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Glacial Arêtes

Arêtes and Pyramidal Peaks in highland areas are glaciers features usually created by erosion, not deposition. Features such as corries, ribbon lakes, U shaped valleys and hanging valleys are typical of upland areas such as the Alps in Europe, the English Lake District and the Southern Alps in New Zealand. When a corrie is formed, its back and side walls tend to be steep and jagged, perhaps almost vertical. When two corries form next to each other, and their adjacent walls are eroded backwards until they meet, a narrow and pointed rock ridge is formed. This is often likened to a knife edge, with near vertical sides and a sharp top edge. This feature is called an arete. 

Pyramidal Peaks 
When three or more corries erode backwards and meet they cannot form an arete; it has steep sides but doesn't have the length to make a ridge. Imagine three corries at the corners of a triangle, eventually all eroding back and meeting in the middle. A sharp pointed pyramid shape is created. This is called a Pyramidal Peak, or Horn, and is a common shape for mountain tops in well glaciated areas. 

This arête is in the Chamonix Valley, France near Mont Blanc Cirques are created when glaciers erode backwards into mountainsides, creating rounded hollows shaped like a shallow bowls. Arêtes are jagged, narrow ridges created where the back walls of two cirque glaciers meet, eroding the ridge on both sides. Horns, such as the famous Matterhorn in Switzerland, are created when several cirque glaciers erode a mountain until all that is left is a steep, pointed peak with sharp, ridge-like arêtes leading up to the top.

Glacial terms
Firn (Névé) – snow that accumulates to form ice
Nunataks – Exposed summits in Ice sheets
Supraglacial debris -  Eroded material on top of the glacier
Englacial debris -  Eroded material in the middle of the glacier
Subglacial debris -  Eroded material underneath the glacier
Frost Shattering – Freeze thaw material that falls onto the glacier
Abraision (striations) – Erosion of the valley floor by subglacial debris
Plucking – Glacier freezes onto loose rock and plucks it away when it moves
Bergschrund – the Crevasse at the head of the glacier in which melt water collects
Rotational movement – the erosion effect that causes Cirques
Arêtes – formed by 2 adjacent cirques eroding backwards
Pyramidal Peaks – formed by 3 or more adjacent cirques eroding backwards
Glacial Troughs – flat valley floors
Ribbon lakes – in glacial troughs
Truncated Spurs – Ridges cut short by glaciers
Hanging valleys - Valleys cut by glaciers with waterfalls
Roche Moutonnee – Resistant rock smooth one side with plucked crags downstream
Rock Drumlins - Resistant rock smooth both sides – Whalebacks
Till Drumlins – Glacial till and debris in groups broad upstream long and thin down
Terminal Moraines – Glacial debris dumped at the end of a glacial
Lateral Moraines – Glacial debris dumped at the side of a glacial
Medial Moraines – Glacial debris from two lateral moraines when glaciers meet
Kettle Lakes – Depressions caused by large ice lump that then melted
Eskers – lines of subglacial meltwater streams deposited debris