Glaciation - Formations Flashcards
Formation of an arête
Snows collects in hollows of a mountain and turns to ice. Ice erodes all sides of the mountain creating corries. The back wall of the corries are eroded back towards each other by the processes of plucking and abrasion until a narrow, jagged, knife like ridge is formed. An arête is formed when 2 corries form back to back.
Formation of a corrie
Snow collects in north facing hollows of a hill or mountain and compacts to form a glacier. The action of gravity allows the ice to move downhill. Ice freezes onto rock and the back wall and as the ice moves plucks rocks away creating a steep back wall. The back wall is also eroded by freeze thaw weathering. Rocks frozen in the base of the hollow act like sandpaper and as the glacier moves deepens the hollow by abrasion. When the glacier melts a pool of water called a tarn is left.
Formation of a pyramidal peak
When 3 or 4 corries erode back to back around a mountain the arêtes between the mountain rise to form a central peak called a pyramidal peak. Plucking of the back walls makes the corries larger until the top of the mountain is eroded back and a sharp central peak remains. The exposed rock is also eroded by freeze thaw weathering.
Formation of a U shape valley
A glacier moves down the main V shape valley and erodes it by plucking where ice freezes onto fragments of rock and pulls them away and by abrasion where rocks in the ice act like sandpaper and scrape the land surface. As a result the valley becomes wider, straighter and wider. When the glacier melts it becomes a U shape. It has very straight sides and a fairly flat floor, any rivers running through this valley are called misfit streams.
Formation of a hanging valley
A hanging valley is formed when a large glacier joins with a small tributary glacier. The large glacier moves down the V shaped valley and by the processes of plucking and abrasion deepens the valley floor and straightens the valley sides making it U shape. The interlocking spurs become truncated spurs. The same processes take place in the small tributary glacier. The smaller glacier makes a small valley which is left hanging from the truncated spurs leading into the U shape valley. Often a waterfall is left coming fro, the hanging valley into the U shape valley.
Formation of a ribbon lake
A ribbon lake is formed when a large glacier moves down a U shape valley and erodes everything in its way. By the processes of plucking and abrasion the valley is made deeper and wider. The large glacier moves over a section of softer bedrock that is more easily plucked and abraded than the harder bedrock. When this happens, an over deepened depression is made in the softer bedrock. After the ice age, the depression is filled with glacial meltwater.
