Fech fech is a very fine powder caused by the erosion of clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

-limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 terrain. This pulverized soil is common under a thin crust in deserts. 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

Quicksand

Quicksand is a colloid hydrogel consisting of fine granular matter , clay, and water.Water circulation underground can focus in an area with the optimal mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column...

-like substance.

Fech fech is common in the Qattara Depression

Qattara Depression

The Qattara Depression is a depression in the north west of Egypt in the Matruh Governorate and is part of the Libyan Desert. It lies below sea level and is covered with salt pans, sand dunes and salt marshes. The region extends between latitudes of 28°35' and 30°25' North and longitudes of 26°20'...

, making that portion of the Sahara Desert impassable by most vehicles.