Page 96 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 79

Plate 3.2 A nivation hollow in Old Man Range, New Zealand.
(Photograph by Stefan Grab)

occurs in conjunction with permafrost or seasonally existing depression. Once initiated under a snow patch,
frozen ground, when it is usually referred to as gelifluc- a nivation hollow (Plate 3.2) increases its size and tends
tion. It frequently operates with frost creep, and it is hard to collect more snow each year, so providing an example
to distinguish the action of the two processes. Gelifluc- of positive feedback (p. 18).
tion is an important process in periglacial environments,
especially on silty soils, owing to the common saturation
of the soil that results from the restricted drainage asso- Glacier processes
ciated with a permafrost layer or seasonally frozen water
table, and owing to moisture delivered by the thawing of A glacier is a large mass of ice formed of compressed
snow and ice. snow that moves slowly under its own weight. Glaciers
are often classed as warm (or temperate) and cold
(or polar), according to the temperature of the ice.
Nivation Warm glaciers have ice at pressure melting point except
near the surface, where cooling occurs in winter. Cold
This process is associated with late-lying or peren- glaciers have a considerable portion of ice below pressure
nial snow patches. It is a local denudation brought melting point. However, glaciologists now recognize that
about by the combined effects of frost action (freeze– warm and cold ice may occur within the same glacier
thaw weathering, particularly the annual freeze), chemi- or ice sheet. The Antarctic sheet, for instance, consists
cal weathering, gelifluction, frost creep, and meltwater mainly of cold ice, but basal layers of warm ice are present
flow (see Thorn and Hall 2002). It is most vigorous in places. A more useful distinction may be between
in subarctic and alpine environments, where it leads warm-based glaciers, with a basal layer at pressure melt-
to the forming of nivation hollows as snow patches ing point, and cold-based glaciers, with a basal layer
eat into hillsides. Snow patches often start in a small below pressure melting point.