patterned ground

(geology) Any of several well-defined, generally symmetrical forms, such as circles, polygons, and steps, that are characteristic of surficial material subject to intensive frost action.

The arrangement of stones into polygons, isolated circles, concentrations of circles known as nets, steps, and stripes. Polygons and circles are more common on level surfaces, stripes generally form on slopes, but there is no delimiting declivity where one ends and the other begins. The patterns are made of coarser stones, separated by much smaller stones known as fines.

The formation of patterned ground has been ascribed to the formation of convection currents within the active layer, (although some of the rock fragments may be boulders and extremely heavy), and to the sinking of the surface layer on drying, and thus becoming more dense, in summer. Others attribute patterned ground to the upward injection of slow-freezing, waterlogged silts at particular points. See also involution. Once there has been some sorting between coarse and fine sediments, the coarser sediments would freeze first, doming up the finer areas. Any larger material on the domes would then roll downslope to the coarser areas; thus the sorting would be accentuated. Patterned ground may also be due to frost heaving, and the drying and shrinking of surface layers. Patterned ground is most common in periglacial areas, such as Spitzbergen, but polygons can develop during severe winter freezing.