Post
by Digit » Sat Jan 15, 2011 4:17 pm
Better late than never I suppose, but I thought this might be of interest...
Geochemistry, Sources and Transport of the Stonehenge Bluestones
O Williams-Thorpe & R S Thorpe
Stonehenge on Salisbury Plain, UK, is famous for its construction from large lintelled sarsen stones, and also because it has been proposed that some of its stones — the bluestones which are foreign to the geology of Salisbury Plain — were brought to the site by humans from a distant source in Preseli, South Wales. The bluestones include hard dolerites (mostly ‘spotted’) and rhyolites, and softer structurally unsuitable sandstones and basic tuffs. Chemical analysis of eleven dolerites originated at three sources in Preseli within a small area (ca. 2 km2), while the rhyolite monoliths are from four different sources including localities in northern Preseli and perhaps on the north Pembrokeshire coast, between 10 and 30 km apart. Opaque mineralogy of the dolerites supports the conclusion of a Preseli source, while modal analysis of a sandstone fragment excavated at Stonehenge shows that it is not from the Cosheston or Senni Beds of South Wales, as has been suggested. This variety of source implies selection of material from a mixed (glacial) source, not at a carefully human-chosen outcrop. Glacial erratic material from south-west Wales has been identified as far east as Cardiff, and early (Anglian) glaciation of the Bristol/Bath area is indicated by an erratic find and glacial landforms. The apparent lack of glacial erratics between Bristol and Stonehenge (except perhaps for the Boles Barrow boulder) and in rivers draining Salisbury Plain, is consistent with the irregular deposition of ‘free’ boulders at the edge of extensive ice sheets. Bluestone fragments on Salisbury Plain without clear archaeological context, and pieces incorporated, sometimes apparently accidentally, in monuments of Neolithic age onwards (some predating the bluestone erections at Stonehenge) may be remnants of erratics. Clearance of boulders from Salisbury Plain for agricultural purposes is clearly described by the geologist J. A. de Luc, and a boulder consistent in appearance with an erratic was found at Stonehenge in the 1920s.
It is concluded that the bluestones of Stonehenge were available locally to the builders, and were transported from south Wales not by humans, but by glacial activity of perhaps the Anglian period (ca. 400,000 years BP) or earlier. This conclusion has prompted re-examination of other suggestions of long-distance transport of megaliths. The sarsen stones at Stonehenge need not have been brought from 30 km to the north as has been suggested, since recent surveys show small concentrations of sarsens near Stonehenge, the remnant of boulders largely cleared during 18th–19th centuries. Calculations of the manpower required to construct Stonehenge need to be re-assessed in view of the absence of long-distance stone transport. Other megaliths in Britain and in northern Europe show no evidence for stone transport of greater than ca. 5 km, and reveal a preference for use of erratics in some glaciated areas. In at least some cases the availability of stone has dictated the location of the monuments. It is therefore inappropriate to interpret the positions of megaliths in terms of social or economic territories without first examining the geological constraints on their siting.
PBA 77, 133–161 © The British Academy 1992
Roy.
First people deny a thing, then they belittle it, then they say it was known all along! Von Humboldt
