Composite-wedge pseudomorphs in Flanders, Belgium

Large-scale polygonal crop marks underlain by sand-filled wedge structures cover extensive surfaces on the West Flanders plateau (Flanders, Belgium). Wedge structures (similar to 1.0 m wide, up to >= 1.6 m deep) observed at several sites are interpreted as composite-wedge pseudomorphs, formed by thermal contraction cracking in a permafrost environment. The composite infilling points to former relatively dry conditions on the plateaus. Superimposed wedge structures may indicate discrete growth stages. The structures probably formed during the Pleniglacial (comprising Marine Isotope Stages 2-4), although an older age is not excluded. The widespread occurrence of composite-wedge pseudomorphs on the plateaus conflicts with the literature suggesting that ice-wedge pseudomorphs are dominant in Flanders. As a consequence, palaeoenvironmental reconstructions may have been biased towards environmental conditions suitable for ice-wedge formation. Ice wedges probably formed mainly in depressions, whereas composite wedges may have been common on the plateaus. Copyright (c) 2006 John Wiley & Sons, Ltd.