Sunday, 5 June 2011

Glaciers Part 2 - Processes and Landforms linked with Valley Glaciers

Lauterbrunnen valley in Switzerland

A valley glacier is a glacier which extends from a larger body of ice, most commonly a corrie, and moves downslope taking the path of the existing river channel but, over time, the glacier cuts its own wider, deeper and straighter trough.

Valley glaciers are powerful agents of erosion and are directly responsible for the deepening of pre-existing river valleys and so, thereby, they contributed greatly to the spectacular scenery found in many mountainous areas. Due to their erosive ability they have exaggerated, in what can be described as quite a dramatic manner, the vertical difference between the frost-shattered peaks and the valley floors that lay below. This can be clearly seen in mountainous regions in temperate latitiudes as large areas are clear of ice and the landforms have been created relatively recently so  they have not been obliterated by further denudation (stripping of surface cover - applies to both vegetation and soils). The Lauterbrunnen valley in central Switzerland is a classic example. The pre-glacial river valley has been greatly straightened, widened and deepened by the presence of ice and its erosive ability. The resulting glacial trough has a deep U shaped cross-section and the present river is lost within the cavernous spaces of the glacier-carved valley.  The former interlocking spurs have been planed off to give the valley a straight plan with their remnants left behind in the form of high points on the valley sides which are known as truncated spurs. Streams are left hanging from the top of the glacial trough  and cascade down as waterfalls to the valley floor far below.
This image shows not only a truncated spur and how flat the valley floor is but it
also demonstrates the common U-shaped cross section
Although a smooth valley cross profile is created, the dominant feature of the long profile of a glaciated valley is, often, its irregularity. The sudden drop that occurs at the head of the valley is known as the trough end. When the glacier was present, it would have exploited weaknesses within rocks, mainly joints, and variations in resistance between rock strata's as it moved down the valley. The more resistant rock outcrops form into steps or rock bars, whereas less resistant rock erodes into rock basins. Water on the land later fills up the basins to form ribbon lakes, some of which are of great depth and considerable length.

 Exposed bedrock surfaces are normally scarred with fine grooves or more deeply scratched by striations on their upstream sides and quarried by plucking on their downstream sides - making them more jagged. Rock that has eroded is transported, only to be deposited once the carrying capacity of the glacier is reduced due to melting. A ridge deposited across the valley floor at the maximum point reached by the ice forms a terminal moraine. This can be the first in a series of ridges, or recessional moraines with each one representing a point in the valley where the glacier remained stationary for a period of time. The floor of the glacial trough is, often, covered with glacial debris which can be classed as ground moraine.

A moraine is simply a pile of deposits and these depostional landforms can be classified into one of 4 groups depending on their location.
  • Terminal moraines are found at the furthest point reached by the glacier (known as the terminus end)
  • Lateral moraines are found along either side of the glacier
  • Medial moraines occur at the meeting point of two glaciers
  • Ground moraines are disorganised piles  of rocks of various sizes, shapes and rock types
Where ice is still moving freely, some of the mounds of boulder clay are shaped into egg-shaped hills called drumlins. Drumlins are elongated hills of glacial deposits that can be anything up to 1km long and 500m wide. Drumlins often occur in groups known as drumlin swarms (or a basket of eggs like found in the Vale of Eden). They form when the glacier is carrying too much sediement and so deposits it meaning the the debris that the drumlins is comprised of is the sediment which accumulated under the ancient glacier or is accumulating under glaciers at present. This also means that the long axis of the drumlin will indicate the direction the glacier is/was moving in. 

Drumlins in the Vale of Eden, Lake District

A diagramatical plan view of a drumlin
However, in most places the hummocks and mounds are irregular heaps of glacial till and so, in general, these landforms of deposition are more minor landscape features in comparison to those created by glacial erosion.

So, hopefully, this outlines some of the main features found in valley glaciers and the impacts of erosion and deposition and so next I think I will move on to discuss just why valley glaciers have such a great erosive ability with reference to the types of erosion that actually take place.

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