Glacial Systems - Depositional Landforms

The main types of depositional landforms are summarised below:

• Moraines – these are composed of rocky debris that has been removed from the valley sides and floor by weathering and erosion and carried downhill by glaciers. There are a number of different forms of moraine:
  • Terminal moraine – a crescent shaped mound or ridge of moraine found at the snout of the glacier and which represents the furthest advance of the ice. It marks the point at which the snout remained fixed for some time while continuing movement of fresh ice and debris down the mountain was continuously exposed at this point of terminal ablation to accumulate into a considerable body of material.
  • Push moraine – when an advancing glacier pushes a pile of previously deposited glacial material into a linear ridge. These are found in areas where fluctuating climate has resulted in glacial advance followed by retreat and then advance again.
  • Lateral moraine – moraine running along the edge of the glacier where it meets the valley side. These develop from the accumulation of frost-shattered rock debris falling from valley sides and accumulating at the base of the rock face on the surface of the glacier. When the ice melts the material forms accumulations of unsorted rock debris in a linear slump against the valley sides.
  • Medial moraine – when two glaciers meet the lateral moraines of the adjacent glaciers merge and extend down the middle of the enlarged glacier. When the ice melts the unsorted rock debris is left as a ridge down the middle of the once-glaciated valley.
  • Recessional moraine – these are similar to terminal moraine, however rather than being located at the furthest advance of the glacier they form where the glacier snout remained at the same point for sufficient time to accumulate a significant mound of debris as ice (and debris) flow continued to arrive here before melting. This then resides across the valley as the glacier retreats.
• Drumlins – large hill-sized oval mounds caused by glaciers dropping their basal debris load as a result of friction between the ice and the underlying geology. As the glacier continues to advance around the mound of deposited material they are narrowed and straightened. They are typically found in clusters, known as ‘drumlin swarms’ with the blunter end (stoss) facing the direction from which the ice advanced and a tapering end (lee) in the direction the ice was going. The interior composition of drumlins varies; some have unsorted cores of silts, sands and boulders. Others have fluvial deposits indicating that some may have been formed by fluvioglacial processes rather than simply glacial deposition.
• Erratics – these are rock fragments and boulders that differ from the local geology. They are transported within the glacier and deposited on top of different geology when the glacier melts and retreats.
• Till plains – when a large section of ice detaches from the main body of the glacier and melts, the suspended debris will be deposited and form a large plain of unsorted till.

There are other features associated with glacial erosion and deposition, but they are the consequence of the action of flowing meltwater rather than the main body of solid ice, and are known as fluvioglacial landforms and are covered in the related content sheet.