Glacial erosional features Flashcards
Corrie Formation
Snow builds up over time in a north/north east hollow as it faces away from the sun. Over time, this snow compacts to form a nevee and then over time glacial ice. Plucking then happens which is when ice freezes into the bedrock, pulling loose rocks away from the back wall, making it steeper. Abrasion then occurs at the bottom of the hollow as rocks in the ice wear away the floor making it deeper. Frost shattering action then occurs when water gets into rock cracks, which expands, breaking rock. Gravity then pulls the heavy glacier from its hollow. Meltwater lubricates the glacier. This speeds up the movement of the glacier creating cracks at the bottom of the hollow. A rotational movement of the glacier can over deepen the hollow creating a deep bowl and corrie lip.
Arete formation
After this happens to two corries back to back, it leaves a sharp edge running between them called an arete.
Pyramidal Peak
After this happens to three corries back to back, it leaves a point.
Frost Shattering
Frost shattering action then occurs when water gets into rock cracks, which expands, breaking rock.
Plucking
Plucking then happens which is when ice freezes into the bedrock, pulling loose rocks away from the back wall, making it steeper.
Abrasion
Abrasion then occurs at the bottom of the hollow as rocks in the ice wear away the floor making it deeper.
Formation of a Glacial Trough
A glacier forms on a north or north east facing slope. Snow accumulates and is compressed into firn then glacial ice. The glacial ice then moves down hill due to gravity and occupies a V-shaped valley with interlocking spurs. Moraine embebbed in the glacier rubs away at the valley sides and base making it deeper and wider, This process is called abrasion and truncates the interlocking spurs. The glacier also tears rocks away from the valley sides and loosens rocks, this process is called plucking. Loose rocks enter the ice due to frost - shattering, this is when water enters the cracks in the rocks, freezes and expands, causes rock to break up. Where the valley floor has been over deepened by abrasion a ribbon lake may form.
Formation of a Drumlin
Drumlins are elongated hills made up of unsorted glacial deposits.
Drumlins are formed as the glacier becomes overloaded with sediment and deposits it,
streamlining the sediment as it flows over it. The steep ‘stoss’ slope faces up-valley and the more gently-sloping ‘lee’ slope faces
down-valley.
If there is a small obstacle on the ground, this may act as a trigger point and till clay can build up around it.
Drumlins may be reshaped by further ice movements after being originally deposited.
Drumlins are found in swarms or in a ‘basket or eggs’ topography.
Formation of esker
Eskers are meandering ridges formed by meltwater streams in or underneath a glacier.
They are therefore made up of meltwater sands and gravels. These are sorted by size with large stones at the base because larger stones are dropped first by flowing water.
The stones also tend to be more rounded than glacial deposits, because of the action of
flowing water rounding the edges by erosion.
As the glacier melts, sub-glacial streams flow through tunnels and the load is deposited on
the channel bed
Formation of a U-shaped valley
Snow and ice build up during the Ice Age(s) caused valley glaciers to move
downhill under gravity from their source in the mountains (corrie glacier) following existing (V-shaped) river valleys.
The huge weight /volume of ice combined with processes (plucking, abrasion, rotational
sliding) widened and deepened these valleys.
As the valley glacier advanced it abraded the former interlocking spurs leaving
truncated spurs and steepened the sides of the valley. The resulting U-shape of the valley left behind when the ice melted and disappeared may
vary according to rock hardness and the intensity of erosion. Tributary valleys have smaller less powerful glaciers which result in less deep U-shaped
valleys ‘hanging’ above the main valley. For example, Glen Coe.
Formation of Terminal Moraine
Terminal moraine is a ridge across the valley and made up of glacial deposits/till/boulder clay; which is unsorted. As the glacier moves downhill it acts like a bulldozer, pushing sediment in front of its snout as it goes. On reaching warmer lower altitudes, or when temperatures rise, the glacier melts, losing power and depositing the moraine. Terminal moraine marks the furthest point that the glacier reaches. Once the ice has retreated, the terminal (or end) moraine can often form a natural dam, creating a ribbon lake. Ablation exceeds accumulation
Formation of a hanging valley
Hanging valleys result from differential erosion between a deeper main glacier and smaller tributary glaciers. As the glacier moves down a main valley it erodes it by plucking and abrasion. This creates a main valley that is deep and wide. A smaller glacier moves down a tributary valley but does not erode it as deeply as the main valley as there is less ice and less erosive power. When the ice melts the floor of the smaller, tributary valley is left above the main valley floor as less ice means less vertical erosion. This is often marked by a waterfall.
