Erosional processes
Glaciers are responsible for sculpting and forming many features of the landscape that we see today, both as a result of their erosional powers and also from the deposition of materials that get entrained within them as they move. The main erosional processes at work in glacial environments are freeze thaw, plucking and abrasion.
Freeze thawThis is where the repeated cycle of freezing and melting of water within a crack in a rock's surface, enlarges it until it finally splits. This is because when water freezes its volume increases by around 9% meaning it forces the crack apart.
|
Plucking
This is where rock fragments weakened by cycles of freeze thaw get frozen to the underside of the glacier and 'plucked' from the bed before being transported away. Abrasion This is where rock fragments that have become embedded within the glacier rub along the bedrock, eroding it. When a glacier has retreated you often see scratches and gouges in the surface of rocks, which are called striations. These are caused by abrasion. |
Erosional landforms
These features are created by the erosional power of glaciers and although less apparent when covered in snow and ice, they are revealed once one has retreated, as shown in the illustrations below.
Arete
An arete is a thin, sharp edged ridge of rock that is formed when two adjacent glaciers (often corries) running back to back erode the rock between them until it becomes a steep and narrow ridge. Glacial troughs/U shaped valleys
As a glacier starts moving downslope it chooses the steepest path, which often follows the course of a V shaped river valley. As glaciers have more erosional power than rivers they are able to deepen and widen the valley giving it more of a U shape, with a flat bottom and steeper sides. A misfit stream is a stream that is out of proportion with the valley that it runs through. I.e. The stream at its current strength does not have the erosional power to have created the valley. A hanging valley is a valley left 'hanging' above the main glacial trough, as it is unable to match the rate of erosion of the main trough due to having less ice within it or a harder bed. You will often find waterfalls at these sites.(see fig. 5.) A truncated spur is a lateral ridge that runs down a hill or mountain which has been cut off by the flow of a glacier, ending the spur in an inverted V shape (see fig. 5). |
Corrie/cirque
A corrie is a deep, steep, three sided bowl that resembles an amphitheatre. They are often found at the start of a glacier. A corrie is formed when snow settles in a depression and conditions allow it to remain all year round. The following years snow falls on it, compacting it and eventually after this cycle continues for many years glacier ice is formed (see glaciers section). Once glacier ice has formed gravitational forces act upon it, but as it is trapped in a depression it is unable to move down hill and instead a circular motion called rotational slip is created. Through erosional processes like freeze thaw, plucking and abrasion the corrie becomes wider and deeper, although some ice is able to escape it and start moving downhill. A tarn is simply a circular lake that forms in a corrie after a glacier has retreated. Pyramidal peak/horn
Pyramidal peaks are found where three or more corrie glaciers run back to back and over the course of many years are able to erode a mountains summit into a sharp point. A prime example is the Matterhorn in the Swiss Alps. Ribbon lakes
Ribbon lakes are long thin lakes that form in areas of a valley floor that are made up of softer rock. These areas are more easily eroded than the surrounding harder rock, so a deeper depression is carved out. Once the glacier retreats these deeper troughs fill with water to and form lakes. |
Depositional Landforms
These are formed when material that gets entrained within the glacier is deposited. There are three different ways that material is entrained within a glacier:
Glacial deposits are called till or boulder clay and differ from river deposits in that they are unsorted and unstratified (not in layers). This means that material of all shapes and sizes, from large boulders to fine sediment, is deposited at the same time with no regularity. This unsorted quality means that glacial deposits are not bonded together very well so can be easily eroded. Material is also angular as once entrained in the glacier rock fragments have little chance to collide (attrition) and smooth off sharp edges as they would if transported by water.
These are formed when material that gets entrained within the glacier is deposited. There are three different ways that material is entrained within a glacier:
- Supraglacial is where material, mainly from rock falls resulting from weathering of the sides of the valley, is transported on top of a glacier.
- Englacial is where material is transported within the the ice of the glacier. This includes material that was originally supraglacial, but has since been covered by layers of snow and ice.
- Subglacial is where material is transported underneath the glacier. This material may have been plucked from the bedrock or have made its way down from the top of the glacier.
Glacial deposits are called till or boulder clay and differ from river deposits in that they are unsorted and unstratified (not in layers). This means that material of all shapes and sizes, from large boulders to fine sediment, is deposited at the same time with no regularity. This unsorted quality means that glacial deposits are not bonded together very well so can be easily eroded. Material is also angular as once entrained in the glacier rock fragments have little chance to collide (attrition) and smooth off sharp edges as they would if transported by water.
Morraines
As a glacier retreats material is deposited in the form of moraines at various points of its course. Lateral moraines form at the sides of a glacier and are often made up of material that has fallen onto the glacier as it erodes the sides of the valley. Medial moraines form when the lateral moraines of two glaciers merge. The existence of medial moraines show that a glacier has multiple sources. Ground moraines are material deposited across the floor of the glacier in a flat, non distinct form. Terminal moraines form at the snout of a glacier and mark the furthest point that ice has got to. Recessional moraines form at the snout of the glacier as it recedes back up the valley. Push moraines occur when a glacier that is in retreat starts to advance again, pushing forward material that has already been deposited. Supraglacial moraines (Material on the surface of the glacier) and englacial moraines (material frozen within the glacier) only exist when a glacier is present. |
Drumlins
Drumlins are a type of subglacial bedform; an elongated, streamlined hill that typically resembles an egg half buried along its length. They are usually formed from deposited glacial till, though the nature of this can vary greatly. They can measure up to a kilometre or so in length, 500 metres wide and 50m high. Drumlins are not normally found on their own, but usually as a field or swarm consisting of between 10 and 1000 individual drumlins. How they are formed is still open to debate with the only thing certain being that it is something to do with the flow of the glacier, as the lee slope of the drumlin always declines in the direction of ice flow. There are some suggestions that a hard rock outcrop towards the stoss end could lead to the deposition of material in this specific shape, particularly when the ice is under stress, such as when a surge is occurring or the glacier is overloaded with material. The pressure of the ice and its forward motion may alone be powerful enough to form these shapes when moving over a soft sedimentary bed. It is hard to know for certain how drumlins are formed due to their formation occurring at the bed of the glacier where access is almost impossible. |