The characteristics of valleys can be changed by glaciation.
The build up of ice and the formation of corries.
The changes to the river valley characteristics: U-shaped valleys, truncated spurs, hanging valleys, ribbon lakes, moraines, pyramidal peaks.
Students are not expected to know about ice ages or the processes involved in ice formation beyond the build up and compaction of snow. Knowledge of case studies will not be required in the exam, but there are opportunities to link landforms and environments with the mountain tourism study in Unit 7.2.
Students are expected to be able to describe the landforms specified, and explain their formation in terms of the processes involved. They should be able to construct annotated diagrams of the landforms and recognise the landforms specified on OS maps and in photographs.
What are the page references in ‘Tomorrow’s Geography?
Glaciers develop where the temperatures are cold enough to allow snow to accumulate over a period of years. Favourable conditions are found around the poles and at high altitudes in lower latitudes, i.e. mountainous regions such Northern Scandinavia and the Alps. Enough snow must fall each winter to ensure that it doesn't all melt in the summer. This way, the amount of snow lying on the ground gets deeper each year as new snow is added to the remains of last years fall.
On north facing slopes it may survive all year without melting, whereas all the snow may melt on south facing slopes ( in the northern hemisphere ). This is because north facing slopes get much less direct sunshine than south facing ones, and thus remain cooler.
Plucking - glacier ice freezes into cracks in rocks and when the glacier moves it pulls out chunks to leave a jagged surface
Abrasion - rocks stuck in the ice grind away the bedrock under the glacier
Freeze Thaw - water in cracks in the rock freezes and expands forcing open the gap. When the ice melts more water can get into the crack and freeze again. After many cycles of freezing and thawing lumps of rock are broken off the surface
A corrie (also called a cirque or cwm) starts as a snow patch on a cold mountain side. The temperatures must be low enough to allow the snow to remain all year round. Under these conditions snow can accumulate and the snow patch will grow in size and depth each year. Due to temperature changes and pressure, the snow becomes altered to ice, and when sufficient ice has accumulated it will begin to move downhill under the influence of gravity
Below the ice there is a scouring action produced by the rotational movement of the ice as it moves down hill, and abrasion from the rock debris held in the ice. The rocks embedded in the ice scrape away at the underlying surface, slowly producing a curved hollow. Most of the erosion occurs in the centre of the ice, with less at the back, sides and front.
This causes a deepening of the hollow in which the snow has accumulated, a steepening of the walls behind and to its sides, and an 'armchair' shaped features is gradually produced
Helvellyn and Red Tarn in winter (English Lake District).
Active corrie formation Active corrie with snow and ice deepest in the centre, and steep back and side walls
Aretes and Pyramidal Peaks In highland areas the most obvious glaciers features are usually those 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. Aretes 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 Pyramidal Peak
A pyramidal peak (far right), two aretes (middle) and a valley glacier (left) in the Chamonix valley, France. (The peak is Mont Blanc)
Ribbon Lakes When a glacier moves along its valley, changes in the rate of flow caused by extension or compression may lead to increased deepening of sections of the valley floor. Areas of softer rock may also experience increased deepening. When the glacier retreats, the deepened sections fill with melt water and become lakes. These lakes remain long after glaciation has ended, supplied by rainfall and subsequent streams and rivers. The English Lake district owes its character to these narrow ribbon lakes along its valley floors.
U shaped Valley 1 U Shaped Valley When a glacier erodes its valley , a classic U shape is formed, the side walls tending to be steep and possibly curving inwards at the base, and the valley floor almost flat. U shaped valleys often start life as river valleys that existed before glaciation occurred. The glaciers then followed the existing V shaped valleys, eroding and deepening them as the ice moved. Over time the valleys became straightened, widened and deepened, keeping the steep sides and acquiring a flat base. U shaped valleys are also known as glacial troughs. The flat floor is roughly shaped by the ice which tends to cut down more evenly than flowing water. A thick layer of glacial debris (ground moraine) is deposited as the ice retreats, smothering any minor irregularities, and creating a well drained and fertile soil
U shaped Valley 2 Glacial U shaped valley with steep sides, flat floor, and truncated spurs (middle left)
Moraine has unsorted angular material of all sizes dumped together.
Terminal Moraine is a ridge across the valley marking the former snout.
Ground Moraine is bumpy or hummocky spread over a wide flattish Till Plain .
Outwash Plains are large flat areas of meltwater sands and gravels downslope of the terminal moraine. All fluvioglacial deposits are rounded in nature and sorted into alternate layers depending upon the season of deposition.
Moraine Dammed Lakes occur where terminal moraine has prevented some meltwater from leaving the valley
Draw an arrow to show the direction the ice was moving (1 mark)
Explain the formation of a corrie (6 marks)
Subject Contents Section Contents Printer Friendly Version Highlight Words < < Prev Next > > Geography : Glaciation : Glacial Test Question 7 from 1998 Exam The diagram shows a Corrie (Cirque, Cwm) occupied by ice in the last Ice Age. 1. Name the features shown at A and B. [ 2 ] 2. Draw an arrow to show the direction in which the ice is moving [ 1 ] 3. Explain the formation of the corrie (you might use diagrams to help you). [ You would have 18 lines to complete your answer ]. [ 6