“A glacier is a mass of snow and ice that moves slowly over the land away from its place of accumulation”. – Worcester.
W.D. Thornbury defines a glacier “as a mass of ice and firm formed by re-crystallization of snow and melt water which lies entirely or largely on land and gives evidence of either present or former movement”.
There are various forms of glaciers. They are of different sizes and of different origins. That is why different names have been given to individual types. A morphological classification of glaciers has been presented by
Ahlmann which is as under:
A. Continuous Sheets of glaciers from which the ice moves in all directions.
1. Continental glaciers covering extensive areas.
2. Glacier caps which are smaller than continental glaciers.
3. Highland glaciers which are found on the mountain peaks.
B. Glaciers confined to more or less marked courses which control their main movement.
Independent glaciers and outlets of ice from glaciers of group A.
4. Valley glaciers of the alpine type.
5. Transaction glaciers which cover a whole valley system.
6. Cirque glaciers which occupy localized niches on mountain sides.
7. Wall-sided glaciers which cover a valley side or part of it.
8. Glaciers’ tongues afloat.
C. Glacier ice which spreads in large or small cake-like sheets over level ground at the foot of glaciated areas. Such glaciers are not independent. They are connected with some other types of glaciers.
9. Piedmont glaciers formed by the joining together of the lower parts of 4, 5 or 7 above.
10. Foot glaciers, which are the lower ends and more extended portions of types 4, 5 and 7 as above.
11. Shelf ice.
Continental glaciers are ice caps, whereas the glaciers belonging to Group B are ice streams. Group C glaciers are transitional types. Ice caps move in various directions because of the differential pressure within the ice mass itself.
Motion in ice caps is not controlled by the underlying topography. Ice streams move under the influence of gravity. Their movement is controlled by the underlying topography.
Ahlmann gives another classification of glaciers on the basis of their thermal characteristics:
A. Temperate glaciers
B. Polar glaciers or Cold glaciers
1. High polar glaciers
2. Sub-polar glaciers
According to Ahlmann, polar glaciers are those which throughout the year are theoretically below the freezing point. However, only high polar glaciers have this characteristic. But sub-polar glaciers during the summer months have temperatures above the freezing point.
It may be noted that few glaciers attain the above mentioned temperature characteristics. Remember that in reality various parts of a glacier may have different temperatures.
It may be stated that it is the thermal characteristics within an individual glacier that influence its rate and type of movement, the way it erodes and the amount of melt water produced by it.