Thaw modification of frost-fissure wedges,Richards Island,Pleistocene Mackenzie Delta,Western Arctic Canada

Thaw modification is the general process whereby frost-fissure wedges are modified during thaw, and by which frost-fissure pseudomorphs may develop. Specific processes of thaw modification are inferred from ice-wedge pseudomorphs, composite-wedge pseudomorphs and deformed sand wedges in the Pleistocene Mackenzie Delta: i.e. thermal erosion, collapse, subsidence, refreezing, loading, buoyancy, spreading, folding and shearing. Thaw modification is believed to result in selective preservation of pseudomorphs and wedges. Sand wedges are more likely to be preserved than are ice-wedge pseudomorphs or compositewedge pseudomorphs, because only those sand wedges that penetrate massive ice or icy sediments are prone to thaw modification. Furthermore, whereas ice wedges preferentially develop in ice-rich, fine-grained sediments (thaw-sensitive), their pseudomorphs appear to be selectively preserved in ice-poor, coarse-grained sediments (thaw-stable).