Sorted polygon in glacial debris covering floor of Bull Pass in Southern Victoria Land, Antarctica. Both coarse and fine-grained materials can be found in polygonal depressions, and both raised and depressed centers can occur. These polygons have coarse, raised centers, coarse marginal trenches, and coarse-depleted, gently sloping shoulders.
Sorted polygons are among a number of enigmatic landforms developed in periglacial environments.
A leading explanation for these types of sorted polygons is that the upper regolith in periglacial environments convects, or experiences diapirism, driven either by temperature or density gradients. Many granite and dolerite boulders in moderately to old glacial moraines show evidence of cavernous weathering. In the former, silica or other minerals are precipitated into microfractures, strengthening the outer portion of the rock against the attack of other physical and chemical processes. This site is investigating the effects of wind-driven sand on natural materials exposed for long periods of time in the cold desert environment. The rocks on the right actually perturb the wind flow sufficiently to modify the amount and nature of abrasion on the lower portions of this rack, but two nearby racks are not similarly affected, which permits the effects on the lower samples to be determined.