Glacial Erosion Landforms

Distinctive glacial landforms result from different physical processes.

Key words

1. Arête - A sharp, knife-like ridge formed between two corries cutting back by processes of erosion and freeze thaw. Bulldozing Ice pushes material of all shapes and sizes as it moves slowly forward.
2. Corrie (Also called cirque) - Armchair-shaped hollow in the mountainside formed by glacial erosion, rotational slip and freeze-thaw weathering. This is where the valley glacier begins. When the ice melts, it can leave a small circular lake called a tarn.
3. Pyramidal peak  - Where several corries cut back to meet at a central point, the mountain takes the form of a steep pyramid.

The processes of glaciers have shaped many parts of the UK landscape, including Snowdonia in North Wales and the Lake District in Cumbria. Major features created by glacial erosion include corries, arêtes, pyramidal peaks, truncated spurs, glacial troughs, ribbon lakes and hanging valleys.

Corries
Corries are bowl shaped hollows with a steep back wall and hollow, forming an armchair shape. They form in hollows where snow can accumulate.  The snow compacts into ice and this accumulates over many years to compact into Névé then ice. This moves downhill because of gravity, the mass of the ice, water at its base and the slope it is on.  It will move in a rotational movement because of the slope and the overlying pressure.
The ice freezes to the back wall and when it moves it plucks rock out steepening the back wall.  Freeze Thaw and frost shatter above the hollow on exposed rocks shatters the rock and deliver shattered rock known as scree to the ice (both on top of the ice, within it and under it).
This material from plucking and frost shatter is then moved along under the ice abrading the hollow by scratching the surface rock. This creates a steep back wall and a hollow known as a corrie or cirque.   At the front edge of the corrie the ice thins out at is speeds up on its journey down valley, and this area is eroded less and crevasses form.  This leaves a lip of rock. When the ice melts a corrie lake can form.
In France and Switzerland Corries (the Scottish word) are known as Cirques (because of their near CIRcular shape), Coombe in England and in Wales it is Cwm. Grisedale Tarn in the Lake District is a great example of a Corrie lake.

Arêtes and pyramidal peaks

How Corries are created

Where 2 corries occur back to back, they can erode backwards through the processes outlined above.  As these corries erode backwards they steepen the back walls in both corries, which eventually leaves a steep knife edged ridge called an Arête.  Where 3 or more corries erode backwards towards one another, this can create a Pyramidal peak, a steep-sided pointed mountain like the Matterhorn. Striding edge in the Lake District is a fabulous example of an Arête. 

Glacial erosion landforms - truncated spurs, glacial troughs, ribbon lakes and hanging valleys

As corrie glaciers leave their source regions and descend down old river valleys they can make huge changes to the landscape. One of the major changes they make is to the V-shaped valleys characteristic of the upper reaches of river valleys. The glaciers basically alter this V shape into a U, by creating a steep sided, wide valley in the shape of the letter U. They are formed by a valley glacier that moves down the valley because of gravity.

U shaped valleys or glacial troughs

As the glacier moves down the valley it plucks the rock from beneath and those rocks then rub against the bed of the valley, eroding it further.  This deepens and widens the valley. At the front end of the glacier it acts like a bulldozer, shifting and removing soil, plucking rock from interlocking spurs and truncating them. This creates Truncated Spurs, which are interlocking spurs without the land that interlocks!  Originally, interlocking spurs are created as a river erodes the upper valley it cuts down into the rock and meanders in and out of the surrounding rock. During glaciations this rock is removed by descending ice sheets.
Eroded rocks also play an integral role, as this material is used as a tool to abrade the valley sides and floor further.
DIFFERENTIAL rates of erosion can occur.  This could be because of varying strengths of the bedrock or because there is thicker ice in one region of the glacier than another or because there is more moraine abrading the ground in one region than another. This means that some parts of the valley floor are over deepened allowing the creation of Ribbon Lakes.  These are long and thin lakes that collect from melt water and rain water after the glacier has melted.  

Glacial Trough Features: Hanging Valleys
Within glacial valleys there are main glaciers and smaller tributary glaciers (just like with rivers).  The main glacier can erode its valley to a much greater extent because they are wider, deeper; have more mass and more moraines to use as erosive tools.  The tributary valley glaciers are smaller, have less mass and moraine hence erode their valley less.  This means that the main valley is deeper, wider and steeper, and this becomes really evident post glaciation, when the tributary glacier is left hanging high above the main valley.  When rivers return, they often form waterfalls in these hanging valleys.

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