The target area of the SRDS survey is the Sado river basin (southern Alentejo, Portugal), a large, sedimentary basin with an area of 7692 km² and a mean altitude of 127 meters asl. The Sado flows for 180 kilometers in a northerly direction from its source in the Serra da Vigia (230 m asl) before changing its course and flowing west towards the Atlantic ocean. Southern Alentejo forms part of the Iberian Pyrite Belt, largely composed of late Devonian and Cambrian volcanic rocks (tuffs, rhyolites, etc) overlain by metamorphic and sedimentary deposits (including siliceous formations, jaspers, cherts, schists, grauvaques and conglomerates). These deposits form the economic basis of a once-thriving and still active mining industry (Barriga and Carvalho 1997; Martins, Alves, and Costa 2002-2003). The sediments filling the hydrological basin are mostly detritic and continental, overlying marine sediments of Miocene age. Plio-Pleistocene sediments deposited on the broad erosional surface that formed as the sea retreated (the planalto deposits) were in turn dissected and reworked by the river system that began forming during the Pleistocene. The region is caracterised by poorly developed soils (mainly derived from bedrock schists).

The history and topography of the target region is heavily influenced by tectonic processes. Subsidence of the Sado basin and the formation of the Grândola horst at the beginning of the Quaternary (Pimentel & Azevedo 1991) resulted in distinctly different sedimentary histories in the northern and southern halves of the Sado drainage system. The northern part of the drainage, from Grândola to the estuary, forms part of the litoral platform; sediments in this sector are predominantly composed of fine sands, easily remobilised by aeolian processes. Fluvial terraces in the northern sector are difficult to identify or age (Pimentel 1989). The southern part of the drainage, from Grândola to the Serra da Vigia, is essentially an extended erosion surface upon which a braided fluvial system formed during the Plio-Pleistocene, depositing a succession of sandy, detritic sediments cloaked with coarse rañas (Pimentel 1989, Pimentel and Azevedo 1991). South of Grândola, the Sado river is weakly incised forming two, heavily dissected terraces (Pimentel 1989, Pimentel and Azevedo 1991).

Raw material sources

Primary sources of raw material suitable for knapping are rare in the survey region. There are no known sources of flint, although chert is occasionally encountered in very small quantities. Jasper occurs in outcrops on the margins of the Sado basin, especially in the south of the survey region. Other potential raw materials including quartz, quartzite, grauwacke and jasper cobbles, occur in secondary contexts throughout the drainage system.