Depositional Landforms
Geography: Edexcel A Level, topic 2.4.5
Glacial Moraines
A range of glacial depositional processes have created a variety of distinctive landforms in many lowland glaciated areas. Moraines consist of unconsolidated and unsorted angular materials.
End moraines
- Terminal and Recessional moraines are collectively called 'end moraines'.
- Terminal moraine marks the maximum position of the ice.
- This consists of debris dumped at the snout of a glacier.
- They are usually arc-shaped and can be tens of metres high and kilometres long.
- E.g. the Cromer ridge in Norfolk.
- Recessional moraines mark the stand-still position of a glacier as it gradually melted during deglaciation.
Lateral moraines
- Lateral moraines are located along the edge of valley glaciers where the ice meets the valley wall.
- This is where weathering and mass movement constantly drop debris onto the ice.
- During deglaciation, the debris is dumped in a hummocky line along the valley edge.
Medial moraine
- Medial moraine is produced when two valley glaciers meet to form one larger glacier.
- Their respective lateral moraines merge into a medial moraine running down the middle of the larger glacier.
- On deglaciation, there is a line of moraine in the centre of the glacial valley.
Drumlins and Till Plains
A drumlin is an oval-shaped hill often formed of moraine that has been deposited as a glacier moves forwards. They indicate the direction that the glacier was flowing in.
Drumlin characteristics
- A drumlin is an oval-shaped hill, often of unsorted till, shaped like an egg half-buried along its long axis.
- Drumlins can be up to 50 m tall and around 200-1,000 m long.
- The long axis runs parallel to the direction of ice flow, with its steeper ‘stoss’ end pointing up-ice and its gently sloping ‘lee’ end pointing down-ice.
- Drumlins can be either rock-moulded or made entirely of glacial sediments.
- They are often found in groups or swarms where the landscape is described as ‘a basket of eggs’ topography.
Drumlin formation
- Geographers still do not know how drumlins are formed exactly beneath the ice.
- The most widely accepted theory suggests that different sediments, with different strengths, behave in different ways when stress is applied by the overlying ice.
- This leads to zones of stronger sediments able to resist deformation and these act as cores which develop into drumlins.
- E.g. In the Aire Gap, North Yorkshire, Cumbria.
Till plains
- Till is the unconsolidated sediment deposited by a glacier - it is a mix of clay, boulders and gravel.
- Lodgement till is deposited under the ice and is usually structureless.
- Ablation till is deposited by melting ice and usually has some evidence of deposition by running meltwater.
- Boulder clay is the name given to the till deposits in many lowland areas of the British Isles. Boulder clay ‘blankets’ the underlying topography.
- E.g. Coastal East Yorkshire and Norfolk.
Evidence of Ice Extent and Movement
Glacial depositional processes have created distinctive landforms in many lowland glaciated areas. These can be looked at to understand the extent of ice cover in the past and the direction ice masses moved in.
Evidence for maximum extent of ice
- Terminal moraines mark the extreme edge of a glacial advance.
Evidence for direction of movement
- The orientation of drumlins and crag and tails provide evidence of the direction of movement by their angle of long axis.
- Erratics also provide evidence of the direction of movement.
Test yourself on Depositional Landforms
At which location on the glacier would you find lateral moraine?
Which of theses are types of glacial moraine?
How tall can a drumlin be?
What do the orientation of drumlins and crag and tails tell us about past glaciers?