Examine how landscape evidence can be used to identify the direction and extent of ice movement
There are many different types of landscape evidence that can be used to identify the direction and extent of ice movement over a region. The main focus of this essay is to cover all of these aspects and make an assumption as to how useful they are as evidence. The landscape features that I will be identifying will be the result Glacial Deposition. There are many useful sources of information that I will be able to use in supporting my account.
Glacial deposition is a main source of evidence as to the direction of ice movement. Drift is a term used to refer collectively to all glacial deposits. The material that has been transported englacially by the movement of ice will at one stage have been deposited. These deposits include boulders, clay, gravels and sands which may be subdivided into Till. Till includes all the material deposited directly by the ice and glacifluvial material. Glacifluvial material includes deposits which may have been deposited initially by the ice and then later picked up and redeposited by meltwater. This deposition occurs in lowland areas and in upland valleys. A study of drift deposits helps to explain the nature and extent of the ice advance, the frequency of the ice advances, sources and directions of ice movement and postglacial chronology. This is why drift is such a vital piece of evidence used to explain ice movement as can be clearly identified on the landscape.
Till often refers to all materials deposited by the ice but it is more accurately used to mean an unsorted mixture of rocks clays and sands. The composition of till reflects the character of the rocks over which it has passed; East Anglia, for example is covered by chalky till because the ice passed over a chalk escarpement. Most of the material transported sub-glacially by the till is deposited as ground moraine. The East Anglian Till Sheet completely masks the underlying sedimentary bedrock. The average thickness of the till is about 30m, though in the north it exceeds 70m. it comprises a variety of different till deposited by separate advances during each of the major glaciations. A threefold sequence of drift deposits associated with the Anglian Glaciation has been located in a cliff section at Corton in north-east Suffolk. This information is an excellent account as to the appearance of erratics form northern Britain and Scandinavia which indicates an early advance of ice form the north. The upper layer of bluish-grey, chalky Lowestoft Till suggests that the parent ice mass carried these deposits from the chalk scarplands to the west.
Another type of till can also be included to account for the direction and extent of ice movement. Ablation till refers to material carried anglacially or supraglacially that has been deposited when the glacier melted. The orientation of the long axis of the till lines up with the direction of ice flow which produces a distinct line that can visually explain where the ice has come from. Initially the stones will be deposited horizontally but if there is a subsequent glacier re-advance the stone will be pushed forward from behind tilting them upwards.
Till fabric analysis is another source of information which explains how the orientation of deposited material attempts suggests where the ice sheets originated from. In Glen Rossa on the Isle of Arran, a sample of 50 stones was taken from the till deposits. Their orientation indicated that the direction of the glacier flow NNW/SSE. The stones had been pushed upwards to an angle of 25*, indicating a glacier re-advance after the initial deposition. Most of the stones matched up with the lithology on the mainland of Scotland.
This evidence will be more apparent in areas where the mass of the ice was thicker as more material could be transported either englacially or supraglacially which will have had a greater effect on the shaping of the landscape. This is because areas with a denser proportion of ice submerging the landscape will have greater downward forces acting on the land. The landscape will then be subject to various stresses causing deformation resulting in the creation of features such as drumlins. Furthermore, the material transported will be larger so these deposits will be more clearly identified.
Some of the landform characteristics of glacial deposition include erratics that are boulders picked up and carried by the ice, often for many kilometres to be deposited in areas of completely different lithology. By determining where the boulders came from, it is possible to track ice movements and also the extent of their movements. For example, volcanic material form Ailsa Craig in the Firth of the Clyde has been found 250kms to the south of the Lancashire plain, while some deposits on the north Norfolk coast originated in southern Norway. This is a very good piece of evidence as it is the only explanation that can be used to elucidate how large boulders of solid material can be transported without the aid of any human intervention for hundreds of kilometres. This is also a good explanation as to the spatial potential the ice had. By recording the lithology of an area of land and then measuring how far material with the same lithology has been able to travel gives an indication as to the extent of the advance of the ice.
The final piece of evidence that can be used to justify ice movement over prolonged periods and distances are Moraines. A moraine is a type of landform that develops when the debris carried by a glacier is deposited. It is not therefore the material that is being transported by the glacier. The only exception to this is medial moraine which is a term which refers to a landform both on the glacier and in the valley after glacial recession. To mark the extent of the glacier movement, terminal or end moraine is a source that can be used to mark the maximum advance of the glacier ice sheet. This is a high mound of material which extends across a valley, or lowland area at right angles. A series of terminal moraines stretches for hundreds of kilometres across the North German Lowlands. Six different groups have been identified, the most predominant being the Pomerian, Frankfurt-Oder and Brandenburg. Glen Rossa in Scotland is a more local example of a terminal moraine that extends most of the way across the floor of the valley. It marks the limit of the Loch Lomand advance. Lateral moraine can help to identify how debris derived from frost shattering of the valley sides can be carried along side the glacier. When the glacier melts, however, it leaves an embankment of material along the valley side. Other moraine deposits, such as medial, recessional and push moraines are also good sources of supporting evidence for glacier movement.
From the analysis of till to the existence of moraines, it is clear that the landscape is an excellent source of evidence for identifying the direction and extent of ice movement. The unusual appearance of erratics on the Yorkshire Dales are great historical landmarks that provide geologists with resources that tell more about the history of the area than any textbook and the distance that they cover can be used to mark the extent of the ice movement. These and many more landscape features are windows through which the past can be viewed past and will continue to be a great resource for researchers in the future.
