Abstract

Ventifacts occuring on extensive wind-deflated surfaces throughout ice-free Wright Valley, Antarctica, are the product of complex evolutionary processes. Initially, the wind removes the −2.0 to 1.5 ϕ grain-size fraction of valley soils to produce a lag gravel. The lag gravel then continues to evolve at a reduced rate as coarser granule and gravel fractions are removed. The morphology of ventifacts forming the lag gravels suggests that the distribution of wind-polished faces is determined largely by the shape of the original unpolished rock fragments. Once ventifaction is initiated, the number of faces per clast declines rapidly as minor faces are removed by the polishing. As ventification proceeds, the trend is reversed as the ventifacts are reoriented by the wind and new faces form. At approximately 2.1 m.y. the number of faces per ventifact begins to exceed that of the original unpolished rock fragments. In the early, more active stages of ventifact development, ventifacts tend to be oriented either transverse or parallel to the wind direction. Salt weathering is also a major factor in determining the morphology of the Wright Valley ventifacts, proceeding more slowly than ventifaction; its effects are not apparent until approximately 0.5 to 2.1 m.y. Salt weathering begins by pitting and flaking the underside of the ventifacts; by this means a ventifact may be reduced to a hollow shell which ultimately collapses under its own weight.

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