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The characteristics of glacial till reflect the erosional, transportation and depositional history of the constituent sediment. Glacial till is a heterogeneous mixture of rock fragments ranging in size from clay to boulders and is deposited directly from glacial ice without water transport. (Strahler A. H. & Strahler A. N. 1976). Ice sheets deposited in the Pleistocene period may be more than 30 m deep and inevitably the constituents of glacial till will reflect the retreats, advances and stationary phases of the ice which have taken place during the history of a glacier since its formation.
I shall outline the various forms of glacial till, giving consideration to the type of prevailing climatic conditions which give rise to their formation and deposition to illustrate how the constituent sediment does provide evidence for glacial activity over time. As glaciers are so effective at erosion and transport, large quantities of debris is also associated with them. According to its location with respect to the glacier, such debris transported as ice mass may be divided into three main categories. There are three main positions that a glacier can transport debris, englacial debris which occurs within the glacier, supra glacial debris which occurs on the glacier surface and subglacial debris which occurs on the base of the glacier, (please look at figure 15. 12 below).
While debris is being transported it may remain in any one of these positions until it is deposited by the ice directly or it may end up being reworked by melt water. Deposition of the transported material is a complex process, but the fraction deposited directly from the ice is called till. It consists of a wide range of grain sizes, so it is often referred to as boulder clay. It also posses very little stratification and frequently contains far travelled erratic material, which tends to have class with edges and corners blunted by abrasion. (Briggs D. Et. Al. 1997) There are two main different types of till which have been recognised, lodgement till and ablation till.
Lodgement till is laid down sub glacially when debris is released directly from the sole of the ice. Ablation till accumulates initially in a supra glacial position and is later lowered to the ground surface by under melting. Ablation till can be further subdivided into melt out till and flow till. Melt out till is a direct product of ablation continuing beneath a cover of detritus and flow till consists of debris that has built up on ice and after saturation with melt water becomes so unstable that it flows or slumps into near by hollows. There are various ways that subglacial debris may build up, one way is due to alternating bands of dirty and cleaner ice causing material to become attached to the ice differently and material can also become lodged into openings or crevasses within the ground. Due to this action material is then in an englacial position where few glacial actions take place because it will not be moving in and out of basal ice.
The source of supra glacial debris mainly comes from the glacier de stabilising adjacent rock walls, so material will fall on to glacier surface. The surface of a glacier will also receive air borne material supplied by various sources, which will become lodged to the surface of the glacier due to its mottled surface. Debris which will later form till will enter a glacier by crevasses, entrained in melt water and by pressure causing mass absorption. In areas particularly where a glacier opens out in to a plan, the till will be deposited into swarms of rounded hummocks called drumlins.
Sometimes these drumlins will occur in a regular pattern, so a term known as the basket of eggs relief is used to a glacial landscape of this type. Drumlins are streamlined hills which may vary in size from only a few meters in height to over 50 m. They are commonly between 1 and 2 km in length and about 0. 5 km wide. Generally drumlins lie with their long axes parallel to the inferred direction of ice movement and they have an approximately ellipsoid plan this can clearly bee seen in the figure 14 below. (Figure 14, Waugh D. 2000) There are several views on how drumlins are formed, but the most widely accepted view is that they have been formed when the ice became overloaded with material, so that the capacity of the glacier was reduced. The competence of the glacier may have been reduced due to changes in the velocity related to a pattern of extending-compressing flow or a change in the velocity of the glacier. Once that material has been deposited it may have been moulded into a streamlined shape due to a later ice movement.
The most recent theory was made in 1999 and it is based on evidence that drumlins can be composed of both till and glacifluvial sediments. The view is they are sub glacially deformed masses of pre- existing sediment to which more sediment may be added by the melting out of debris form the glacier base (Even D. 1999). Below is a picture of some drumlins form the Kejimkujik Lake. (web), The above picture of Drumlins is the hallmark of Kejimkujik National Park is of numerous streamlined elliptical hills called drumlins. The low tapering tail points in the direction of ice flow; their trend records the main southeast course of the ice sheet across Nova Scotia. Carved by strong glacial action from a thick blanket of drift, the drumlins form islands and peninsulas in many lakes. This line of drumlins is thought to have been lodged on a bedrock ridge. (web) The poorly sorted, class-rounded lodgement till and well sorted, coarser, angular ablation till represent the basal and overlaying supra glacial deposits of the same ice advance, which show a degree of bedding.
The lodgement of till occurs through the net debris release from moving basal ice. This will then form large till sheets quite often known as plains, where pressure- melting is spread over a large area or basal flow has diminished to an extent where basil sheer increases. The basal deformation may shift from the debris rock to the debris-ice boundary due to changes in geotechnical properties of the till, such as dilatant or an increase in the volume to void ratio. Both of these processes may cause a deformation within the till as upper layers conform to the basal ice. This will lead to large scale till block thrusting and streamlining. This mainly explains the mechanism for the formation of drumlins 101 - 2 m high and 102 - 3 m long, (Briggs D. 1997).
Closely related to drumlins are a whole series of other streamlined forms described under a broad heading of fluted ground moraine. Some of the largest flutes are up to 20 km long, 100 m wide and 25 m high. These are generally much smaller than this and they are usually found just beyond the glacial front. Moraine is a type of landform that develops when the debris carried by a glacier is deposited.
It is clear then that it is therefore not the actual material that is being transported by the glacier, with the exception of the medial moraine. Medial moraine is a name used to refer to a land form both on the glacier and in the valley after glacial recession. There are five...
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Research essay sample on Supra Glacial Debris Debris Which Occurs Till
