Cryospheric systems - Fundamentals of the Physical Environment - page 374

Plate 15.17 Prominent ice wedges in permafrost in the North West Territories, Canada.
Photo: Ken Atkinson

ognizes five types based on morphology and landscape
position. On nearly horizontal surfaces occur circles , poly-
gons and nets , whilst on slopes are found steps and stripes .
Each type can be further subdivided into non-sorted and
sorted forms based on the absence or presence of promi-
nent sorting between stones and finer material. Lundqvist
(1962) relates the variety of patterned ground to factors
of slope, stoniness, soil texture, vegetation, soil ice and soil
moisture ( Figure 15.20 ). Further complexity comes in hav-
ing to differentiate between active and relict features.
Earth hummocks and ice-wedge polygons are two
widely distributed periglacial features that will be discus-
sed in this section. Earth hummocks belong to the
category of non-sorted nets. They are nearly circular on
level ground but become elongated on slopes. They are
1-2 m in diameter and about 50 cm in height from the
top of the hummock to the interhummock trough. Where
composed predominantly of peat, they are referred to as
thufurs . Usually the permafrost table is a mirror image of
the surface, being shallowest under the trough and deepest
under the top of the hummock. The top of the hummocks

Micro-relief caused by soil freezing:
hummocks and ice-wedge polygons

The presence of permafrost causes displacements of the
soil and produces a unique microtopography. Cryoturba-
tion refers to soil mixing whereby soil surfaces become
unstable, soil materials are mixed internally, and soil
horizons are disrupted and displaced. The disruptive
effects of the freezing of the soil are caused by volume
change. First is the 9 per cent volume change due to the
phase change of water into ice. However, frost heave often
reflects volume increases of 40-80 per cent. This is caused
by the processes of ice segregation discussed on p. 374,
which give accumulations of ice from migrating soil water
that are able to lift the surface of the ground, and which
can sort stones from fines in the soil. Thus heaving and
sorting are ubiquitous processes in periglacial regions
( Figure 15.19 ).
Patterned ground is the collective term for the distinc-
tive micro-relief produced by such processes. A widely
used classification is that of Washburn (1979) who rec-