Patterned Ground

Abstract

Patterned ground is terrain exhibiting surface patterning, primarily in the form of circles, polygons, irregular networks, or stripes. Two types are distinguished: sorted patterns delimited by alternating soil and clasts, and nonsorted patterns defined by microrelief or alternating vegetated and unvegetated ground. Most patterns form through recurrent freezing and thawing of moist soil in periglacial environments. Small sorted forms reflect separation of stony soil into fine and coarse domains by differential frost heave and/or differential needle-ice growth. Larger sorted patterns are probably produced by a combination of differential (annual) frost heave and buoyancy-driven soil circulation during thaw. Small sorted patterns reflect shallow soil freezing, but large sorted patterns are often associated with permafrost. The most widespread nonsorted patterns are ice-wedge polygons, earth hummocks, and nonsorted circles and stripes. Ice-wedge polygons develop through recurrent thermal contraction cracking of permafrost and freezing of meltwater in cracks. Earth hummocks are small vegetated soil mounds in both permafrost and nonpermafrost environments. Their formation has been attributed to circulatory soil movement above depressions in the permafrost table, differential frost heave, and upward injection of soil through dilation cracks. Nonsorted circles and stripes are thought to reflect circulatory soil movements in areas of alternating vegetated and vegetation-free ground in permafrost areas.