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This paper presents the first intensive integrated field and laboratory study of an area of typical silcrete occurrence in the UK, and enables parallels to be drawn with in situ silcretes in the neighbouring Paris Basin and other parts of northern Europe. Silcrete distribution in the eastern South Downs is localized and discontinuous, with occurrence principally as dislocated boulders on the Chalk. These boulders mainly occur between the Cuckmere and Goldstone valleys, with greatest concentrations around Stanmer, Falmer, Rottingdean, and in the Goldstone valley. Many occur at higher levels in the landscape, on or near interfluves usually peripheral to outliers of Palaeocene sediments and in close association with Claywith-Flints. At lower levels they occur on valley floors and on the coastal plain within Quaternary sediments. Boulders\ud commonly have a-axis dimensions of 0·5–2 m and thicknesses of 0·3–0·6 m. Silcretes in higher positions typically exhibit angular tabular or prismatic shapes whilst those in derived settings are more rounded, suggesting weathering and erosion during transport from a localized high level silcrete lens (or lenses). Three varieties of silcrete have been identified. Pale grey ‘saccharoid sarsens’ are most widespread, and exhibit a simple grain-supported (GS-) fabric with predominantly sand-sized\ud quartzose sediment cemented by optically continuous quartz overgrowths and minor micro- and cryptocrystalline silica.\ud Brown ‘hard sarsens’ occur as smaller blocks, mainly in the Seaford Head area. These also exhibit a GS-fabric but with more fine sediment and a greater variety of cement types. Textural and geochemical evidence suggests there is gradation between the saccharoid and hard varieties of sarsen. Conglomeratic ‘puddingstone’ is common in the Goldstone valley but rare elsewhere, and shows closest similarity to saccharoid sarsen in texture and cementation. Silcrete micromorphology is universally\ud simple and uniform, with fabrics suggestive of groundwater or drainage-line silicification. The uniformity of sediments within all three types of silcrete indicates a common host, suggested to be the Upnor Formation of the Lambeth Group (Palaeocene). Deposition of the host sediment appears to have been followed by illuviation of clay-rich material via the primary fabric and along cracks to form geopetal caps, drapes and vein-like structures. During silicification this finer material has been replaced by less well-ordered silica whilst optically continuous quartz overgrowths characterize ‘purer’ areas. The silcrete was also affected by late-stage influxes of iron oxides, clay minerals and occasionally calcite. On the basis of distributional and other evidence it is suggested that silicification occurred during the Neogene or Quaternary, in association with acid leaching of Lambeth Group sediments probably under temperate conditions. The timing of silcrete formation in the eastern South Downs is thus much later than proposed for other parts of the UK

This study utilises geochemical provenancing of silcrete raw materials, in combination with chaîne opératoire analyses, to explore lithic procurement and behavioural patterns in the northern Kalahari Desert during the Middle Stone Age (MSA). New data from the sites of Rhino Cave, Corner Cave, and ≠Gi in northwest Botswana, combined with earlier results from White Paintings Shelter, reveal that the long distance transport of silcrete for stone tool manufacture was a repeated and extensively used behaviour in this region. Silcrete was imported over distances of up to 295 km to all four sites, from locations along the Boteti River and around Lake Ngami. Significantly, closer known sources of silcrete of equivalent quality were largely bypassed. Silcrete artefacts were transported at various stages of production (as partially and fully prepared cores, blanks, and finished tools) and, with the exception of ≠Gi, in large volumes. The import occurred despite the abundance of locally available raw materials, which were also used to manufacture the same tool types. On the basis of regional palaeoenvironmental data, the timing of the majority of silcrete import from the Boteti River and Lake Ngami is constrained to regionally drier periods of the MSA. The results of our investigation challenge key assumptions underlying predictive models of human mobility that use distance-decay curves and drop-off rates. MSA peoples in the Kalahari appear to have been more mobile than anticipated, and repeatedly made costly choices with regard to both raw material selection and items to be transported. We conclude that (i) base transport cost has been overemphasised as a restrictive factor in predictive models, and (ii) factors such as source availability and preference, raw material quality, and potential sociocultural influences significantly shaped prehistoric landscape use choices.3

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Groundwater silcretes have been recognized recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further the understanding of the micro-fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In-depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of siltand clay-sized particles. Finer-grained silica cements occurred in isolated patches and as vein-or cap-like geopetal features. These caps and veins formed through the silicification of illuviated clay-rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine-grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferruginization, characterized by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub-basins), suggesting that there may be a common mechanism for silcrete formation across north-west Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific 'era' of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene.

The sources of the stone used to construct Stonehenge around 2500 BCE have been debated for over four centuries. The smaller “bluestones” near the center of the monument have been traced to Wales, but the origins of the sarsen (silcrete) megaliths that form the primary architecture of Stonehenge remain unknown. Here, we use geochemical data to show that 50 of the 52 sarsens at the monument share a consistent chemistry and, by inference, originated from a common source area. We then compare the geochemical signature of a core extracted from Stone 58 at Stonehenge with equivalent data for sarsens from across southern Britain. From this, we identify West Woods, Wiltshire, 25 km north of Stonehenge, as the most probable source area for the majority of sarsens at the monument.

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