ABSTRACT

Loess consists chiefly of quartz particles with diameters of about 20-50µ. It is commonly thought to have formed in periglacial areas and in deserts. Smalley (1966b) has suggested that direct glacial grinding could produce suitable particles. The existence of dust storms suggests that fine particles also form in hot, sandy deserts; these could be the result of interparticle contacts where a certain critical kinetic energy is involved.

There are no loess deposits within sandy desert areas, but they may occur at the desert margins. The major loess provinces to which a desert origin has been ascribed either are not true loess or are explicable in terms of glacial origin. Of the minor deposits, only that of the Negev seems to have a well established desert origin. The quartz particles in the Negev deposit probably formed by simple impact. This mechanism does not seem adequate for the Chinese loess, and it is suggested that the widespread Pleistocene glaciation of China was responsible for the bulk of the fine particles. Thus although loess particles can form mechanically in deserts, they have not done so on a scale sufficient to produce major loess deposits. Desert loess deposits should be distinguishable from glacial oess deposits by their size and their mineralogical nature.

Stresses produced by temperature changes can break rocks provided that the rock particles are not below a certain critical size. Most quartz is introduced into the sedimentary system as sand-size particles (about 500µ); if the critical thermal break size is above 500µ, then the thermal breakage process will not produce loess-size particles.