Geochemical fingerprinting the sarsen stones at Stonehenge

Description

Overview
The sources of the stone used to construct Stonehenge have been debated by archaeologists and geologists for over a century. The smaller Bluestones, derived from west Wales, have attracted most attention. In comparison, virtually no work has been done on the sources of the larger sarsen stones (silcretes) used to construct the central Trilithon Horseshoe, outer Circle and peripheral settings. Conventional wisdom suggests that, given their size, the sarsens were all sourced from the Marlborough Downs. However, petrological, mineralogical and laser-scanning analyses indicate considerable variability among the Stonehenge sarsens, making this assumption questionable.

The most accurate means of determining the provenance of any stone artefact is geochemical fingerprinting, whereby the elemental chemistry of the artefact is matched against that of potential source areas. For Stonehenge, this would require two stages:

1. an initial analysis of the sarsen stones at the monument,followed by
2. equivalent analyses of sarsen boulders across their range of natural occurrence (i.e. south of a line from Devon to Suffolk).

Project aims
This project, funded by British Academy/Leverhulme Small Research Grant SG170610, addresses the first of these stages. The primary aim is to determine, for the first time, the chemical variability of the sarsen stones at Stonehenge. This will allow us to assess whether, as appears likely from laser scanning data, there are stones with differing chemistries (and hence potentially different sources) at the monument. 
Our second aim is to undertake pilot chemical analyses of sarsen boulders from selected areas of southern England, including clusters close to Stonehenge (e.g. Marlborough Downs) and more distant (e.g. Hants, Sussex, Kent, Suffolk). This will allow us to scope, for the first time, the chemical variability in natural sarsen occurrences, and more efficiently target areas for intensive investigation in the future.

To avoid the need to core at the monument, we plan to analyse samples from two collections of sarsen fragments from Stonehenge. These derive from two sets of excavations in 2008 (led by project partners Tim Darvill and Mike Parker-Pearson) and represent a selection of sarsens from the monument. The first, between the sarsen Circle and Trilithon Horseshoe, uncovered sarsen debris from stone-breaking, possibly used as packing materials. The second, to the north of Stonehenge, revealed extensive working debris from two areas where sarsens were dressed prior to erection.

We will analyse each sarsen fragment in these collections using non-invasive pXRF, to identify broad chemical groupings. Based on this, we will select 100 samples from across these groups for combined ICP-MS and ICP-AES analyses. A similar approach will be used to select 40 samples for pilot analysis from the extensive collection of sarsens from southern England held at the University of Brighton. ICP-MS/AES data will be assessed using standard geochemical provenancing approaches to define any chemical groups within the Stonehenge samples, and provisionally match these groups against source areas. Following these analyses we will select samples for further investigation, including via a novel combination of petrological, cathodoluminescence and Qemscan analyses of polished thin-sections, to crosscheck any potential geochemical match.

Key findings

The project is in progress, with a completion date of March 2019. Key results and links to publications will be added here as the project progresses.
StatusActive
Effective start/end date1/10/1731/03/19

Keywords

  • Stonehenge
  • silcrete
  • geochemistry