Glacial Erosion

Glacial erosion tends to occur in upland regions and is carried out by the two main processes:

1- Abrasion- occurs where the material the glacier is carrying rubs away at the valley floor and sides. It can be likened to the effect of sandpaper or a giant file. The coarser material may leave scratches on the rock know as striations. The finer debris smoothes and polishes rock surfaces. The debris involved in abrasion is often worn down by the process into a very fine material known as rock flour.

2- Plucking- involves the glacier freezing onto and into rock outcrops. As the ice moves forward it pulls away masses of rock. Plucking is mainly found at the base of the glacier where pressure and friction often result in the melting of the ice. It is also marked in well-jointed rocks and in those where the surface has been weakend by freeze-thaw action. Plucking leaves a very jaggered landscape.

Freeze-thaw action/Frost shattering- occurs in areas where temperatures rise during the day but drop below zero during the night for a substantial part of the winter. Water which enters the cracks in the rocks freezes overnight. As ice occupies more space than water this exerts much pressure on the crack, as this process continues to happen, the crack widens & weakens until eventually pieces of rock break off. On steep slopes this leads to the collection of material at the base known as scree. In a glacial valley, much of this material falls from the valley side onto the edges of the glacier and some finds it way to the base of the ice via many crevasses which cross the glaciers surface.

Nivation- is a series of processes that operate underneath a patch of snow in hollows, particularly on the north and east facing slopes. Freeze thaw action and possibly chemical weathering, operating under the snow, cause the underlying rock to disintrigrate. As some of the snow melts in spring, the weathered particles are moved downslope by the meltwater and by solifluction. Over some period of time, this leads to the formation of nivation hollows which when enlarged can be the beginning of corries.

Landforms created by glacial erosion include: corries, aretes, pyramidal peaks, glacial troughs, hanging valleys and truncated spurs.

A corrie is a arm-chair shaped rock hollow, with steep back walls and an over deepened basin with a rock lip. It often contains a small lake known as a tarn. In the british isles corries are mainly found on the north, northeast and east facing slopes where reduced insolation allows more accumulation of snow.

When two corries develop in highland regions they will produce other erosional features. When corries lie back to back or alongside each other, enlargement will often leave a narrow, steepsided ridge between the two hollows known as an arete. An example of this is Striding Edge on Helvellyn in the Lake District.

If two or more corries develop on a mountain, the central mass will survive as a pyramidal peak, which has a sharp appearence due to frost shattering. An example of this is the Matterhorn in the Alps.

It is a result of several interacting process; the original process believed to be nivation, which acts upon a shallow, periglacial hollow and enlarges it into an embryo corrie. As the hollow grows, the snow becomes thicker (due to accumulation) and is compressed to form a firn and then ice.

The back wall becomes steeper through the action of plucking. The rotational movement of the ice together with the debris supplied by plucking and frost shattering on the back wall, abrades the floor of the hollow which over-deepens the corrie.

As the hollow deepns, the thinner ice at its edge does not produce the same amount of downcutting and a rock lip develops on the threshold of the feature. Some thresholds have there height increased by morainc deposits formed when the glaciers snout was in that position. After the last ice has melted the corrie fills with meltwater and rainwater to form a small lake known as a tarn.

Glaciers flow down pre-existing valleys as they move from upland areas. Thet striaghten, widen and deepen these valleys, changing the original V-shape into a U-shaped valley, typical of glacial erosion. The action of ice combined with huge amounts of meltwater and subglacial debris, has a far greater erosive power than that of water.

As both extending and compressing flow are present the amount of erosion varies down the valley. Where compressing flow is present the glacier will over-deepen parts of the valley floor, leading to the formation of rock basins. It is also suggested that over-deepening is caused by increased erosion at the confluence of glaciers, areas of weaker rock or zones of well jointed rocks.

The main features of glacial throughs are:

• usually fairly straight and wide base with steeps sides - U-shape
• stepped long profile with alternating steps and rock basins
• some glacial valleys end abruptly at thier heads in a steep wall known as a trough end, which lie a number of corries.
• rock basins filled with ribbon lakes i.e. Wast Water in the Lake District

The main features of glacial throughs continued are:

• over deepening below the present sea level- this lead to the formation of fjords when sea levels rose after the ice ages and submerged the lower parts of the glacial valleys, for example on the coasts of Norway and southwest New Zealand.
• hanging valleys on the sides of the main valley. These are either pre-existing tributary river valleys which where not glaciated, or tributary glacial valleys. In tributary glacial valleys there would have been less ice and therefore less erosion than in the main valley. The tributary valley floor was therefore left higher than that of the main valley when the ice retreated.
• areas of land projecting from the river-valley side (spurs) have been removed by the glacier producing truncated spurs.
• small areas of rock on the valley floor are not always completely removed and this leaves roches moutonnees. These have an upsteam side polished by abrasion and downstream side made jaggered by plucking.
• after ice retreat, many glacial troughs were filled with shallow lakes which were later infilled and thier sides were modified by frost shattering and the development of screes which altered the glacial U-shape i.e. Great Langdale, Lake District.