The intense erosive action of a glacier produces vast volumes of sediments. Very fine material, called rock flour, is produced by the scraping and grinding of rock surfaces, whilst larger particles are produced by plucking and freeze-thaw. Massive rocks that fell from the valley walls onto the moving ice can also be transported, and then deposited hundreds of miles from their origins.
Geographers classify glacial sediment according to it's mode of deposition. The collective name for all the sediments and debris deposited under glacial conditions is Glacial Drift. Sediments that were deposited by melting ice or by glacial streams are called Fluvio-glacial. Debris deposited directly by the glacier, such as moraine and intra-glacial material dropped 'in situ' by retreating ice, is known as Till
The range of sediment sizes and processes of deposition produce a wide range of landforms.
We will look at the following landforms:
Kames and Kettles
Read the articles about each of the features listed above, and make sure that you understand what each feature looks like, and how it is formed.
Once you understand each feature, make brief notes on each, explaining its origins and appearance.
To test your knowledge of depositional features, answer the following questions as if you were in an examination. All the answers should be in sentences, using geographical terms whenever possible, so instead of saying 'the powdery stuff', say 'rock flour'.
1. Drumlins and Crag and Tail features have a similar shape. How would you tell them apart if you saw one in the field?
2. Till is sometimes called 'boulder clay', and individual rocks within it are often described as 'sub-angular'. Why do you think the term 'boulder clay' is sometimes used, and what does 'sub-angular' mean?
3. What can be discovered about a glacier by studying varves within its sediments?
4. Starting at the source of the glacier, put these types of moraine into the order in which you'd first encounter them as you travelled down the mountain side to the valley floor below the glacier. Give reasons for the order in which you list them.
Lateral Moraine, Terminal Moraine, Recessional Moraine, Medial Moraine, Push Moraine, Ground Moraine.
5. In what ways do you think that past glacial deposition effects human activity in your country today? Hint: Think about agriculture, communications and tourism to get you started
You will now have to carry out some research of your own...
a) Find a map of an area that has been glaciated in the past. It needs to be at a scale that will show landforms clearly, so use a 1:50,000 or 1:25,000 scale. Try to identify all the depositional features we have discussed, and select the best examples to keep as case studies to use in the exams.
b) Glacial deposition is not always a slow and regular process. Sometimes there may be a violent and sudden glacial outburst flood caused by water escaping from sub-glacial lakes, cavities within the ice, or local volcanic melting. Two terms are associated with glacial outbursts; Jokulhlaup and Lahar . Find out what these terms mean, and use the Internet to discover when the last major glacial outburst happened in Iceland.
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