Grain Structure of Sand Dunes and its Relation to their Water Content

Abstract

THE stratification of wind-accumulated sand beds is well-known, and the evidence of it can often be seen on the vertical faces of sand-pits. Beadnell¹ has shown that the laminar structure of fresh non-cohesive dune sand can also be observed, if the dry dune is soaked with water to allow a vertical face to be exposed without collapse. The strata he thus exposed dipped at the angle of repose of sand, which experiments show to lie between 30½° and 35¾° to the horizontal². In this case the succeeding layers have been formed by sand which, having been driven over the surface of the dune, has come to rest temporarily sheltered on the upper part of the slope, and has accumulated there, increasing the angle of the slope until shear occurs. An avalanche then takes place down the slope from top to bottom (Fig. 1). Fresh sand deposits formed in this way are characterized by their loose packing and by their extreme softness. I have called³ such a mode of deposition 'encroachment'.