Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 246

FIGURE 5.6 A stone polygon with needle ice development. (Photo by J. D. Vitek.)
Patterned Ground
Patterning of rocks, soil, and vegetation into various geometric forms is a common fea-
ture of high mountain landscapes (Fig. 5.7) These patterns fall into three basic categor-
ies: polygons, circles, and lines or stripes. These “surface markings” or “structure soils”
as they have been called, range in size from tiny features measured in centimeters to
large-scale forms several meters across. Because of their striking geometric arrange-
ment and curious nature, they have attracted a great deal of attention (Troll 1958;
Washburn 1956; Gleason et al. 1986; Odegard et al. 1988; Warburton and Caine 1999;
Haugland and Owen 2005; Haugland 2006). The exact mechanism of their origin is still
controversial. Washburn (1956) listed 19 different theories that have been proposed to
account for their development! One problem is that similar patterns can be caused by
different processes. The polygon, for example, is one of the most ubiquitous forms in
nature. It can be created by thermal contraction upon freezing, by the drying and crack-
ing of soil in a mud puddle or dry lakebed, or when molten lava solidifies. It is not sur-
prising, then, that the process of freezing and thawing also creates polygons.
A useful contribution to the study of patterned ground was the classification devised
by A. L. Washburn in 1956. It is based on only two criteria: geometric form and the
presence or absence of sorting (the segregation of rocks and fines). The basic types of
patterned ground are sorted and nonsorted circles, polygons, nets, steps, and stripes. A
pattern is sorted if particle size varies from one part of the feature to another. A sorted
circle, for example, has finer material in the center and larger particles around the peri-
meter. Frost sorting depends on (1) the amount of available moisture (best in saturated
soils), (2) the rate of freezing (best with slow migration), (3) the particle size distribu-