Geoscience Reference
In-Depth Information
Pavements and stone mantles
Julie E. Laity
9.1
Introduction
or hamada and Sharon (1962) uses the term hamada in the
context of a serir.
The formation of desert pavements has been inten-
sively investigated over many decades, particularly in the
arid southwestern United States and Israel, where modern
models of pavement formation were largely developed.
This interest shows little sign of waning. The expansion
of research to new regions, such as the UAE, Patago-
nia and Australia, has provided additional data, which
show that stony mantle formation, while sharing many
processes in common, also varies spatially, reflecting dif-
fering parent materials, weathering processes and aeolian
environments.
In arid and semi-arid environments, soils commonly con-
tain rock fragments, whose size and cover affect a range of
geomorphic processes, including wind and water erosion,
infiltration, evaporation, runoff generation and soil forma-
tion processes. A mosaic of soils and geomorphic surfaces
develops, ranging in age from those only recently formed
to relict features from the Pleistocene or even earlier. This
chapter examines the nature of two widespread covers:
stony mantles and rocky or bouldery surfaces (hamadas).
Mantles of stony material have been given many names
according to their geographic location, including stone
pavement (desert pavement), reg, serir, gobi and gibber
plain. The degree to which these terms are synonymous is
unclear, as a systematic global comparison of the differ-
ent named surfaces has not been conducted. Considerable
research efforts have been devoted to understanding the
nature and formation of stone pavements. This interest is
merited, as they are estimated to be the most extensive
of desert surfaces, covering more than 50 % of the land
(Evenari, 1985). By contrast, hamadas have received
much less attention and these features are poorly defined.
Although hamadas and stone pavements are discussed
separately in this chapter, their distinction is often blurred.
As Mabbutt (1977) points out, hamadas may become
smoothed over time as fragments weather and intersti-
tial soil develops through aeolian and fluvial inputs, or
wash and animal movements redistribute material. There-
fore, hamada slopes may be transitional to pavements,
both spatially (an upper hamada slope and lower pave-
ment region) and temporally (hamadas are weathered to
form pavements). In some cases, no distinction between
the two types of surfaces is made in the literature. Twidale
(1994) considers the gibber plain to be equivalent to a reg
9.2
Surface types: hamadas and
stony surfaces
9.2.1
Hamada
Hamada
(
hammada
) is an Arabic word denoting difficult
bouldery terrain that is 'unfruitful' to cross. In Australia,
the term
stony tableland
has also been employed (Mab-
butt, 1977). Although hamada surfaces are extensive, their
development and geomorphic significance have received
little study. Additionally, they are not always clearly de-
fined and there is some contradiction in the use of ter-
minology. Sharon (1962, p. 130) described hamada in
the Negev and Sinai Deserts as being 'covered by quite
closely backed angular rock fragments forming a desert
pavement', and later noted that the surface should be clas-
sified as serir (p. 132), although 'hamada' followed local
Israeli and Arab usage. Zohary (1945, p. 14) used hamada
in the context of 'gravelly desert plains', more similar to
