Page 270 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 253

Plate 10.4 Glacial trough with valley glaciers at head, East Greenland.
(Photograph by Mike Hambrey)

Water Mass balances may also be drawn up for continental
gain
ice sheets and ice caps. In an ice sheet, the accumu-
lation zone lies in the central, elevated portion and is
Accumulation Water surrounded by a skirting ablation zone at lower eleva-
zone loss tion. In Antarctica, the situation is more complicated
Firn line because some ice streams suffer net ablation in the arid
interior and net accumulation nearer to the wetter coasts.
Ablation
zone
Glacier floor

Glacier surface at Quaternary glaciations
start of budget year
It is important to realize that the current distribution of
Figure 10.3 Glacier mass balance: schematic changes ice is much smaller than its distribution during glacial
in the geometry of a glacier during an equilibrium stages over the last million years or so. Oxygen iso-
budget year. tope data from deep-sea cores (and loess sequences) has
Source: Adapted from Marcus (1969) revealed a sequence of alternating frigid conditions and
warm interludes known as glacial and interglacial stages
(Figure 10.4). The coldest conditions occurred at high
latitudes, but the entire Earth seems to have cooled down,
net losses of water in the ablation zone and the glacier with snowlines lower than at present even in the tropics
will retain its overall shape and volume from year to year. (Figure 10.5). Palaeoglaciology deals with the reconstruc-
If there is either a net gain or a net loss of water from tion of these Quaternary, and older, ice sheets, mainly by
the entire glacier, then attendant changes in glacier shape analysing the nature and distribution of glacial landforms
and volume and in the position of the firn line will result. (see Glasser and Bennett 2004).