Formation from desert sand

The area of actual erg and dune formation is delimited by the 150 mm/yr isohyet. This precipitation boundary appears to have shifted strongly in the recent past. Between 20,000 and 13,000 yr BP, the southern limit of active dune formation in the Sahara desert was 800 km south of its present position and most of the now sparely vegetated Sahelian zone was an area of active dune formation at that time. These dunes, mostly of the longitudinal type, are now fixed by vegetation, but their aeolian parentage is still obvious from their well-sorted material. A similar story can be told for the Kalahari sands. Overgrazing in recent times has reactivated aeolian transport in many regions with sands. 

Sherman mining area is located in north of Shaanxi province. Its west is Mu Us Desert covered with flat aeoUan sand layer-the most significant aquifer and east is the Loess Plateau with serious hydraulic cutting. The stratums, typically from top to bottom, are sand layer (aquifer, mainly distributed in the west), Lishi formation (loess, widespread in the whole area), Baode formation (laterite, widespread in the whole area) and Yanan formation (the major coal-bearing strata). The sand aquifer has suitable permeability coefficient to hold water not to run... 

The dust content of the atmosphere may have a wide variety of effects. Airborne ashes and soot usually contain water-soluble salts, especially sulfates, often sulfuric acid as well, and thus accelerate corrosion. On the other hand, alkaline dusts from the lime and cement industries may form hard incrustations and may thus sometimes have a protective effect. Mineral dusts like sand, e.g., during sandstorms in desert areas, may produce damage through abrasion or, by the mere formation of deposits, may inhibit the function of mechanical or electrical switching elements. 

A characteristic of desert landscape in Australia and South Africa is a silica deposit termed silcrete, which may contain as much as 99% silica (Williamson [1957]). It has been suggested that this may be formed by the cementation of sand by upward movement of silica-rich waters, but this mode of formation certainly does not apply to all silcrete deposits (Jackson [1957]). Silica in the form of white powder has been noted on the surface of Hungarian solonetz soils. The KOH extracts from these soils contain silica and alumina in the ratio of 2 1, indicating degradation which can apparently be halted by organic maturing (Szabolcs and Darab [1958]). 

---