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. Desert loess deposits should be distinguishable from glacial loess 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 critical size. Most quartz is introduced into the sedimentary system as sand-size particles (about 500mu ); if the critical thermal break size is above 500mu , then the thermal breakage process will not produce loess-sized particles.