Geology Reference
In-Depth Information
Box 11.2
TYPES OF PATTERNED GROUND
Circles
Circles occur individually or in sets. They are usually
0.5to3mindiameter.
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
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
hemmed by ice-wedge troughs.
Plate 11.2
Stone circles, Kongsfjord, Spitsbergen.
(
Photograph by Wilfred H. Theakstone
)
of less than a couple of metres.
Earth hummocks
(also
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
and 1-2 m in diameter and form mainly in fine-grained
material in cold environments where ample moisture
and seasonal frost penetration permanently displace
Nets
Nets are a transitional form between circles and
polygons. They are typically small with a diameter
