Topic 2 - EQ3 - Glaciation Flashcards
How are hanging valleys formed?
Smaller tributary glacier in a tributary valley does not have the same mass as a much larger glacier = does not erode the valley to the same extent (horizontally but importantly vertically) as the larger glacier in the main valley = when the ice melts the floor of the smaller valley is left ‘hanging’ above the deeper main valley (often with a waterfall between the two)
How are corries formed?
Initially small nivation hollow has been expanded over time by freeze thaw and plucking = rock entrained in the glacier that has formed from compacted snow in the nivation hollow = rotational ice movement of cirque glacier enlarges hollow creating a large hollow and a steep back wall with steep sides = glacier melts revealing a corrie often with a body of water called a tarn infilling the hollow
How are arêtes formed?
Freeze thaw, plucking and abrasion on the back wall of two corries on a mountainside mean they erode backwards and towards one another = creates a narrow ridge between two corries (arêtes)
How are pyramidal peaks formed?
Freeze thaw, plucking and abrasion on the back wall of three cirque glaciers on a mountainside mean they erode backwards and towards one another = a pointed mountain peak with three (or more) cirque glaciers or corries
How are glacial troughs (U shaped valleys) formed?
A v-shaped river valley is greatly widened and deepened due to powerful plucking and abrasion by a glacier moving through the valley = a U shaped valley with steep sides and wide and flat floors (often accompanied by a misfit stream flowing through the middle)
How are truncated spurs formed?
Valley glaciers (much less flexible than rivers) remove the ends of interlocking spurs in the path of glacial erosion by plucking and abrasion as they move through a river valley = rocky valley side with truncated spurs no longer interlocking
How are ribbon lakes formed?
Areas of increased plucking and abrasion by the valley glacier deepens part of the valley floor as a result of either the confluence of glaciers or it being an area of weaker rocks = long and narrow ribbon lake along floor of a glacial trough
What are macro-scale erosional glacial landforms?
-Corries
-Hanging valleys
-Arêtes
-Pyramidal peaks
-Glacial troughs
-Truncated spurs
-Hanging valleys
-Ribbon lakes
How are roche moutonnees formed?
Ice moves over a resistant rock outcrop and increased pressure causes melting and basal sliding and up-valley stoss side is smoothed by abrasion - on leeward side pressure is reduced and re-freezing and plucking takes place creating a steep jagged lee slope (as opposed to smooth upward stoss)
How are knock and lochans formed?
Intense at the base of the glacier excavates and removes areas of weaker rock = forms over-deepened hollows that often fill with meltwater and precipitation following ice retreat (lochans) but more resistant bedrock is left un-eroded and appear as small rock hills (knocks)
Landscape left is characterised by alternating small rock hills and hollows infilled with water
How are crag and tails formed?
Large mass of hard rock is resistant to ice scouring (weaker rock eroded) and creates a rocky outcrop with a steep stoss side and a reduced glacier velocity on the lee protects the softer rock and allows deposition - but the sheltering effect diminishes with distance, creating a sloping deposited tail.
How are striations (chattermarks) formed?
Abrasion by debris embedded in the base of the glacier scratches the rock surface as it passes over bare rock leaving scratches/grooves.
What are meso-scale erosional glacial landforms?
-Roche moutonnees
-Knock and lochans
-Crag and tails
What are micro-scale erosional glacial landforms?
-Striations/chattermarks
Which erosional glacial landform can show the extent of former ice? How?
U-shaped valleys (glacial troughs)
On some valley walls it is possible to identify a trimline. Below the trimline there is evidence of glacial abrasion (striations and polished rock surfaces) whilst above there is not such evidence. Therefore, the trimline indicates the extent and the height to which the U shaped valley was filled with ice.
What are the limitations with using U shaped valleys to show former ice extent?
-Trimlines only show localised extent, they do not show extent across the whole glacier (something which can vary greatly).
-Tributary glaciers (often localised) can also make trimlines higher than the main body of ice made them.
-E.g. Baltro glacier in Pakistan is the longest on earth at 63km and so trimlines would vary greatly between 1st and 63rd km.
How do corries show former ice movement?
In Britain and the N Hemisphere, corries are nearly always orientated between NW to NE (most common) to SE and corrie orientation indicates direction of ice movement. (e.g. in Lake District indicates that ice moved in from North and East).
As glacier moves it turns larger rock fragments to point in direction of ice movement so till fabric analysis can measure the orientation of a large sample of clasts and identify former ice movement.
What are the limitations with using corries to show former ice movement?
If corrie orientation is to be used to reconstruct former ice movement, the sample size must be large to be conclusive as there will be various anomalies (especially at high altitudes where it is so cold that carries can sometimes face any direction)
How do roche moutonnees (and striations) show former ice movement?
The abraded, smooth and polished upstream/stoss side indicates the direction in which the glacier has come from - the steep jagged downward/lee side is the side that the glacier moved over second with reduced pressure enabling plucking. Therefore, by identifying the stoss and lee the direction of ice movement can be calculated.
Striations/chattermarks often appear on the polished stoss side. Striations are often parallel to ice flow and so further show the direction of ice movement.
What are the roche moutonnees (and striations) with using corries to show former ice movement?
-Roche moutonnees are miso-scale and striations micro-scale and so only really show ice movement in one very specific location. Need to connect up related roche moutonnees to paint an accurate picture of the movement of a mass of ice.
-Roche moutonnees can be exposed to heavy weathering (especially problematic on polished side as can mask striations and make them less clear) and multiple glaciations (can paint a contrasting picture if two ice masses move in different directions)
-Human activity can disturb a roche moutonnee in busy areas (e.g. honister pass)
How do crag and tails show former ice movement?
The crag is the upward/stoss side of ice movement and the tail is the downward/lee. The longer lee is therefore where ice moved over second.
What are the limitations with using crag and tails to show former ice movement?
Tails are especially vulnerable to change as they are formed of less resistant material. Crag and tails may be modified by multiple glaciations, post-glacial weathering and erosion, as well as human activities such as farming, the building of infrastructure/settlements and mining. E.g. Edinburgh’s royal mile is the tail of a crag and tail with Edinburgh castle on the crag + Arthur’s seat is another famous crag within viewing distance
What are the two types of ways that glaciers deposit material?
Lodgement till = ‘spread’ onto valley floor BENEATH the ice by glaciers
Ablation till = ‘dropped’ by a glacier as it MELTS, mainly deposited by the SNOUT (as this is where most ablation happens) because as the ice around the debris melts it is deposited
What size of material do glaciers deposit?
Everything from boulders (hence why we have erratics) to pebbles and clay
What landforms are deposits (till) often deposited as?
Moraines (terminal, medial, lateral and recessional)
What is a lateral moraine?
A ridge of moraine (deposited material) along the edge of a valley floor (often parallel to ice flow) - formed of debris dropped by the glacier as a melts and so only form in the ablation zone
What is a medial moraine?
A ridge of moraine (deposited material) down the middle of the valley floor (often parallel to ice flow) caused by the merging of lateral moraines at the intersection of two tributary glaciers
