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In
common with other Saharan regions, the Free Zone of Western Sahara is
characterised by an arid environment containing numerous indicators of
past humid conditions. In the north, dense networks of drainage
channels focus runoff into a number of occasionally active wadis which
flow into the Saguia el-Hamra, a large ephemeral river. Other wadis are
filled with accumulated sediment and appear to be inactive. Sand and
gravel plains, elevated plateaux, hills (occurring in isolation and in
ranges) and extensive playa surfaces are major features of the
landscape. Topography is more subdued in the southern regions, in which
groups of hills rise from a generally flat landscape of sand and gravel
plains in which palaeochannels are visible in satellite imagery.
Surface geology is dominated by limestone in the north-east, sandstone
in the northern and central areas, and granite in central and southern
areas, although there is considerable heterogeneity in the surface
geology in all regions. Basalt dykes are common in both northern and
southern areas, and the far south-east of the territory extends into a
region of dunes and sand sheets. Rainfall today is higher than in other Saharan regions located at similar latitudes. While there are currently no meteorological stations in the Free Zone, climatological maps from the middle of the twentieth century indicate a maximum mean annual rainfall of around 50 mm in central and northern regions (Dubief, 1959). Rainfall is associated with both mid-latitude and monsoonal systems, and is sufficiently abundant for the local Sahrawi people to have a concept of drought and to practice mobile pastoralism. While the timings of past transitions from humid to arid conditions and vice versa are yet to be determined in Western Sahara, the archaeological record, palaeo-environmental reconstruction from adjacent regions, and preliminary radiocarbon dates suggest that climatic variations reflected those of the greater Saharan region throughout the Quaternary. The presence of extensive sheets of fluvial gravels containing Acheulean and Mousterian lithics on the margins of major wadis and on the flanks of the surrounding hills suggests an extensive river system in the Pleistocene (Brooks et al. 2003, 64). Accelerator Mass Spectrometry (AMS) dating of humic material from a rockshelter at Irghayra in the Northern Sector of the Free Zone yielded an uncalibrated radiocarbon date of 6210 ± 80 BP, during the last Holocene humid phase in the Sahara when greater summer solar insolation at middle latitudes was associated with a stronger monsoon and winter rainfall appears to have been greater in many Saharan regions (Rodrigues et al., 2000). This humid phase was interrupted by a number of regional and local arid episodes, the most intense and widespread of which occurred around 8 kyr BP (Gasse, 2002). Bulinus truncatus shells from a Northern Secor playa site (site L3) yielded an uncalibrated date of 4,020 ± 40 b.p. (Brooks et al. 2003), coinciding broadly with the transition from arid to humid conditions throughout the greater Saharan region (de Menocal et al, 2000). The precise timing of this transition varied geographically, with desiccation progressing more rapidly in eastern and central regions, where aridification commenced in the 7th millennium BP and accelerated around 6 kyr BP, when evidence from across the northern hemisphere is indicative of a cold, arid episode (see Brooks, 2006 for a review). Monsoon collapse occurred around 6400 BP in the Gilf Kebir, followed by the cessation of winter rains around 5300 BP (Linstädter and Kröpelin, 2004). In the central Sahara monsoon rains appear to have persisted until the end of the 6th millennium BP, while winter rains in the Fezzan region of Libya continued until the earl y 3rd millennium BP (Cremaschi and di Lernia, 2001; Cremaschi et al., 2006). Evidence from palaeolake sediments in more westerly regions indicates transitions to aridity by around 4.5 kyr on the northern margins of the Sahara, by around 5 kyr in the southern Sahara, and after about 4 kyr BP in the Sahel (Gasse, 2002). In the coastal region of central Mauritania, Kocurek et al. (1991) found evidence for wetter conditions than at present from 11,000 b.p. to 3,000 b.p. with the possibility of a dry interval between 7,000 and 6,000 b.p. Lancaster et al. (2002) identify a period of increased aeolian activity after 5 kyr BP in Mauritania, at latitudes located some 100-300 km south of the southern limit of Western Sahara. While the two dates acquired for Western Sahara are consistent with regional trends, the environmental evolution of Western Sahara is likely to have been complex, due to its extension into the monsoon zone coupled with its proximity to the Atlantic and (in the north) the Atlas mountains, resulting in this region being subject to influences from a variety of different climatic regimes and hydrogeological systems. Furthermore, environmental trajectories might be expected to be different in the northern and southern sectors of the Free Zone. Ongoing environmental work is aimed at developing chronologies of environmental change from a variety of indicators in the Free Zone, in order to provide a context within which social and cultural changes may be interpreted. References
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