Features of erosion. These slide-shows are all on the Prepwork folder if you wish to copy any notes from them; we will not be stopping in class for you to do this. FEATURES OF GLACIAL EROSION.
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These slide-shows are all on the Prepwork folder if you wish to copy any notes from them; we will not be stopping in class for you to do this
FEATURES OF GLACIAL EROSION
These are the features you will need to be able to describe, explain their formation, draw a labelled sketch of and recognise on an OS map.
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.
Read the section about corries in the booklet, pages 28 and 29.
There is a bullet-pointed list of actions explaining in order how a corrie is created.
Learn this at home!
Also learn how to draw and label sketches like a),b) and c).
Features of erosion
NOTE;- the labels on the first two sketches are not just naming the features, like in sketch c).
They describe what is happening in the sketch, naming the processes.
This makes these labels -and the diagram- worth more marks than one where bits only are named!
Answer these questions in the usual way;-
Q1. Where is the bergshrund and what causes it?
A. It is at the top of the back-wall, where the ice is ripping away from the rock because of gravity pulling the ice downhill.
Q2.What erosion process greatly affects the backwall, and how does this affect the rock texture here?
A.Plucking is the main process here and it leaves the rock jagged and rough.
Q3.The debris created by plucking is active elsewhere in the corrie- where and with what results?
A.Abrasion uses the plucking products to smooth, deepen and enlarge the floor/ basin of the corrie.
Q4. Describe how corries, aretes and pyramidal peaks are all related features.
A. To create an arete, there has to be more than one corrie eroding the land between them into an arete. The eroded corrie back-walls eat into the summit of the mountain creating a pyramidal peak, where several aretes come together.
Formation of a corrie
Take a handout of the diagram and stick it into your jotter.
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.
On an OS map, corries are shown as horseshoe-shaped features.
Note the east-facing aspect; snow lasts longer before melting.
Note the contours are very close together, especially on the steep backwall. Note also the bare rock symbol.
An arete is a narrow, sharp-edged ridge which forms the side walls of corries or separates different glacial valleys.
Features of erosion
On a map an arete is hard to see. It is a long ridge between to corries or valleys.
The red lines mark the spines of the three aretes.
These corries and arete are in the Cairngorms.
Striding Edge arete on
Pyramidal peaks are also called horns.
A VALLEY GLACIER
The next few slides will help to explain the formation of this feature.
Stages in the formation of a U-shaped valley
Take the handout and use page 284 of ‘Wider World’ to add the labels.
These are the actions that form this feature.
a) A glacier flows into an earlier 'V' shaped valley.
b) The glacier abrades and plucks the sides and floor of the river valley.
c) The valley is greatly deepened, widened and straightened.
d) When the ice melts the valley is 'U' shaped.
e) It now has very steep sides and a fairly flat floor.
f) Any rivers are called 'misfit streams’ because they are far too small to have cut the valley.
Former hill spurs are ‘truncated’- their ends are cut off by the ice action to form steep, sheer cliffs.
The yellow lines show where the spurs were before they were chopped off! Note how they defined a V-shaped valley.
A U-shaped valley in Canada.
When a glaciated valley is submerged or drowned by a rise in sea level, a fiord is formed.
The sea lochs of western Scotland are the best examples of fiords in the British Isles.
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?
U-shaped valleys have few contours on their floors.
There is a hanging valley here.
Note the very steep sides.
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.
The English Lake
District owes its
character to these
narrow ribbon lakes
along its valley
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.
An alluvial fan
(This is really a depositional feature.)
alluvium = silt deposited by a river
A crag and tail
Plug of volcanic rock
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.
Now play the glacial erosion dominoes game!
There is a mapping question sheet to try at this point.
You will need a copy of the questions and the map extract supplied.
A ruler may also be quite useful.
The extract is of the area you will need to know for the Rural Land Use section of the course.
Identify the features marked on the diagram by matching them to the names of features listed below.
Arête; Hanging Valley; Corrie;
'U' shaped Valley; Alluvial Fan; Pyramidal Peak;
Corrie Tarn; Misfit Stream;
Ribbon Lake; Truncated Spur; Screes.