Cryospheric systems - Fundamentals of the Physical Environment - page 376

(a) Cryostatic pressure
1.

Ground horizontal Ground sloping

GROUND POOR IN BOULDERS

Peat hummocks (thufur) Stripe hummocks

Frozen

Unfrozen

Permafrost

2. Non-sorted circles or mudboils

Earth nets Miniature steps

Frozen

(b) Sorting by upfreezing
1. Mixed grain size soil with upfreezing of larger stones

GROUND RICH IN BOULDERS

Stone pits Stone banked steps

os t t a bl e

Sorted stone circles Stone stripes

2. Upwelling of fines, sorting of stones, load deformation

Sorted stone circles

3. Formation of s orted stone polygons or circles

Stone garland

Stone stripes

Figure 15.19 Development of (a) non-sorted circles by
cryostatic pressure and (b) sorted circles by upfreezing.
Source: After Selby (1985)

Bedrock

Figure 15.20 Relationships between types of patterned
ground, vegetation and slope.
Source: After Lundqvist (1962)

indicated by the fact that experimental sites bulldozed
after fires in 1968 show no signs of hummock regrowth.
Ice-wedge polygons are a special category of a broader
group of polygonal shaped surface features found
throughout the Arctic ( Plate 15.19 ). Polygons are contrac-
tion features, with desiccation cracking probably being the
major cause of fissuring. Subaerial desiccation can result

from drainage, evaporation or water movement to places
of ice formation. Freeze-drying or thermal contraction is
a secondary and related process and the two processes
would reinforce each other. Permafrost cracking is a
special case of thermal contraction; in the permafrost