Glaciation Learning Intentions Be able to understand the processes which occurred during the last ice age. Be able to describe and explain the formation of features formed during glacial periods. 1
Water enters crack Processes Of Weathering Freeze-Thaw Ice freezes and exerts pressure from inside rock Rock breaks up over time 2
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Conditions and processes which encourage the formation of scree slopes. Steep and bare rock faces with lines of weakness/well-jointed carboniferous limestone. Cold climate where temperatures often fall below freezing point at night. The two factors above allow physical weathering to take place in the form of freeze-thaw action/frost shattering, where water collects in the rock fractures, freezes and expands by about 9% exerting great pressure on even the hardest rock. Repeated freeze-thaw action splits the rock into large sharp fragments which break off and are moved downhill by gravity to accumulate at the base of steep slopes as a scree or talus slope as large heaps of rock debris. Biological weathering Root wedging Animals burrows 4
Processes Of Erosion Plucking Water can enter cracks in the rock and freeze, as it does so it freezes the glacier and rock together. As the glacier moves it literally pulls the rock out and is likely to occur more when temperatures fluctuate around 0ºC. 5
Processes Of Erosion Plucking LITHOSPHERE The water from this process often comes from the friction between ice moving over rock. Another source of the water is a result of melting of the ice; during the day it melts, but refreezes at night. Plucked Rock 6
Processes Of Erosion Abrasion This is when rock under the ice or stuck in the bottom of it, scrape away at the surface of the bed rock surface. The rock comes from the debris produced by Freeze Thaw and Plucking. The bedrock is scratched, polished, smoothed and eventually worn away by the scouring action. 7
Processes Of Erosion Abrasion Striations or scratches can be left on the rock 8
Features of erosion CORRIES Snow collects in hollows, especially on the less sunny north and east facing slopes, turns to glacial ice and moves downwards under the force of gravity Rocks are plucked out and the hollow is widened by abrasion to become a corrie. A corrie is a deep, rounded hollow with a steep back wall. 9
Corries Task 3 Q2 LITHOSPHERE Match the letters to the descriptions. Flat Base Horse shoe shape Steep sides b a c d Cliffs/ Outcrops/Scree
S N Layers Ice Hollow Corrie Development 1
Corrie Formation Stage 1 Snow collects in hollows high up in the mountains, usually in north facing slopes. Pressure causes the lower layers of ice to turn to ice. Gravity and reduced friction cause the ice to move downhill. The hollow in which the snow and ice collected is eroded by the ice to form a much deeper, steeper hollow called a Corrie. 12
Corrie Development 2 LITHOSPHERE FT PL RS L AB
Corrie Formation Stage 2 As meltwater under the ice seeps into cracks the rock in the hollow is weathered by freeze thaw action. Then when the meltwater freezes onto the rock plucking takes place. The backwall and sidewalls of the hollow are eroded quickly by these two processes. Abrasion due to rocks taken in by frost shattering and plucking deepens the hollow. The glacier slips in a rotating movement, which means that it is eroding the most at the back wall and base. So with less erosion at the base so a lip starts to form, which has deposits of rocks left on it by the glacier 14
Corrie Development 3 LITHOSPHERE Lip Scree Lochan
Corrie Formation Stage 3 When the ice melts a corrie can clearly be seen. It is an armchair shaped hollow in the mountainside with steep sidewalls and steep backwall and a flat base. Sometimes after glaciation, corries fill with meltwater to form corrie lochs or tarns. 16
Features of erosion Formation of a corrie 17
Features of erosion Back wall scree lip Once the glacier retreats, the corrie may be filled with water. A small, generally circular loch is formed. This is known as a tarn or corrie lochan. 18
Features of erosion Red Tarn 19
Features of erosion ARETE An arete is a narrow, sharp-edged ridge which forms the side walls of corries or separates different glacial valleys. 20
Arete Formation Aretes are formed when two corries are found back to back or side by side in the mountain area. Plucking and freeze thaw action erode the backwalls of both corries towards each other. The land between them gets narrower and narrower until all that remains is a steep knife-edged ridge of rock. 21
Features of erosion Striding Edge Striding Edge arete on Helvellyn, Lake District 22
Features of erosion MATTERHORN Pyramidal peaks are also called horns. 23
Pyramidal Peak Diagram Of Pyramidal Peak Erosion Pyramidal Peaks b c c a
Pyramidal Peak Formation Found where 3 or more corries are located in the same mountainside. Plucking and freeze-thaw action of the backwalls of each corries occurs to such an extent that the rounded summit is eroded into a sharp peak or point. The rock erodes back so that the mountain becomes steeper and the three corrie walls come close enough to converge This leaves a pointed and jagged (freeze thaw) peak eg. The Eiger (The Alps) 25
