Glacial Erosion. How do Glaciers Move?. Internal deformation Extending Flow Compressive Flow Basal Sliding Regelation Surging Lateral Shearing Creep Rotational Flow. 1. Processes of Glacial Erosion 1. Abrasion 2. Plucking 3. Pressure Release (Dilitation) 4. Subglacial Water Erosion.
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3. Pressure Release (Dilitation)
4. Subglacial Water Erosion
With it's load of abrasive rock fragments, the base of the glacier acts like a belt sander, scraping across the rock, eroding it, producing characteristic erosional features, and creating a supply of material that leads eventually to the formation of depositional features as well. This scraping process is called Abrasion.
When a glacier moves across the underlying rock, the process of abrasion wears it away. It is the fragments of rock held in the ice that do the abrading, scraping across the rock surface like nails across a wooden desk top. Larger rock fragments leave deep scratch marks behind them. These scratch marks are straight parallel lines that reveal the direction of ice movement.
Freshly exposed striations have a preferred orientation of rock grains. By lightly running a finger along the striation it is possible to discover that when moving one way along it, the rock feels smooth, but when moving the other way it feels more coarse. The moving ice leaves the rock grains aligned with the direction of movement, so when the striation feels smooth, your finger is moving in the direction of ice flow.
The Grooves on Kelley's Island have been the source of debate for over 100 years. Some say they were cut by glacier ice, others say by jets of subglacial water. Note the curved forms suggesting fluid flow.
Occasionally, a moving glacier may become stuck on its bed. This occurs when for some reason a reduction in pressure causes liquid water to freeze, attaching the moving ice to the bedrock. As the ice continues to move an immense pulling force is applied to the attached rock which may then fracture and be plucked from its position. It involves the removal of much larger fragments of rock than abrasion.
When a glacier erodes its valley a classic U shape is formed, the side walls tending to be steep and possibly curving inwards at the base, and the valley floor almost flat.
U shaped valleys often start life as river valleys that existed before glaciation occurred. The glaciers then followed the existing V shaped valleys, eroding and deepening them as the ice moved. Over time the valleys became straightened, widened and deepened, keeping the steep sides and acquiring a flat base. U shaped valleys are also known as Glacial Troughs.
The flat floor is roughly shaped by the ice which tends to cut down more evenly than flowing water. A thick layer of glacial debris (ground moraine) is deposited as the ice retreats, smothering any minor irregularities, and creating a well drained and fertile soil.
In mountainous areas, the low lying flat valley floors are frequently used for farming, transport routes and habitation. They offer the easiest routes through the mountains, are warmer than the higher ground, and have good water supplies. The flatness of the ground is particularly advantageous for rail and road systems where steep inclines are best avoided.
The glacial-shaped valley trends north through the Brooks Range. For scale, look at the road on the floor of the valley. Atigun Glacier and one of its moraines are in the foreground.
When a corrie is formed, its back and side walls tend to be steep and jagged, perhaps almost vertical. When two corries form next to each other, and their adjacent walls are eroded backwards until they meet, a narrow and pointed rock ridge is formed. This is often likened to a knife edge, with near vertical sides and a sharp top edge. This feature is called an arete
Fjord - A glacial trough whose floor is occupied by the sea.Common in uplifted mid-latitudes coasts, in Norway.
Fjords:Characterized by: steep sides, overdeepened rock basins,shallow thresholds at the coast. Glaciers exloit: pre-existing river valleys and underlying weaknesses in bedrock
They are the product of different rates of erosion between the main valley and the valleys that enter it along its sides. The floors of the tributary valleys are eroded and deepened at a slower rate than the floor of the main valley, so the difference between the depths of the two valleys steadily increases over time. The tributaries are left high above the main valley, hanging on the edges, their rivers and streams entering the main valley by either a series of small waterfalls or a single impressive fall.
In the diagram above, the ice was flowing from left to right. The long axis of the drumlin is the line A-B, the point of maximum width is the line C-D, and the highest point on the landform is at E. Not all drumlins will show such a distinct difference in slope angle between the stoss end and lee slope, but the stoss end will always be the steeper of the two.
Roche Moutonnee are outcrops of resistant bed rock with a gentle abraded slope on what would have been the upstream side of the ice (stoss slope) and a steep rougher slope on the downstream side (lee slope). The name is French and translates into English as 'sheep rocks', a good description of them when seen from a distance. The smooth upstream slope is probably caused by abrasion as the ice advances over the rock, and the rough 'tail' is due to the action of plucking where ice has attached to the rock and literally pulled rock fragments away. Plucking could occur because as the ice moved up the stoss slope there was a reduction in pressure, allowing liquid water to re-freeze and attach the ice to the underlying rocks.
A pass or saddle between
2 mountain peaks
Bergschrund Explorer on Skillet Glacier in 1936. Bergschrund is visible as the dark band of ice in the background.
A landscape of ice-moulded rock knobs with intervening lochans which have been eroded along lines of structural weakness. This type aite is found in NW Scotland (eg Barra) and also is Canadian and Scandanavian Shields