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Flashcards in Glacial erosion Deck (27)
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How does the erosion technique of abrasion work?

Material carried by the glacier rubs away the valley floor and the sides - like sandpaper.
Coarser material carried may leave scratches in the rock - striations or large gouges.
The finer debris smooths and polishes rock surfaces.


What is rock flour?

The debris involved with abrasion is often worn down to make a fine material - rock flour.


How does the erosion technique of plucking work?
Where is it commonly found?

Glacier freezes onto rocks.
Ice moves forwards - pulls away bits of rock.
Mainly found at the base of a glacier - pressure and friction often result in the melting of the ice.
Very jagged landscape.


How does the weathering technique of free-thaw/frost disintegration work?

Water enters the cracks in the rock and freezes overnight.
Ice occupies more space than water (10% more) - increased pressure on the crack - widens.
Pieces of rock breaks off.
Optimum at 0C - mostly periglacial areas.


What is scree?

A collection of material at the base of a steep slope, usually as a result of freeze-thaw weathering.
In a glacial valley, much of this material falls from the valley side onto the edges of a glacier.
Some bits finds its way to the base of the ice via crevasses.


What is nivation?

Takes place under patches of snow in hollows.
Freeze-thaw action under the snow, causing the underlying rock to disintegrate.


What are the landforms produced as a result of glacial erosion? 9

1. Corries.
2. Aretes.
3. Pyramidal peak.
4. Glacial troughs.
5. Hanging valleys.
6. Truncated spurs.
7. Crag and tail.
8. Roche mountee.
9. Ribbon lakes.


What is the explanation for the formation of a corrie?

Form in hollows - snow accumulation.
Snow - ice - neve - moves downhill.
Hollow is deepened by nivation - grows into a corrie/cirque glacier.
Rotational movement - due to gradient and the overlying pressure.
Ice freezes to the back wall - plucking steepens it.
Freeze Thaw/frost shatter above the hollow on exposed rocks shatters the rock and deliver the scree to the ice - results in the Bergschrund crevasse - abrasion.
Water trickling down the Bergschrund encourages even more freeze thaw action - more corrie growth.
Moraine at the base creates a rock basin by abrasion.
Rock lip is left from the decreased rate of erosion.
Lip heightened by the moraine deposition.
A small lake created at the postglacial stage - tarn.


What does a corrie look like?

Armchair shaped hollow with a steep back wall.
Also has an over-deepened basin with a rock lip.


What is an example of a corrie and a corrie lake?

Ben Lui in Scotland.
Grisedale Tarn in the Lake District.


What is the explanation for the formation of an arete?

Where 2 corries occur back to back, they can erode backwards by abrasion.
Backwards erosion steepen the back walls in both corries - steep knife edge.


What is an example of an Arete?

Striding Edge, Lake District.
Steep sides - 300m.


What is the explanation for the formation of a pyramidal peak?

3+ corries erode backwards towards one another.
A steep-sided pointed mountain.


What is an example of a pyramidal peak?

The Matterhorn, Alps.


How are glacial troughs formed?

As corrie glaciers leave their source regions and descend down old river valleys, they make a U shaped valley.


What leads to the formation of rock basins?

When compressing flow is present, the glacier will over deepen parts pf the valley floor - rock basins.


What are the major features of a glacial trough?

1. U shaped - formed from gravity pulling down the glacier.
2. Stepped long profile with alternating steps and rock basins.
3. Trough ends - glacial valleys end abruptly with their heads in a steep wall.
4. Rock basins filled with ribbon lakes e.g. West Water, Lake District.
5. Fjords (submerging of lower parts of glacial valleys) e.g. Milford Sound, New Zealand.
6. Steep edged truncated spurs.
7. Hanging valleys.


How are hanging valleys formed?

Located on the side of the main valley.
These are either pre-existing tributary river valleys that weren't glaciated, or tributary glacial valleys.
In tributary glacial valleys - less ice - less erosion than in the main valley.
The tributary valley floor is left higher than the main valley when the ice retreated.
When rivers return, they often form waterfalls in these hanging valleys.


What is an example of a hanging valley?

Fisher Gill into Thirlmere, Lake District.


How are truncated spurs formed?

Areas of land projecting from the river-valley side (interlocking spurs) have been removed to produce truncated spurs by abrasion.
During glaciations this rock is removed by descending ice sheets.


What is an example of truncated spurs?

Either side of Fisher Gill, Lake District.


How are striations formed?

Long scratches in the rock where moraine has been dragged over the bedrock.
Run parallel to the direction of ice movement - can be used to calculate the direction of ice movement once the ice has retreated.


How are roche mountonnees formed?

Small areas of rock of the valley floor that are not always completely removed.
They have a smooth upstream side polished by abrasion (STOSS) - PMP rises.
Jagged down slope (LEE) formed by plucking (as the water refreezes).


What is an example of a roche mountonnee?

Coniston area of Lake District.


How is crag and tail formed?

Resistant rock on the STOSS (upslope side) and a gently sloping tail (on the LEE side) of less resistant rock.
Forms when a glacier passes over resilient rock.
Erodes the softer material (limestone) - abrasion.


What is an example of a crag and tail?

The rock on which Edinburgh Castle is built.
Resistant rock - basalt.


How are ribbon lakes formed?

Erratic erosion - due to either alternating bands of resistant and non-resistant rock, or thicker ice.
Sharp edged boulders erode the erode the softer rock when the glacier moves by abrasion - rock basin.
Hard rock erodes less - rock bars - acts as a dam.
Water accumulates - ribbon lake.
Ice - compressing flow.
EXAMPLE: Loch Ness, Scotland.