Deep valley LITHOSPHERE b U Shaped Valley d Steep sides a Straight valley c Loch in wide valley
U-shaped Valley Misfit Steam 27
U-shaped Valley Ribbon Lake 28
Features of erosion A VALLEY GLACIER The next few slides will help to explain the formation of this feature. 29
U Shaped Valleys Ice starts to flow out of corries or off ice caps It flows down V shaped river valleys
Ice moves down river valley Usual erosion occurs Abrasion deepens Plucking erodes sides back Glacier also bulldozes rock out of the way U shaped valley develops
U-shaped valley formation When a glacier erodes its valley, a classic U shape is formed, the side walls tend to be steep and possibly curving inwards at the base, and the valley floor almost flat. U shaped valleys start life as V shaped river valleys that existed before glaciation. Ice moves down the V shaped valley and glaciers erode it and change its shape as the ice moves. Plucking and freeze-thaw action truncate spurs of rock and steepen the valley sides and abrasion deepens and flattens the valley floor. Over time the valley also becomes straighter. 32
Misfit Stream After glaciation when the ice melts, the river which originally flowed on the floor of the V-shaped valley may once again begin to flow through the U-shaped valley. It is now called a Misfit Stream. It is so called because it looks out of place in the now large valley and also because it did not contribute to its formation. 33
Ribbon Lake At some points in the U-shaped valley, glaciers erode more deeply than elsewhere. This might have been because the rock was softer and more easily eroded or because the ice was thicker at this point and therefore more powerful. Where ice did this, it would create an overdeepened hollow which after glaciation filled with meltwater to become a Ribbon Lake. The lake takes on the same shape as the valley in which it was formed so tends to be long and quite narrow. 34
Truncated Spurs LITHOSPHERE Truncated Spur Steep Cliffs Overlooking U shaped valleys Often Scree collects underneath
Truncated Spurs LITHOSPHERE Bulldozed by glaciers Abrasion & plucking help with erosion Post glaciation freeze-thaw has created more scree below
Truncated Spurs LITHOSPHERE l Cliff Face Scree k m Steep Sides U Shaped Valley
These are deep U shaped valleys formed by the erosion of valley glaciers As glaciers emerge from the lip of their corries they merge and form a more powerful glacier which erodes more readily They follow the course of former V shaped river valleys As they progress down the valley through their own weight (gravity) they erode like a corrie glacier The top of the valley sides are eroded by freeze thaw weathering, mid height of the valley by plucking and the base by abrasion This means that the former V shaped valley will widen, steepen and deepens When a U shaped valley cuts across a water shed it is called a glacial breech The interlocking spurs of the river valley literally have their noses cut off by the ice, leaving a ridge descending into the valley which suddenly steepens, sometimes into an outcrop or a cliff face The steepest part of the valley is often at the trough head (start) where the descending corrie glaciers move quickly under gravity. Other more eroded points tend to be a result of the glacier moving over weaker rock which abrades readily, deepening the base This over deepening can fill in with water leaving a ribbon loch eg, Loch Avon (Cairngorms)
Features of erosion Stages in the formation of a U-shaped valley 39
Features of erosion A U-shaped valley in Canada. 40
When a glaciated valley by the coast is submerged or drowned by a rise in sea level, a fiord is formed. Features of erosion The sea lochs of western Scotland are the best examples of fiords in the British Isles. Fiord/fjord 41
Features of erosion A hanging valley Vertical erosion in the main glacier is far greater than in the tributary glaciers. So the valleys are not the same depth. After the glacier has retreated, rivers flowing down the tributary join the main valley via a waterfall Can you spot the river delta,too? 42
Features of erosion U-shaped valleys have few contours on their floors. Note the very steep sides. Truncated spur There is a hanging valley here. waterfall Misfit stream 43
Features of erosion When a glacier moves along the valley, some parts are deepened more than others. When the glacier retreats, the deepest parts fill with water and become lakes. Ribbon lakes The English Lake District owes its character to these narrow ribbon lakes along its valley floors. 44
Erosion/ deposition An alluvial fan alluvium = silt deposited by a river Caused when a stream falling from a side valley reaches flatter ground on the valley floor. Material is dropped at the break of slope to form this fan shape. (This is really a depositional feature.) 45
Erosion/ deposition A crag and tail Edinburgh Castle Plug of volcanic rock tail These are partly erosional, partly depositional features. The rock face facing the ice is steepened by glacial erosion. Softer rock on the other slope is protected from erosion to form a tail of boulder clay. 46