Large-scale deformation of laminae in ancient sandstone of supposed eolian origin has perplexed geologists for many years. Other workers have described several types of deformation related to lee-side avalanching, but none at the scale observed in some ancient eolian sandstone. Our observations on the surface and in trenches of a transverse dune in the Killpecker dune field in southwestern Wyoming suggest that incorporation of snow into dunes may provide such a mechanism.

During periods of snowfall, large snow cornices form on the crests of dunes. In the spring the cornices are covered by blown dry sand which is remobilized after most other snow has melted. Subsequent warming causes the sand-covered cornices to melt, become unstable, and slide at least part way down the slipface. Both folding and brecciation take place in the sand covering the snow during melting and sliding. Folding of laminae in sand under the snow also occurs. Further burial of this deformed mass of snow and sand results in its incorporation into the internal structure of the dune. Collapse breccia is formed where climatic conditions and depth of burial permit the continued melting of incorporated snow. In some cases, however, the covering sand provides sufficient insulation to prev nt further melting and the snow becomes a permanent or semipermanent part of the dune.

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