Glacial Landscapes (AQA A Level Geography)

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

Landforms of glacial erosion

• 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

Formation of a roche moutonnée 

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

The Great Stone of Fourstones - erratics are random rocks that vary in size!

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

Glacial till                                                                                   Glacial outwash plain