Geoscience Reference
In-Depth Information
Remarkable polygonal patterns, ranging from centimetres to tens of metres across,
are distinctive features of flat Arctic landscapes, resulting from volume changes in the
surface blanket on freezing and thawing (Fig. 17). In cross-section the polygon cracks
and ridges correspond to downward-narrowing wedges (often visible also in the walls
of gravel pits in Southern England). Thaw lakes are also a feature of flat areas under
conditions of Arctic frozen ground (Fig. 18). They appear to be linked to the formation
of the polygonal features, but can amalgamate to become kilometres across and may
periodically discharge their muddy soup of disturbed sediments down even very gentle
slopes.
Not only can these frozen ground processes be studied in Arctic areas today, but
they have left characteristic traces in many of the landscapes of Southern England.
Some examples from Norfolk are illustrated in Chapter 8 (Figs 306 and 307), and these
provide specific examples of the result of ancient freeze-thaw processes on a small
scale. However, the more we examine the wider features of present-day landscapes
across Southern England, the more it becomes clear that most have been considerably
modified by the general operation of frozen ground processes during the last 2 million
years. These processes are likely to have been responsible for the retreat of significant
slopes and even for the lowering of surfaces that have almost no perceptible slope.
FIG
17.
Polygonal frozen ground patterns on the Arctic coastal plain near Barrow,
Alaska. (Copyright Landform Slides - Ken Gardner)
