Glacial Transport
Glaciers are capable of transporting very large volumes of material over very large distances. The rate of transport will be dependent on both the supply of material and the velocity of the glacier. The rock material carried by a glacier is known as moraine. As well as material added to the glacier as it erodes the bedrock at its base and sides, material may come from a variety of sources, these include amongst others:
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rockfalls from weathering of the surrounding slopes; |
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wind-blown materials, including dust and volcanic ash |
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avalanches |
The material carried by a glacier can be classified as either supraglacial, englacial or subglacial moraine.
1. Supraglacial - this is material carried on top of the ice e.g. that falling on to the ice from weathering of surrounding slopes or that is wind blown. If this material is covered by further accumulation of snow or falls down crevasses in the surface of the ice, it may eventually become englacial.
2. Englacial - this is material carried within the glacier itself
3. Subglacial - this is material carried along in the base of the glacier. Much of this is likely to have been derived from glacial erosion, however some may have been englacial material that has gradually worked its way down through the ice. Surface meltwater streams that flow down crevasses may also provide material which becomes subglacial. Where compressional flow occurs, this material may however be thrust upwards to become englacial or even supraglacial.
Linear accumulations of moraine along the edges of the glacier surface are known as linear moraine whereas as an accumulation of material towards the centre of the glacier surface, away from the edges is known as medial moraine (see diagram below). Medial moraines form at the confluence of two glaciers where a tributary glacier joins up with the main glacier.
Direction and extent of glacial transport.
Material left behind when glaciers retreat helps to give an indication of where the glacier has travelled from as well as the method by which the material was transported. Glacial erratics can give a good indication of the capacity of a glacier to transport large blocks of material over long distances. An erratic is a rock that is found on rocks of different lithology having been transported glacially and stranded when the glacier retreated. These may vary in size from small rocks to large boulders. Many of these rocks can be traced back to their source giving an indication of the extent of the glacial transport. Unique granite blocks originating from the island of Ailsa Craig (Firth of Clyde) have been found on the South-West Lancashire Plain indicating a movement of 240km (Clowes and Comfort, 1991).