patterned ground

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patterned ground An assemblage of small-scale, geometric features typically found at the surface of a regolith that has been disturbed by frost action. The group includes circles, polygons, and nets, which normally occur on level or gently sloping surfaces, and steps and stripes, which are found on steeper gradients. Both sorted and non-sorted varieties are recognized. The sorted varieties are typically outlined by coarse, stony material, and so are termed ‘stone circles’, ‘stone polygons’, ‘stone nets’, ‘stone steps’, and ‘stone stripes’. The origin of patterned ground involves a complex interaction of several geomorphological processes, including ground cracking, frost sorting, frost heaving, and mass movement. The ‘ice-wedge polygon’ is an important member. It is usually 15–30 m in diameter and bounded by ice wedges up to 3 m wide and about 10 m deep which occupy contraction cracks that form under very low temperatures. The wedges define raised zones (when freezing is active) or depressions (owing to thaw). The ‘stone garland’ is a variety of sorted step, which ends in a stony riser (less than 1 m high) supporting a relatively bare tread (less than 8 m long) upslope. It is found on gradients of 5–15° (in Alaska) and may be caused by a combination of frost pull and frost push, which heave stones to the surface, and mass movement. Patterned ground may also be found in areas underlain by montmorillonitic soils experiencing markedly seasonal rainfall, where the microrelief forms are called gilgai.

patterned ground An assemblage of small-scale, geometric features typically found at the surface of a regolith that has been disturbed by frost action. The group includes circles, polygons, and nets, which normally occur on level or gently sloping surfaces, and steps and stripes which are found on steeper gradients. Both sorted and non-sorted varieties are recognized. The sorted varieties are typically outlined by coarse, stony material, and so are termed ‘stone circles’, ‘stone polygons’, ‘stone nets’, ‘stone steps’, and ‘stone stripes’. The origin of patterned ground involves a complex interaction of several geomorphological processes, including ground cracking, frost sorting, frost heaving, and mass movement. The ‘ice-wedge polygon’ is an important member. It is usually 15–30 m in diameter and bounded by ice wedges up to 3 m wide and about 10 m deep which occupy contraction cracks that form under very low temperatures. The wedges define raised zones (when freezing is active) or depressions (due to thaw). The ‘stone garland’ is a variety of sorted step, which ends in a stony riser (less than 1 m high) supporting a relatively bare tread (less than 8 m long) up-slope. It is found on gradients of 5–15° (in Alaska) and may be due to a combination of frost pull and frost push, which heave stones to the surface, and mass movement. Patterned ground may also be found in areas underlain by montmorillonitic soils experiencing markedly seasonal rainfall, where the microrelief forms are called gilgai.