Fech fech ( ar, فش فش) is a very fine powder caused by the erosion of
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
-
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
terrain and it is most commonly found in
desert
A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
s. It consists of a
surface horizon of pulverized soil with low particle cohesion
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.
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 is a colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a ...
-like substance.
Fech-fech is classified into two types:
* Fech-fech that developed during the
Holocene
The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...
, in lake mud or fluvio-lacustrine
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s.
* Fech-fech that developed from shale.
Fech fech is common in the
Qattara Depression
The Qattara Depression ( ar, منخفض القطارة, Munḫafaḍ al-Qaṭṭārah) is a depression in northwestern Egypt, specifically in the Matruh Governorate. The depression is part of the Western Desert of Egypt.
The Qattara Depressi ...
, making that portion of the
Sahara Desert impassable by most vehicles.
See also
*
Dry quicksand
Dry quicksand is loose sand whose bulk density is reduced by blowing air through it and which yields easily to weight or pressure. It acts similarly to normal quicksand, but it does not contain any water and does not operate on the same principle ...
References
Sediments
Soil
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