier joins the main glacier, two lateral moraines merge to produce a single line of sediment that runs down the centre of the main glacier. On melting, the medial moraine forms a ridge down the centre of the valley.
Terminal moraine – huge amounts of material pile up at the snout of a glacier to form a high ridge, often tens of metres high across the valley. The terminal moraine represents the furthest extent of the glacier’s advance.