HAIRPIN EROSIONAL MARKS, HORSESHOE VORTICES AND SUBGLACIAL EROSION

Crescentic erosional marks wrapped around obstacles and extending into straight grooves are termed hairpin erosional marks. They are U-shaped to semi-circular in cross-section, are deepest around the upstream shoulders of the obstacles, and range over seven orders of magnitude in scale. Some are asymmetrical with one groove better developed than the other. The straight grooves may broaden and shallow rapidly in the downflow direction. Upstream grooves merge with, rather than cross, downstream crescentic marks. Grooves are distinct from glacial abrasion forms and many are not striated. Hairpin erosional marks are interpreted as being products of scour by horseshoe vortices in turbulent fluids. This interpretation is supported by the close similarity of these marks to scours produced experimentally and in nature by horseshoe vortices. Estimated Reynolds Numbers show that such vortices are probable around obstacles in wind and flowing water, but are extremely unlikely in ice. Aeolian hairpin erosional marks related to yardangs are illustrated to support this conclusion. The surface texture, cross-sectional shapes, and spatial patterns of hairpin erosional marks also discount formation by ice. A model for the evolution of isolated erosional drumlins from an initial field of hairpin erosional marks extends earlier research on a meltwater hypothesis for drumlins.