Page 116 - Sedimentology and Stratigraphy

Distribution of Glacial Environments 103

Fig. 7.1 Snowfall adds to the mass of a glacier in the accumulation zone and as the glacier advances downslope it enters the
ablation zone where mass is lost due to ice melting. Glacial advance or retreat is governed by the balance between these
two processes.

positions of the head and snout remain fixed. A cooling or mountains that protrude above the ice as areas of
of the climate reduces the rate of melting and there will bare rock are called nunataks (Fig. 7.2). In polar
be glacial advance down the valley, whereas under a regions the ice extends from the highlands of the
warmer climate the melting will exceed the rate of land areas down to sea level, where glaciers feed ice
addition of snow and there will be glacial retreat
(but note that the ice is still moving downslope within
the glacier). Mountain glaciers do not usually reach
sea level in temperate areas, except in places where
there is high precipitation, which adds a lot of mate-
rial at the head of the glacier: these glaciers move
rapidly downslope and loss may be both by melting
and calving of icebergs into the sea.
Polar glaciers occur at the north and south poles,
which are regions of low precipitation (Antarctica is
the driest continent): the addition to the glaciers from
snow is quite small each year, but the year-round low
temperatures mean that little melting occurs. Perma-
nent ice in the polar continental areas forms large ice
sheets and domed ice caps covering tens to hundreds Fig. 7.2 Hills and ridges of bare rock (known as nunataks)
of thousands of square kilometres. These may com- surrounded by glaciers and ice sheets in a high-latitude
pletely or partially bury the topography, and the hills polar glacial area.