Page 303 - Fundamentals of Geomorphology

286 PROCESS AND FORM

Box 11.2

TYPES OF PATTERNED GROUND

Circles
Circles occur individually or in sets. They are usually
0.5to3min diameter. Sorted circles have fine mate-
rial at the centre and a rim of stones, the stones
being large in larger circles (Plate 11.2). The debris
island is a particular type of sorted stone circle in
which a core of fine material is girded by blocks and
boulders on steep, debris-covered slopes. Non-sorted
circles are dome-shaped, lack stony borders, and are
fringed by vegetation. Circles are not restricted to areas
of permafrost, and unsorted sorts are recorded from
non-periglacial environments.

Polygons

Polygons occur in sets. Non-sorted polygons range
in size from about a metre across to large tundra or
ice-wedge polygons that may be a hundred metres or
more across. Sorted polygons are at most 10 m across
and the borders of the polygons are formed of stones
with finer material between them (Plate 11.3a). They
are usually associated with flat land, while non-sorted
polygons may occur on fairly steep slopes. Furrows or
cracks edge non-sorted polygons (Figure 11.3). The
best-developed polygons occur in regions with frosty
climates, but polygons are known from hot deserts. Ice-
wedge polygons are exclusively found in permafrost
zones, the ice-wedges often occurring at the edges of
large, non-sorted polygons. Two kinds of ice-wedge Plate 11.2 Stone circles, Kongsfjord, Spitsbergen.
(Photograph by Wilfred H. Theakstone)
polygons are recognized. The first is a saucer-shaped
polygon with a low centre, which may hold standing
water in summer, and marginal ridges on either side
of the ice-wedge trough. The second has a high centre of less than a couple of metres. Earth hummocks (also
hemmed by ice-wedge troughs. called thúfur and pounus) consist of a domed core
of mineral soil crowned by vegetation and are a com-
mon type of unsorted net. They are about 0.5 m high
Nets
and 1–2 m in diameter and form mainly in fine-grained
Nets are a transitional form between circles and material in cold environments where ample moisture
polygons. They are typically small with a diameter and seasonal frost penetration permanently displace