Formation of miniature sorted patterns by shallow ground freezing: a field experiment

At an experimental site near sea level in Scotland, a miniature sorted net with cell diameters of 56-172 mm and stony borders 4-9 cm deep developed in frost-susceptible regolith over two winters. Pattern formation was evident after only 8 freeze-thaw cycles, during which the minimum surface temperature was -5.3 degrees C and freezing extended to a maximum depth of 16-17 cm. The primary process of pattern development was repeated differential growth of near-surface needle ice, causing updoming of a cover of coarse debris. During subsequent thaw, bending and melting of ice needles resulted in net migration of upheaved clasts towards dome margins, causing progressive concentration of clasts in stony borders surrounding finer cells. This differentiation of near-surface soil texture appears self-sustaining, promoting preferential growth of ice lenses and thus preferential heave in fine cells. The experiment suggests that miniature frost-sorted patterns are not necessarily diagnostic of present or former periglacial environments, but may develop on unvegetated soil of suitable composition in a wide range of mid- or even low-latitude environments where shallow ground freezing is accompanied by the growth of needle ice.