Glacial deposition introduction
Ice behaves in the same way as all the other agents of erosion:
- it wears away the land surface- erosion
- it carries away the materials eroded- transportation
- It dumps elsewhere the materials it is carrying- deposition
Valley glaciers erode with so much power, and ice sheets erode such great expanses of land, that large amounts of loose rock are available for transport. Glaciers can transport enormous loads.
Erratic, is the name given to a boulder dropped by ice in an area where it does not belong.
All materials transported by glaciers are called moraine. Although most are carried in the glacier's base, some are carried on the surface, which show up as dark lines of moraine on the top of the glacier. Piles of material along the sides are called lateral moraines; those somewhere in the middle of the glacier, formed after valley glaciers join together, are called medial moraines. Two separate lateral moraines unite to form one medial moraine. The material for these moraines is broken off from the rocky peaks above by freeze thaw weathering and it falls down the valley sides on to the top of the ice.
Glacial deposition introduction CONTINUED...
However, not even a glacier can keep on growing for ever. It reaches a point where the ice loss is greater than the amount of new ice supplied. For example, most valley glaciers begin to melt when they reach lower ground where temperatures are higher. Only a few reach the sea before they have completely melted. As the ice melts and thins, its carrying capacity is reduced. When the glacier reaches the point of overload (load greater than carrying capacity), it must deposit some of its load. Any obstacle along its course encourages deposition.
The general name given to all materials deposited by ice is boulder clay. As its name suggests, this is usually clay which contains numerous boulders of many different sizes. It is an unsorted deposit. This means that large and soft rocks, as well as finer particles, are all mixed together. The boulders it contains are described as angular. They have sharp edges, not yet rounded off, as they would have been if they had been transported by rivers. The 'ingredients' of the boulder clay vary greatly according to what the glacier eroded before it reached the area. Sometimes the deposits are more sandy than clayey, which is why some physical geographers prefer to use the term glacial till instead of boulder clay to describe all ice-deposited materials. As the glacier continues to push forward, melting more and more all the time, it leaves a trail of boulder clay behind it which forms a hummocky surface of ground moraine.
In many of the lowland areas of south-west Scotland and north-west England, glacial deposition has produced a distinctive landscape of many low hills, each one typically about 30-40 metres high and 300-400 metres long. These hills all lie in the same direction and have similar shapes- blunt at one end and tapered at the other; in fact, each hill looks like an egg. Each of these hills is a drumlin. Drumlins occur in swarms and are said to for 'basket of eggs' topography, so called because of the appearane of the landscape.
Drumlins form when the ice is pushing forward across a lowland area, but it is overloaded and melting. It does not need much to encourage more deposition; any small obstacle, such as a rock outcrop or mound, is sufficient. Most deposition occurs around the upstream end of the obstacle, which forms the drumlin's blunt end. The rest of the boulder clay that is deposited is then moulded into shape around the obstacle by the moving ice to form the tapered end downstream. The drumlin is another landform from which it is possible to detect the direction of ice movement.
All the remaining load is dropped and dumped at the glacier's snout- the furthest point reached by the ice. This point is marked by a ridge of boulder clay across the valley or lowlands, running parallel to the ice front, and is called a terminal moraine.
Where ice sheets remained stationary for a long time, such as in central Europe during the main ice advances in the Ice Age, sufficient boulder clay was deposited to form ridges more than 200 metres high.
More typically, terminal moraines formed by valley glaciers are between 20 and 40 metres high.
Terminal moraines which cross valleys from natural dams behind which river water can pile up and form lakes. These lakes are also long and thin and are called ribbon lakes.
This tells you that landforms with the same appearance can have different methods of formation.