Glaciated Upland Landscapes: Erosion & Deposition (Edexcel GCSE Geography A)

Glaciated Upland Landscapes - Erosion

• As a glacier moves it erodes the landscape which existed before glaciation

• Several landforms in the glaciated uplands are formed by erosion, including:
  

  • Pyramidal peak 

  • Arête  

  • Corrie/cwm/cirque and tarns

  • Truncated spur

  • Hanging valley

  • Ribbon lake

  • Glacial trough/U-shaped valley

  • Roche moutonnées

Pyramidal peak 

• This is a three-sided, pointed mountain peak

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

• This creates a sharply pointed mountain summit

• Examples include Snowdon in Wales and Buachaille Etive Mòr, Glencoe, Scotland 

Arête  

• Arêtes are narrow, steep-sided ridges

• Formed when two glaciers flow back-to-back

• As each glacier erodes 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 Wales, and Striding Edge in Lake District, England

Corries and tarns

• Corries are formed in hollows where snow can accumulate, usually on a north-facing slope

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

• Formed when the glacial ice moves through gravity, rotational slip, and sheer mass of the ice

• Ice freezes on the back wall of the hollow and as the ice moves, it plucks the rock out, which steepens the back wall

• Freeze-thaw, plucking and abrasion further erode the hollow into a rounded, steep-sided 'armchair' shape with a lip at the bottom end

• Examples include Helvellyn Corrie in the Lake District and Cwm Idwal in Snowdonia

• A tarn is a mountain pool or lake in a corrie after the glacier has melted 

• 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 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. they are an inverted 'V' shape 

• An example is Nant Ffrancon Valley in Snowdonia

Hanging valley

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

• When melting occurs, waterfalls form, dropping onto the valley floor

• An example is Cwm Dyli in Snowdonia

Ribbon lake

• As a glacier flows, it travels over harder 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 Snowdonia

Glacial trough/U-shaped valley

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

  • They start as V-shaped river valleys

  • Then, due to the size and weight of the glacial ice, the glacier erodes the sides and bottom

  • This makes the valley deeper and wider, changing it to a U shape

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

• U-shaped valleys are often drained by streams known as 'misfit streams'

  • They are known as misfit streams because they seem tiny in comparison to the valley

Roche moutonnées

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

• Reversed crag and tail, with plucking occurring on the leeward side 

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

• Erosion over the rock, creates striations across the top of the rock

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

• 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 (sheep's back)

Glaciated Upland Landscapes - Deposition

Moraines

• Unsorted glacial till is deposited in mounds, called moraines

• There are four types of moraines:

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

  • Terminal (end): Material deposited at the snout of the glacier,

    • Ice sheet terminal moraines can be over 200m high

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

    • Scree falling from the valley side is deposited as the glacier retreats

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

  • Recessional: When the glacier retreat pauses, small ridges of till are deposited  

Erratics

• Erratics are random rocks of different sizes and types from the area where 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 at the same rate 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!

Glaciated Upland Landscapes - Interaction of Erosion & Deposition

Drumlins

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

• They form beneath the glacier when the glacier meets an obstruction

  • This results in material being deposited as ground moraine  

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

• 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 has a tapered tail 

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

Crag and tail

• Formed when a ‘crag’, a large section of hard, resistant bedrock, e.g. volcanic rock, protects a section of softer, unconsolidated material, the ‘tail’

• The advancing ice sheet is forced to go around the resistant rock (crag),

  • This leads to melting and refreezing around loose rocks and boulders

  • The rocks and boulders are plucked as the ice mass moves forward

• The leeward or downstream side of less resistant material is protected directly behind the crag

• Velocity and pressure are reduced and deposition occurs

• As the ice mass continues, abrasion of the softer rock occurs

  • The tail is smoothed by abrasion with the plucked crag debris

• An example is Castle Rock (crag) and Royal Mile in Edinburgh (tail)

Crag and tail