Page 115 - Sedimentology and Stratigraphy

7

Glacial Environments

Glaciers are important agents of erosion of bedrock and mechanisms of transport of
detritus in mountain regions. Deposition of this material on land produces characteristic
landforms and distinctive sediment character, but these continental glacial deposits
generally have a low preservation potential in the long term and are rarely incorporated
into the stratigraphic record. Glacial processes which bring sediment into the marine
environment generate deposits that have a much higher chance of long-term preserva-
tion, and recognition of the characteristics of these sediments can provide important
clues about past climates. The polar ice caps contain most of the world’s ice and any
climate variations that result in changes in the volumes of the continental ice caps have a
profound effect on global sea level.

7.1 DISTRIBUTION OF GLACIAL winter is mainly in the form of snow. Accumulating
ENVIRONMENTS snow compacts and starts to form ice especially in the
upper parts of valleys, and a glacier forms if the
Ice accumulates in areas where the addition of snow summer melt is insufficient to remove all of the mass
each year exceeds the losses due to melting, evapora- added each winter. These conditions can exist at any
tion or wind deflation. The climate is clearly a control- latitude if the mountains are high enough. Once
ling factor, as these conditions can be maintained only formed, the weight of snow accumulating in the
in areas where there is either a large amount of winter upper part of the glacier (the accumulation zone of
snow that is not matched by summer thaw, or in places the glacier) causes it to move downslope, where it
that are cold most of the time, irrespective of the reaches lower altitudes and higher temperatures.
amount of precipitation. There are areas of permanent The lower part of the glacier is the ablation zone
ice at almost all latitudes, including within the tropics, where the glacier melts during the summer (Hambrey
and there are two main types of glacial terrains: tempe- & Glasser 2003) (Fig. 7.1). Under stable climatic con-
rate (or mountain) glaciers and polar ice caps. ditions an equilibrium develops between accumula-
Temperate or mountain glaciers form in areas of tion at the head and melting at the front, with the
relatively high altitude where precipitation in the glacier moving downslope all the time, but the