Glacial Processes

Processes of glacial erosion

There are two main glacial erosion processes, namely abrasion, and the other plucking. This usually happens when the glacier is moving and picking up material. Abrasion is when the glacier moves downhill, and the frozen rocks into the bottom and the sides of the glacier scrape the rock beneath. The glacier becomes like sandpaper as it scratches the bedrock and leaves marks called striations behind. Plucking is when the glacier moves along the valley and melts temporarily around the bigger boulders it encounters. It then refreezes around them, so when it moves, the boulders are ripped out of the ground and become part of the glacier.

Corries are created as a result of upland glacial erosion. It is an armchair-shaped hollowed area on the side of the mountain that is formed from snow accumulating and becoming compressed ice which then starts to flow from the area and pluck the rocks. As the glacier continues to move, abrasion causes the hollow to become deeper, wider, and steeper. When the glacier gets heavier and moves out of the hollow with a circular movement called the rotational slip.

There are two erosional features produced by corries: arête, a narrow ridge of land between two corries eroding back to back, and pyramidal peak, which is when three or more corries erode back towards one another at the top of a mountain and leave a pointed peak.

Another feature of erosion is the glacial trough. They are created through glaciers flowing through an old river valley which erodes interlocking spurs to create truncated spurs.

The glacier erodes the V-shaped valley to steepen the sides and flatten the bases to become a U-shaped valley.

Processes of glacial deposition

The process of glacial deposition is when the glacier starts to melt as it travels lower down the mountain, reaches warmer temperatures, and deposits the material it carries. When the glacier starts to melt, the water leaving the glacier deposits sand and gravel, known as glacial outwash. The material is sorted into larger, heavier sand and gravel deposited in the glaciated valley. The lighter, smaller particles, such as clay, are deposited further away in the outwash plain. The mixture of transported sediments in the glacier is called glacial till or boulder clay. The piles of glacial till deposited by the glaciers can form a moraine.

There are three types of moraine, lateral moraine, medial moraine and terminal moraine.

From the type of moraine, it is possible to see how the glacier transported the sediments. Lateral moraine is when the material is deposited along both sides of the glacier, it is made up of material scraped off by the glacier as it moves along its path. A medial moraine is the lateral moraine of two glaciers when they meet, creating a line of moraine in the middle of a new, bigger glacier. The terminal moraine is the material deposited at the end of the glacier, the snout, and is also called the end moraine.

Fig. 5 -Lateral moraine on the left next to the Lower Theodul Glacier in Zermatt, Switzerland

Glacial weathering process

The main type of glacial weathering is freeze-thaw weathering. It happens during the day when the temperature is higher, the snow melts, and water goes into the cracks of the rocks. Then the temperature drops, causing the water in the cracks to expand and the rock to crack. As this process is repeated, the crack becomes bigger, and pieces of rock break off. It often shapes the back wall of a corrie, making it steeper.

Processes of glacial transportation

The process of glacial transportation is when the glacier travels from upland to low land areas and carries material as it moves. When the material is within the glacier, it's called englacial material; if the material is on the surface, it's called supraglacial material; if the material is carried underneath the surface, it's called subglacial material.

Movements such as bulldozing and rotational slips are part of glacial transportation.

Many processes are a part of glacial transportation. When freeze-thaw weathering happens at the glacier's edge, pieces of rock breaking off into bigger chunks of rock fall onto the glacier and get transported. Rocks are also transported when plucked out of the glacier and moving downhill with the ice.

Glacial processes examples

Looking at glacial upland landscapes in the UK now, we can see what kind of glacial processes had occurred to shape the land in such ways and how they are continuing to change to this day due to the current climate.

Lake District

The Lake District in the northwest of the UK has one of the highest mountains in the UK, Helvellyn. Many landscapes around the mountain were shaped by the last ice age. Large glaciers eroded the landscape to carve out arêtes, corries, and glacial troughs that can be seen today. Two arêtes, Striding Edge and Swirral Edge, ascend Helvellyn's summit. The Striding Edge is the back wall of the Red Tarn corrie on the eastern side of Helvellyn. Wastwater scree is an example of a landscape created by the weathering process of freeze-thaw weathering, and small pieces of rock are created from the process.

Fig. 7 - Wastwater scree

Snowdonia

Snowdonia, located in northwest Wales, has the highest mountain in Wales, Snowdon. Snowdonia has examples of glacial erosion, deposition, and transportation from 18,000 years ago in the ice age. There are distinctive glacial features such as corries with steep back walls formed through freeze-thaw weathering and plucking, aretes with steep sides and narrow edges, and a corrie lake. Cym Idwal and Cym Clyd are great examples of corries in Snowdonia. The landscape continues to be modified by weathering. In Cym Idwal, for example, freeze-thaw weathering continues to grind the rocks into smaller pieces and can cause landslips on the steeper slopes.

Bethesda area

Bethesda is a small village north of Snowdonia, famous for the Penrhyn slate quarry. In the post-glacial environment, the area was in shallow waters, and the slate was created from metamorphosed mud made from clay minerals. This happened from the heat and the pressure when Snowdonia underwent tectonic convergence and was squashed and then uplifted. The Penrhyn slate quarry opened in the 1780s and is one of the last remaining quarries, which once produced 100,000 tonnes but now employs less than 200 people. However, it has become a tourist attraction by focusing less on the slate and installing Europe's longest and fastest zipline.