Desert rivers - Climatic Change in Deserts - page 160

Figure 10.1. Pleistocene alluvium of the Awash River, southern Afar Rift, Ethiopia.
(After Williams et al., 1986 .)
defunct river valleys in the Namib and Mauretanian deserts are sometimes lined with
beds of hard secondary calcium carbonate or calcrete (see Chapter 15 ), often rich
in uranium (Netterberg, 1980 ; Dingle et al., 1983 , p. 294). These calcretes represent
former alluvial deposits that were subsequently cemented by groundwater rich in
dissolved calcium carbonate - a legacy of past climatic changes.
The great sandstone plateaux of the eastern Sahara, such as the Gilf Kebir in
hyper-arid south-eastern Libya and south-western Egypt, are deeply dissected by now
inactive river valleys along their crenulated margins (Bagnold, 1933 ; Sandford, 1933 ;
Peel, 1939 ; Peel, 1941 ), as are the more recently discovered sandstone plateaux in
southern Libya to the north-east of Tibesti (Williams and Hall, 1965 ; Pesce, 1968 ;
Griffin, 2006 ; Griffin, 2011 ). Peel ( 1966 ) and many later workers have emphasised
the former efficacy of such fluvial erosion (e.g., Griffin, 2006 ; Griffin, 2011 ;Reid,
2009 ), which they consider strong evidence of formerly wetter or 'pluvial' climates
(see Chapter 12 ). Carol Breed and her colleagues have made brilliant use of shuttle-
imaging radar to map a whole series of late Cenozoic river channels in the eastern
Sahara, some with associated prehistoric stone tool assemblages ranging typologically
from Acheulian to Neolithic or from Early-Middle Pleistocene to Holocene in age
(Breed et al., 1987 ; McHugh et al., 1988 ; McHugh et al., 1989 ).
Many of the great rivers that flow south from the Atlas Mountains, such as the
Saoura, are flanked by alluvial terraces that contain pollen indicative of past changes
in the vegetation cover in their headwaters (Beucher, 1971 ;Maley, 1980 ;Maley, 1981 ).
The rivers that once flowed south from Tibesti into a much-expanded Lake Chad, or
'Mega-Chad', terminate in a delta dating from early to mid- Holocene (Schneider,
1967 ; Ergenzinger, 1968 ; Servant et al., 1969 ; Maley et al., 1970 ; Servant and Servant-
Vildary, 1980 ; Drake and Bristow, 2006 ), when much of the Sahara was studded with
small lakes (Faure, 1962 ; Hugot, 1962 ; Faure et al., 1963 ;Faure, 1966 ;Faure, 1969 ;