Aeolian sands and sandstones
Models for the development of ergs and aeolian cross-bedding are deduced. These theories are partly deductions from the modern, partly explanations of the ancient. Ergs may be non-aggrading without complete sand cover or aggrading with or without complete sand cover. Aeolian cross-bedding generally comprises 2 superimposed orders of nested trough-shaped sets. One is developed by migrating draa, one by migrating dunes. Four orders of bounding surface exist: 2 migration surfaces and 2 modification surfaces. The Yellow Sands of N.E. England represent longitudinal draa. The draa were initiated as sand patches, grew by the migration and climbing of sinuous transverse dunes, and stabilised at equilibrium by the development of capping sand-sheets. A bimodal wind regime pertained. A modern analogue of the Sands at equilibrium is provided by 'whalebacks' in the Egyptian Western Desert. The streamlined shape of the draa, well-packed surface, disposition as a series of baffles and distance from the influx minimised reworking in the Zechstein transgression, which drowned the draa in a matter of years. The Bridgnorth Sandstone accumulated from slipfaced and slipfaceless transverse draa. Dune cross-bedding forms sets 2-4 m thick, 20-40 m wide. Slipfaced draa sets are 6-10 m thick, at least 100 m wide. Characteristics of the Bridgnorth Sandstone, Yellow Sands and other formations of aeolian sandstone are explained and illuminated by the erg and cross-bedding models developed. The Bridgnorth Sandstone is slightly finer than the Yellow Sands, and contains more sandflow lamination, less wind-ripple. These features follow from the contrasting erg and bedform types. It Is suspected that sandflows thicker than 80 mm may be confined to draa-sized slipfaces.