Glacial transportation & deposition
Material that is transported on the surface of the glacier is known as supraglacial debris and the material buried within the glacier is known as englacial debris. Material found at the base of the glacier is known as subglacial and may include rock fragments that have fallen down crevasses and material eroded at the base.
Material in the centre of the glacier is known as medial moraine, material at the side of the glacier is known as lateral moraine and material at the bottom of the glacier is called ground moraine.
The huge amounts of material carried down by a glacier will eventually be deposited. The bulk of this will be the debris released by the melting of the ice at the snout. It is also possible for the ice to become overloaded with material reducing its capacity. This may occur near to the snout, as the glacier melts or in areas where the glacier changes between compressing and extending flow. Material that is deposited directly by the ice is know as till or boulder clay although the latter term is barely used today.
Till is used to describe an unsorted mixture of rocks, clay and sand that was mainly transported as supraglacial or englacial material and deposited when the ice melted. The individual stones tend to be angular to sub-angluar in comparison to beach and river material which is rounded. Till reflects the character of the rocks over which the ice has passed. It not only indicates the passage of ice but the fact that the sea level must have been considerably lower to allow ice to move over the area that later became the North Sea.
Sometimes it is possible to find a large block of rockthat has been moved from one area and deposited in another which has a very different geology, this type of feature is known as a erratic.
Two types of glacial deposits are recognised:
- lodgement till- subglacial material that was deposited by the actively moving glacier. A drumlin is a typical feature formed from this material.
- ablation till- produced at the snout when the ice melts. Terminal (end) , push and recessional moraines are typical features produced by ablation till.
The main features of Drumlins are:
- they are smooth, oval-shaped small hills often resembling the top of an egg.
- they can be as long as 1.5km, and upto 50-60km in height.
- they have a steep end known as a stoss and a gently sloping side known as the lee.
- they are elongated in the direction of the ice advance with the stoss at the upstream end and the lee at the downsteam end.
- they are often found in groups known as swarms and given thier shape this is sometines referred to as a 'basket of eggs topography'.
- they are formed from unsorted till.
- they are found on low land plains such as the central lowlands of Scotland. Many are found at the lower end of a glacier valley.
There is much controversy about the origin of drumlins.
Moraines are lines or a series of mounds of material, mainly running across glacial valleys. The main type is the terminal and end moraine which is found at the snout of the glacier. Terminal moraines show the following features:
- they consist of a ridge of material streching across a glacial valley.
- they are elongated at right angles to the direction of the ice advance.
- they are often steep sided particulary the ice contact side, and reach heights of 50-60km.
- they are often cresent shaped, moulded to the form of the snout.
- they are formed from unsorted ablation material.
As the glacier retreats, it is possible for a series of moraines to be formed along the valley, marking points where the retreat halted for some time. These are known as recessional moraines.
If the climate cools for sometime, leading to a glacial advance, a prevoiusly deposited moraine may be shunted up into a mould known as push moraine. Such features are recognised by the orientation of individual pieces of rock which may have been pushed upwards from their original position.