Determining the source of silcrete sarsen stones

Abstract

Materials used in the construction of man-made structures can reveal information about societies, their environment, culture and mobility. Cultures throughout human history have used natural stone to construct funerary monuments and tool technologies. Stone materials from igneous rocks can be provenanced by petrographic examination and geochemical methods that provide a direct link between the artefact and their source. For sedimentary sarsen stones, provenance is often complicated by diagenetic processes acting on source rocks. Here I use an example of stones used to build 6000-year-old earth and sarsen megalith structures from the River Medway valley (Kent, UK). These megalith structures lie on the strategic and pre-Neolithic North Downs migration route into Britain, but the provenance of the sarsens used in their construction has not been established. The highly competent sarsens are diagenetic groundwater silcretes, resulting from acid leaching of glauconitic clay components, and addition of a quartz cement. Heavy rare earth and high field strength element abundances and ratios are resistant to diagenetic effects and show a geochemical association with Paleogene sandstones from the Upnor Formation of the Lambeth Group at the 95% confidence limit. Detrital zircon and titanite grain compositions confirm this relationship. Preferential erosion and removal of the less competent sandstone around silcretes left large (>2 m), durable and isolated sarsen blocks on top of the chalk downs that were subsequently exploited by Neolithic peoples. Use of local stone to construct the Medway megaliths is consistent with their distinct morphological characteristics that have been used to imply independence from continental European cultures at that time. Combined with careful field collection of possible source rocks and sarsen materials, provenance of stones of sedimentary origin that have been affected by diagenesis can be accurately assessed through a combination of petrography, bulk-rock heavy rare earth element and high field strength element abundances, and detrital mineral compositions.