Glacial Deposition Processes
There are two principle depositional processes:
Lodgement
Occurs beneath the ice mass when subglacial debris, being carried, becomes ‘lodged’ or stuck on the glacier bed. It occurs when friction is greater than the drag force of the glacier and is associated with:
– glaciers carrying huge amounts of debris
– changes of gradient
– loss of glacier mass
Ablation
Where material is deposited as the glacier melts. It can include sub-, en- and supra-glacial material.
Minor processes include deformation, which is less common and associated with weak underlying bedrock. These sediments are defined by the movement of the glacier. Finally flow occurs if high meltwater content causes glacial debris to creep/slide or flow during deposition.
Glacial Deposition Landforms
Moraines
These are large glacial deposits, formed via ablation:
– Terminal – these are deposited at the snout of a glacier and indicate the maximum extent of a glacier (can be lowland)
– Lateral – the form along valley sides, mainly from supraglacial material that has accumulated via freeze-thaw weathering on the valley slopes.
– Medial – where two valley glaciers meet, lateral moraines combine to form medial moraines. Also, as a result of entrainment, lateral moraines can migrate into the middle of the glacier surface.
Macro/Meso
Drumlins
An oval, or egg-shaped hill, made up of glacial till which is aligned in the direction of ice-flow. They are formed primarily due to lodgement, but also associated with ablation and can vary greatly in size from 10-50m high and 200-2,000m long. Usually occur in clusters or swarms on flat valley floors (or lowland areas).
Meso/Macro
Lowland Landforms
Erratics
These are boulders picked up by ice, often for many kilometres to be deposited in areas of completely different lithology (rock-type). They can be deposited via lodgement or ablation.
Micro/Meso
Till Plains
This is an extensive plain created by the melting of a large ice sheet that detached from a glacier. It consists of angular, unsorted till which can be divided into lodgement till and ablation till. The till effectively levels-out the topography (shape of the land) to create a mostly flat landscape.
Macro
Explain how glacial processes form depositional landscapes. (8 marks)
Lodgement occurs beneath the ice mass when subglacial debris, being carried, becomes ‘lodged’ or stuck on the glacier bed and occurs when friction is greater than the drag force of the glacier and is associated with glaciers carrying huge amounts of debris.
Ablation occurs when material is dumped as the glacier melts and can include sub-, en- and supra-glacial material.
Minor processes include deformation, which is less common and associated with weak underlying bedrock. These sediments are defined by the movement of the glacier. Finally flow occurs if high meltwater content causes glacial debris to creep/slide or flow during deposition.
These process create a range of landforms including different moraines (terminal, lateral, medial, recessional and push) – formed via ablation. Drumlins are also formed on either valley floors or lowland areas, primarily as a result of lodgement but also ablation.
Erratics and till plains are both lowland depositional features; the former being large boulders with differing geology from the underlying bedrock and the latter being extensive deposits, formed through both lodgement and ablation.