Erosional and depositional landforms

• What is a corrie?

  An armchair shaped hollow found on the mountainside

  • Steep backwall
  • Deepened basin
  • Corrie lip at the front
• Erosional landforms- macro to micro scale

  Pyramidal peaks

  Glacial trough

  Corries

  Aretes

  
  

  Roche Moutonees

  Striations

  
  

  Ellipsoidal basins
• How does a corrie form?

  • Plucking steepens the backwall
  • Abrasion deepens the hollow
  • The lip consists of moraine deposited by the ice as it moves out of the corrie
  • Meltwater collects at the bottom of the corrie to form a tarn
• Example of a corrie?

  Red Tarn Corrie, Helvellyn, Lake District
• What is an arete?

  A narrow knife edged steep sided ridge found between two corries
• How does an arete form?

  • Glacial erosion causes the corries that are back to back or alongside each other to retreat
  • Plucking steepens the slopes
  • Abrasion smooths the side of the ridge
• Example of an arete?

  Striding Edge, Helvellyn, Lake District
• What is a pyramidal peak?

  An angular, sharply pointed mountain top
• How does a pyramidal peak form?

  If three or more corries develop around a mountain top and their back wall retreats, the remaining mass is steepened by plucking to form a pyramid shape.

  Abrasion smooths out the side of the ridge
• Example of a pyramidal peak?

  Matterhorn, Switzerland
• What is a glacial trough?

  A U-shaped valley with steep straight sides and a flat valley floor. It also typically has hanging valleys and truncated spurs ( a ridge descending to the valley floor formed by a stream- waterfall )
• How does a glacial trough form?


  As glacial ice moves down pre-existing river valleys, it erodes the sides and floor of the valley through plucking and abrasion, causing the shape to become deeper, wider and straighter due to the erosive power of ice
• Example of a glacial trough

  Thirlmere, Lake District
• What is a roche moutonee?

  Asymmetrical projections of resistant rock found on the floor of glacial troughs
• How does a roche moutonee form?

  • Advancing ice places pressure on resistant rock causing pressure melting on the up-valley side
  • This meltwater allows the glacier to slide over the rock.
  • As it does so, the surface of the rock is smoothed out by abrasion, creating a gentle slope on the up-valley side, called the stoss end
  • Once the resistant rock is passed, pressure is reduced on the down-valley side, and meltwater re-freezes, resulting in plucking and the steepening of lee side
• Example of a Roche Moutonee?

  Nant Francon Valley, Snowdonia
• What are striations?

  Scratches or grooves on exposed rocks in glaciated areas
• How are striations formed?

  Striations form from abrasion by debris embedded in the base of the glacier as it passes over bare rock.

  Larger, angular debris produces deeper grooves

  They can indicate the direction of movement of a glacier
• Example of Striations?

  Nant Francon Valley, Snowdonia
• What are ellipsoidal basins?

  Large, deep basins shaped like an ellipse
• How are ellipsoidal basins formed?

  Isostatic lowering causes the surface to be lowered by the weight of the ice mass whilst the ice sheet erodes it
• Example of ellipsoidal basins?

  Great Lakes, USA
• Depositional landforms

  Lateral moraines

  Terminal moraines

  Recessional moraines

  Erratics

  Drumlins

  Till sheets
• What are lateral moraines?

  Ridges of till running along the edge of a glacial trough - a build up at the side of a glacier
• How are lateral moraines formed?

  Weathered material accumulates on top of a glacier. As the glacier melts, this material is deposited.
• Example of a lateral moraine

  Meade Glacier, Alaska
• What is a terminal moraine?

  Crescent-shaped ridges of till extending across a glacial trough - a build up at the end of a glacier
• How is a terminal moraine formed?

  
  

  • They mark the position of the maximum advance of the ice as sediment is deposited at the glacier snout.
  • Their crescent-shape is due to the position of the snout - there is less friction at the centre of the glacier, meaning there is faster movement at the centre than the edges
• Example of a terminal moraine

  Meade Glacier, Alaska
• What is a recessional moraine?

  
  

  • A series of ridges running across a glacial trough - a build up of sediment when the glacier pauses during its retreat
  • They lie parallel to each other and the terminal moraine
• How does a recessional moraine form?

  They form during a temporary pause in retreat as material carried to the snout of the glacier is deposited across the width of the glacial trough.
• Example of a recessional moraine?

  Meade Glacier, Alaska
• What is an erratic?

  Individual pieces of rock, often large boulders, that are geologically out of place
• How is an erratic formed?

  
  

  • Rock is eroded in an area of one type of geology, most likely by plucking or added to the supra-glacial debris by weathering and rockfall
  • It is then transported by a glacier down valley, often long distances
  • It is then deposited during melting in an area of differing geology.
• Example of an erratic?

  Ingleborough, Yorkshire Dales
• What is a drumlin?

  Asymmetrical, elongated hills composed of glacial till. The higher and wider stoss end faces the ice-flow (up valley), while the lee side is more gently tapered
• How are drumlins formed?

  • They may be formed by lodgement of subglacial debris as it melts out of the basal ice layers, or Reshaping of previously deposited material during a subsequent re-advance
  • The thinning of ice as it spreads out over a lowland area, reducing its ability to carry debris.
• Example of a drumlin?

  Risebrigg Hill, North Yorkshire
• What is a till sheet?

  Large, thick masses of unstratified till forming extensive and relatively flat surfaces
• How is a till sheet formed?

  Large amounts of till are deposited at the end of a period of glacial advance, during melting and retreat.