Geoscience Reference
In-Depth Information
Figure 9.1
Outcrop or rock hamada with residual in situ origin, Israel. Note the angularity of the rocks that make up this surface.
A coin (28 mm in diameter) shows the scale of the features. (Photograph courtesy of Antony Orme).
the serir surfaces of other authors, whereas Mabbut (1977)
considered hamada to be formed of residual rocks or boul-
ders. The latter definition appears to be the most common
usage and the way in which the term will be employed in
this chapter.
There are two basic types of hamada surfaces. The
first is an outcrop or rock hamada: throughout much of
the Sahara, hamada has the connotation of a structural
tableland, generally in association with flat-bedded rocks
or horizontal weathered crusts (Figure 9.1).
Structural hamada surfaces may be reinforced by the
deposition of duricrusts, including silcretes or calcretes.
Most rock hamada have a residual in situ origin; i.e. they
have not been transported. They are often mantled by an-
gular and unworked rocks. However, the surface may be
considerably modified by aeolian erosion. Hobbs (1917)
describes the extensive Libyan Hamada as having a blind-
ing polish and being 'ruffled' by sand abrasion, such that
the surface appears like waves on the ocean.
The second type is boulder hamada (Figure 9.2). Some
boulders have a residual origin, as shown by their lack of
sorting, angularity and affinity with the rock below (Mab-
butt, 1977). Others, often rounded in form, have been de-
posited by transport from local sources. Secondary pro-
cesses may modify the boulders, changing their texture
or form. Weathering breaks down the rocks, creating ei-
ther rougher or smoother surfaces. Rounding may occur
in place (Mabbutt, 1977) or the materials may become
(Figure 9.2). Czajka (1972) describes wedge-shaped ven-
tifacts in a boulder hamada on the Argentine Puna Plateau
at altitudes of 1500-5000 m.
Furst (1965) noted that there is sometimes a spatial con-
tinuum between stony surface forms, with rock hamada in
tablelands, boulder hamada downslope and reg at lower
altitudes. Materials are stripped from higher altitudes and
moved to lower elevations.
It is likely that many hamada are of considerable antiq-
uity. These largely planar surfaces are protected from dis-
section and channel development by their rocky character
and moderate gradients, which tend to disperse sediment-
laden water. Like pavements, they are best developed and
maintained when protected from active fluvial erosion.
Dark coatings of rock varnish that cover exposed hamada
document the age of the surface and shed light on cli-
matic transitions from wet environments to progressively
more arid conditions (Cremaschi, 1996; Zerboni, 2008).
In some areas, the varnish-coated surfaces are partially
stripped by wind erosion (Cremaschi, 1996).
9.2.2
Stony surfaces: gobi, serir, gibber plains
and desert pavements
Despite the extent and significance of stony mantles in
global deserts, regional differences have not been system-
atically compared. The literature suggests that they vary
