Fech fech (Arabic: فش فش) is a very fine powder caused by the erosion of clay-limestone terrain and it is most commonly found in deserts.[1] It consists of a surface horizon of pulverized soil with low particle cohesion[2] protected under a thin crust. Fech fech is derived from ancient lake muds or on certain argillaceous rocks and is one of the desert surfaces that produces dust.[3] It is not determinable from the surface and can therefore pose a significant transportation hazard acting as a surprise "trap" as the ground collapses beneath a vehicle, miring it in a quicksand-like substance.
^Goudie, Andrew S.; Middleton, Nicholas J., eds. (2006), "Dust Entrainment, Transport and Deposition", Desert Dust in the Global System, Springer, pp. 13–31, doi:10.1007/3-540-32355-4_2, ISBN978-3-540-32355-6