‘The characteristics of glacial till reflect the erosional, transportational 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 it’s 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, supraglacial 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 clast with edges and corners blunted by abrasion.
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(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 subglacially when debris is released directly from the sole of the ice. Ablation till accumulates initially in a supraglacial position and is later lowered to the ground surface by undermelting. Ablation till can be further subdivided into meltout till and flow till. Meltout 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 supraglacial debris mainly comes from the glacier destabilising 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.
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(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 subglacially deformed masses of pre- existing sediment to which more sediment may be added by the melting out of debris form the glacier base” (Evasn D. 1999).
Below is a picture of some drumlins form the Kejimkujik Lake.
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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.
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The poorly sorted, clast-rounded lodgement till and well sorted, coarser, angular ablation till represent the basal and overlaying supraglacial 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 dilatancy 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).
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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 main types of moraine that can be recognised, lateral moraine, medial moraine, terminal or end moraine, recessional moraines and push moraines. Lateral moraine forms along the edges of the glacier. It is made up of material from the valley walls which is broken up by frost shattering and falls onto the ice surface. It is then carried along the sides of the glacier. When the ice melts it forms a ridge or embankment of material along the valley side. This can clearly be seen in figure 15.1, where in the photograph above, the lateral moraine ridge marks the edge of a past glacier of much greater proportions than the one existing today. The present glacier can be seen top left of the image, and is clearly much reduced in size.
Figure 15.1 )
Medial moraine is found in the centre of a valley. When two glaciers merge, the two edges that meet form the centre line of the new glacier. In consequence two lateral moraines find themselves in the middle of the glacier forming a line of material on the glacier surface. The existence of a medial moraine is evidence that the glacier has more than one source of amterial. When the ice melts it forms a ridge of material along the valley centre, which is clearly seen in figure 15.2. The medial moraine is clearly visible as a dark line along the centre of the glacier in figure 15.2.
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Figure 15.2 )
Recessional moraines form at the end of the glacier so they are found across the valley, not along it. They mark interruptions in the retreat of the ice when the glacier or ice sheet has remained stationary for sufficient time to produce a mound of material, which can clearly be seen in figure 15.3. This is a picture of a recessional moraine in the French Alps. The process for the formation of recessional moraines is the same as for a terminal moraine, but they occur where the retreating ice paused rather than at the furthest extent of the ice. Recessional marines are usually parallel to the terminal moraine.
Figure 15.3 )
Push moraines may develop if the climate becomes poorer after a relatively warm period sufficiently for the ice temporarily to advance again. Then the previously deposited moraine may be shunted up into a mound. This can be easily recognised by the individual stones which have been pushed upwards form their original horizontal positions.
The terminal moraine forms at the snout of the glacier. It marks the furthest extent of the ice, and forms across the valley floor. The terminal moraine resembles a large mound of debris or series of mounds extending across a valley or low land area at right angles to the glacier or ice sheet. It is usually the feature that marks the end of unsorted deposits and the start of fluvially sorted material.
Generally terminal and recessional moraines are steeper on their up slope, ice contact side because that is the side into which the glacier has been pushing. One of the best known examples of this in England is the Cromer Ridge in Norfolk. At Cromer the ridge is 90 m high and 8 km long. Congenital moraines can be a great deal larger and a good example of this is the Bloomington Moraine in Illinois, which is more than 300 km long.
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There are several other landforms that originate from glacial action. Two landforms called kames and eskers fall into this category. These are the product of glacial rivers, see figure 15.13. Kames are features that are produced at the margins of the ice and eskers are primarily forms developed beneath the ice. Kames are made up of irregular undulating mounds of bedded sands and gravels that are a group of alluvial cones or deltas deposited unevenly along the front of a stagnant or gradually decaying ice sheet. Some kames occur as kame terraces because they were formed along the trough between the glacier and the valley side, forming narrow flat popped terrace like ridges. Moulin kames are formed in hollows and perforations in decaying ice, so if they develop along major crevasses they may have an angular dog-leg form.
Figure 15.13
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Eskers are elongated ridges of stratified coarse sands and gravels. It is thought that eskers are fossilised courses of subglacial melt water streams. The river channel is restricted by walls of ice, so there is a lot of transported load and hydrostatic pressure in the same area. Then the bed of the channel builds up because the water has nowhere else to go, therefore material is left above the surrounding land after the retreat of the ice. Eskers in a similar way to kames will form during periods of deglaciation.
Kettles and kettle holes are another glacifluvial landform that can be seen in figure 15.13. These features form from detached blocks of ice left behind as the glacier retreats. These blocks of ice are then partially buried by the glacifluvial deposits left by melt water streams. Then when the ice blocks melt away they leave enclosed depressions in the ground which will then usually fill with water to form kettle-hole lakes. The topography of the land left behind after the glacier has retreated is often called ‘kame and kettle’ topography,
The glacial system resembles the fluvial system, but the forms adopted by the glacial ice flowing across the land surface and its erosive power, lead to a variety of very distinctive landforms.
Larger particle till deposits contain individual stones that are sub-angular, which means that that the particles are not are not rounded like river or beach material and they do not possess sharp edges. The composition of till therefore reflects the character of the rocks over which it has passed in its history. A good example of this is in East Anglia because it is covered by a chalky till due to the glacier passing over a chalk escarpment like the East Anglian Heights. Stones and pebbles that have been carried by the glacier generally become aligned with their long axis parallel to the direction of the ice flow because this offers the least resistance to the ice.
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Study of the depositional elements in the till can provide evidence of changes in climate where ice has been exposed for a period of time, or when additional ice formation has occurred.
In final conclusion, it is true to say that consideration of glacial till characteristics do reflect the erosion forces which have been applied over time, the effects of transportation in the ice flow and when and where deposition has taken place.
References
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• Cook I. Et. Al., 2000 Geography In Focus, First Edition, Causeway Press, Lancaster.
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• Waugh D. 2000, Geography An Integrated Approach, Third Edition, Nelson, Surry.
• Strahler A. H. & Strahler A. N. 1976, Environmental Geoscience: Interaction Between Natural Systems And Man, Wiley International Edition, University Press, Oxford.
