Glacial Landscapes (AQA A Level Geography)

Glacial Erosional Landforms

The glacial cycle of erosion

• There are 3 stages to glacial erosion

  • Youthful

  • Mature

  • Aged

• Youthful

  • This marks the beginning of erosional landforms

  • The shaping and hollowing of a corrie by ice

  • The beginnings of aretes and horns

• Mature

  • Corries are well-formed and begin to meet

  • The glacial valley takes on its ribbon shape with a regular, stepped graded contour

  • Hanging valleys are visible

  • The valley floor begins to deepen and takes on the shape of a trough

• Aged

  • 'U'-shaped valley is clearly defined

  • Development of the outwash plain, including features of drumlins, eskers, kettle holes etc. 

  • Corries converge, mountain summit heights decrease and their peaks become rounded

• Erosional landforms are created when moving masses of glacial ice slide and grind over bedrock

• Glacial ice contains large quantities of unsorted sand, gravel, and rock that was plucked out of the bedrock

• Ice sliding across the bedrock, grind the debris into a fine, but gritty powder called rock flour

• Rock flour polishes the surface of the bedrock to a smooth finish called glacial polish

• The remaining trapped debris and larger rocks, create long grooves, called glacial striations, as they flow over the bedrock

• These striations indicate the direction of ice flow

• Pyramidal peak 

  • As the name suggests, this is a three-sided, pointed mountain peak

  • Formed when three or more back-to-back corrie glaciers carve away at the top of a mountain

  • This creates a sharply pointed mountain summit

  • Examples include Yr Wyddfa (Snowdon) in Wales and Buachaille Etive Mòr, Glencoe, Scotland 

• Arête  

  • Arêtes are knife-edge, steep-sided ridges

  • Formed when two corries cut back into the mountainside 

  • As each corrie glacier erode either side of the ridge, the edges become steeper and the ridge narrower

  • This gives the arête it's a jagged profile

  • Examples include Crib Goch in Eryri National Park, and Striding Edge in Lake District England

• Corrie/cwm/cirque

  • Corrie, cwm and cirques are all the same feature and are deep hollows of accumulated snow and ice 

  • In Wales corries are called cwms and in France they are called cirques 

  • Found at the apex of a glacial valley, on the coldest aspect of the mountain, with the greatest accumulation of snow and ice

  • As the accumulated ice begins to flow; basal/rotational sliding along with plucking and abrasion, hollows the mountain into a bowl-shape

  • Debris is pushed to the edges of the corrie, which acts as a dam (corrie lip) to the accumulating snow

  • As the ice thickens within the hollow, it flows over the corrie lip and downhill as a glacier

  • Plucking and freeze-thaw weathering, steepen the back wall of the corrie, into the familiar armchair shaped landform

  • Examples include Helvellyn Corrie in the Lake District and Cwm Idwal in Eryri National Park (Snowdonia)

• Corrie, tarn or cirque lakes
  

  • Corrie, tarn or cirque lakes form when the ice within the corrie melts 

  • Because of the corrie lip at the bottom end, the meltwater is held in place and a circular body of water is formed

  • Examples include Red Tarn, Helvellyn in the Lake District and Cadair Idris in Eryri National Park (Snowdonia)

• Truncated spur

  • Truncated spurs are past interlocking spur edges of past river action that have been cut-off forming cliff-like edges on the valley side

  • Found between hanging valleys and are an inverted 'V' shape 

  • Formed when past ridges/spurs are cut off by the lower valley glacier as it moves past

  • An example is Nant Ffrancon Valley in Eryri National Park

• Hanging valley

  • These are small tributary glaciers found 'hanging' above the main valley floor

  • When melting occurs, there are waterfalls onto the valley floor

  • An example is Cwm Dyli in Eryri National Park

• Ribbon lake

  • As a glacier flows it travels over hard and softer rock

  • Softer rock is less resistant to erosion, so a glacier will carve a deeper trough over this type of rock

  • When the glacier has melted, water collects in these deeper areas

  • This creates a long, thin lake called a ribbon lake

  • Examples include Lake Windermere in the Lake District and Llyn Ogwen in Eryri National Park

  • The areas of harder rock left behind are called rock steps

• Glacial trough/U-shaped valley

  • Glacial troughs are steep-sided valleys with a flat floor 

  • They start as V-shaped river valleys but due to the size and weight of the glacial ice it changes to a U shape as the glacier erodes the sides and bottom making the valley deeper and wider

  • Examples are found all over the UK, but Nant Ffrancon and Nant Gwynant in Eryri National Park are good examples

• Roche moutonnée

  • A resistant, bare mass of rock, on the valley floor, that has been sculpted by flowing ice

  • The upstream or stoss side of the outcrop, is smoothed due to abrasion by the glacier

  • The moving ice leads to localised pressure melting

  • This eases basal sliding and increases erosion over the rock, creating striations across the top of the rock

  • On the leeward or downstream side, the pressure reduction refreezes the meltwater

  • This bonds the base of the glacier to the outcrop

  • As the glacier continues to flow, loose rocks/boulders are plucked out and a jagged, steep surface is left behind

Glacial Depositional Landforms

Moraines

• Unsorted glacial till that is deposited in mounds are called moraines

• Moraines are termed from their position on the glacier:

  • Terminal: Material deposited at the snout of the glacier

  • Lateral: Material is deposited along both sides of the glacier

  • Medial: Ridge of deposited material in the middle where two glaciers meet and continue to flow downhill together

  • Ground: Material dragged under the base of the glacier and deposited over a wide area on the valley floor

  • Recessional: They show the point of glacial retreat

  • Push: They form if the glacier advances after retreat

Drumlins

• Drumlins are elongated, egg-shaped hills and made of glacial till

• They form beneath the glacier when the glacier meets an obstruction and material is deposited as a ground moraine  

• The moraine is then shaped by the moving ice, which follows the direction of the flow of ice

• The largest ones can be over 1km in length, 0.5km wide and 50m high

• Multiple drumlins are known as swarms or baskets of eggs

• As the material is deposited it builds up to have a round, blunt and steep front (stoss) end 

• The flow of ice over the top of the drumlin, drags the material along and down creating the lee slope

• The lee is gently sloped, elongated and with a tapered tail 

• Examples include The Drumlin Field below Cam Fell in the Yorkshire Dales and Conway Valley, North Wales

Erratics

• Erratics are random rocks of different sizes and types from the area they are found

• There is no pattern to their deposition, and they look completely out of place on the landscape

• Glaciers pick up large rocks and carry them hundreds, sometimes thousands of kilometres from where they originate

• Erratics are carried deep in the ice and do not erode the same as rocks at the edges of the glacier

• An example is the Great Stone of Fourstones, (Big Stone)' on the moors of Tatham Fells, England

Till plains

• During the warmer summer months, glaciers begin to melt, and glacial till is deposited on the valley floor or sides of a moving glacier

• Till is unsorted, irregular debris ranging from clay to boulders of any size and shape 

• Meltwater will also flow out of the glacier's snout forming meltwater rivers

• These rivers carry large amounts of glacial till, which will undergo further erosion through attrition to become outwash 

• This finer till is sorted and when the energy of the river reduces, the outwash is deposited in layers further down the valley on the outwash plain