What is Glacial Erosion?
Glaciers are huge sheets of solid ice and snow that cover a large area of land. Owing to the force applied by the weight of the ice, the glaciers move very slowly almost 2 cm per day. This movement of the chunks of packed ice causes erosion on the land underlying the glacier. This is known as glacial erosion. Simply put, glacial erosion is the curving and shaping of the land beneath a moving glacier.
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Two Main Processes of Glacial Erosion
There are two main processes of glacial erosion. The first one is plucking, which is defined as the erosion and transport of large chunks of rocks. As the glacier moves over the land, water melts below the glacier and seeps into the cracks within the underlying bedrock. This water freezes and melts weakening the bond holding the pieces of the rock in place.
The resulting pieces of rocks then become ready to get plucked from their rocky base and easily get carried alongside the moving glacier. Plucking removes rocks and by itself changes the landscape.
Plucking also tends to contribute to another important process of glacial erosion referred to as abrasion. Abrasion is defined as the erosion that occurs when particles scrape against each other. The massive volume of the glacier, together with plucked rocks and sediment clinged to its belly usually tend to scratch and carve the surface of the rock below.
It is almost as if the moving glacier is sanding the rocks with abrasive sandpaper. As the glacier sands the rock, it leaves behind long scratches that form in the direction of the glacial movement called glacial striations. The occurrence of these scratch marks is a positive indication that a glacier once covered the land.
Various Features of Glacial Erosion
Today glaciers only exist in the coldest parts of our planet. However, during the last ice age, glaciers covered a significant portion of the earth. When the ice gets melted, they leave behind some distinct landforms or features of glacial erosion. A few most striking features of glacial erosion are described hereunder.
A cirque, also known as a corrie, is a valley created because of glacial erosion. The shape of the valley is like an amphitheater and looks like a large cup from above. The highest points of the sides of the cliff-like slopes are called the headwall. The floor of the valley is like a bowl and is a place where large amounts of debris and rock particles created due to glacial erosion are deposited. The US national park of North Cascades has the Upper Thornton Lake Cirque.
If a series of cirques are arranged one above the other at different elevations, it is called a cirque stairway. Germany’s Black Forest has the best-known example of cirque stairway. It is the Zastler Loch which lies below the highest point of the Feldberg, the highest mountain of the forest.
Glacial erosion transforms the narrow and V-shaped valleys created by streams into U-shaped valleys by widening the sides and deepening the bottoms of the valleys. The small boulders that glaciers transport are fond deposited throughout the floor of the valley. The Himalaya and the Alps are full of such U-shaped valleys. The Nant Ffrancon Valley in Wales can be cited as an example of such a valley.
This is a narrow ridge between two valleys. Glacial movements often erode two parallel U-shaped valleys or two glacial cirques headwords resulting in a ridge in between them. This is known as an arête. In California’s Sierra Nevada Mountains, the Clouds Rest is an arête.
Also known as the sheep rock, a rôche Moutonnée is formed due to glacial movement on the bedrock. It creates a rounded knob shaped mountain with a gentle upstream slope which has a polished and striated surface curved out due to glacial scouring.
The downward slope is jagged, steep, and has an irregular surface. It is also an outcome of glacial scouring. The ridge between these two slopes remains perpendicular to the direction of glacial movement.
The scaping of the sand grains and rock particles on the bedrock when the glacier moves leaves stains or striations in the form of gouges and scratches. After the glacier has receded, these marks on the bedrock become visible.
In the Kelleys Island in Ohio, one can see the glacial grooves on the Columbus Limestone. These grooves are as long as 400 ft, as wide as 10 ft and as deep as 35 ft.
Glacial Horn/Pyramidal Peak/Nunatak
When more than one glacier divulge from a common point by eroding cirques, a sharply pointed and angular ridge appears between the cirques. This is known as the glacial horn. An extreme form of a glacial horn is called a pyramidal peak.
As a result of glacial activities, a rocky outcrop becomes apparent in the ice field or a jagged and angular structure made of rock and surrounded by glaciers becomes visible. This is called a nunatak. Since this landform has no flat surface, ice does not accumulate on it, which makes them easily identifiable.
A good example of the pyramidal peak is Mount Matterhorn in Zermatt, and Queen Louise Land of Greenland is an example of densely clustered nunataks.
When a glacier moves over a terrain, it leaves debris and sands on the bedrock, which makes the land less fertile or poorly vegetated. The land opposite this terrain, however, becomes more vegetated, and a line appears between these two landforms as a distinguishing mark. This is known as the trim line. One side of the line has more vegetation than the other side of it, and thus, the line is often very distinct and recognizable.
Massive glaciers create U-shaped valley as has been discussed previously. Such valleys have a broad, flattened bottom and steep sides. However, glaciers that are tributary to the main glacier most of the time carve out V-shaped valleys, which look similar to valleys created by rivers. When the main and the tributary glaciers flow at the same level, the shallower valleys created by the tributary glacier hang above the valley carved out by the main glacier.
This feature is known as hanging valley. Rivers often drop down the edges of hanging valleys to create waterfalls. Yosemite National Park has a waterfall named the Bridal Veil Falls that flows down from a hanging valley.
The word D-Fjord comes from the Norwegian word Norse fjoror, meaning “where one fares through.” It is an elongated, deep and narrow sea or lake drain amid steep land. Glaciers that are up to 3 km thick curve out fjords usually. Through several ice ages, the glaciers formed this landscape. Gravel and sand deposits from the glaciers made certain areas of the fjord shallower.
Fjords are often natural harbours. The countries where fjords are mostly found are Norway, New Zealand, Canada, Chile, Greenland, and the U.S. state of Alaska. There are many famous fjords of which The Naeroyfjord in Norway is the narrowest fjords with merely 250 meters width. It is on the UNESCO World Heritage list. The Geirangerfjord is the home of the Seven Sisters waterfalls. Some of the unique features of fjords are the presence of coral reefs and rocky islands called skerries.