Abstract

Two morphological types of shadow dunes, tongues and ridges, were observed after a recent storm on the west coast of Jylland, Denmark. Sand-shadow tongues form where vegetation is a weak obstruction to the sand-transporting wind, whereas sand-shadow ridges form behind vegetated coastal dunes, which interact more effectively with the wind. The sand-shadow tongues consist of low- to medium-angle-dipping strata, which dip in a uniform direction and reflect almost uninterrupted downwind transport of sand. The longitudinal ridges consist of medium- to high-angle cross-strata which are bidirectional and reflect secondary winds that blow obliquely towards the crest line of the dune. Three kinds of oppositely dipping cross-strata occur: 1) tepee-like cross-strata, 2) chevron-like cross-strata with both "stoss"-side and "lee"-side laminae preserved ("double bundles"), and 3) chevron-like cross-strata with only "lee"-side laminae preserved. Tepee-like cross-strata form during vertical accretion of the dune ridge, whereas the chevron-like pattern forms when the dune crest migrates from side to side. If the angle of climb of the "stoss" side is supercritical, "stoss"-side laminae are preserved and "double bundles" are formed. The cross-strata contain a significant amount of plane-bed stratification due to their formation during high-wind velocities.

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