Excavating a polissoir: what did we want to know and how were we going to find that out?

Once we had found the polissoir, and gotten over our excitement, we had to come up with a strategy for what to do next. What could this amazing artefact tell us about Neolithic people? Of course, we researched other examples to compare it to. In England the only other comparable in situ polissoir is located near Avebury at Fyfield Down; it was discovered in 1962 and four small trenches were opened around it in 1963. Others have been discovered in Scotland and there are numerous examples in France.

The Fyfield polissoir (left) and our Valley of Stones polissoir (right) with Cookie the staffy cross rescue dog for scale. Photo © Mike Bennett Past Participate volunteer.

We decided that it would be helpful if we could date the polissoir; was it created and used at the start of the Neolithic when they were starting to clear the landscape of trees (using polished axes) or was it a later object used for other purposes and, if so, for how long a period of time? We also wondered whether we could discover what types of stone axe were polished there, what types of stone did they use and how did they do it?

There were two ways to answer these questions. One was to look at and analyse the stone itself. The other was to look at and investigate the surrounding area.

Firstly, the stone was vulnerable once it had been uncovered. The surface of the polissoir had been untouched for a long period of time but once we told people about it, people would want to touch it, and this could potentially interfere with any surface traces. Historic England conducted a series of high-resolution laser scans, to look at the markings on the surface and to record it for posterity. The results of this scan can be seen in this 3D image.

We also asked Dr Christina Tsoraki to take casts of the worked surface and examine them for traces of microwear, which have the potential to show what materials were being polished and how they were being worked. This is a technique usually reserved for excavated artefacts from archaeological contexts and not left exposed to the elements, but it was felt due to the rarity of the discovery it was worth trying.

Once the moulding and scans were complete it was time to excavate. This was the only way we would find any associated artefacts and was also the best way in which volunteers could be involved. Through excavation we hoped to find remnants of stone tools; dressing flakes or other detritus made when processing axes on the polissoir. The types of stone that these were made from would demonstrate whether only flint found locally was being used, or if they were importing exotic types of rock stones from much further afield. If we found large enough flakes they might provide insights into the types of tools being finished on the site. It was also possible that there were other archaeological features nearby that might provide evidence for how else the site was being used.

We put together a very comprehensive plan of action - it had to pass approval by Historic England and Natural England. The plan was to excavate in 25cm squares, in a chequer board effect, going down in 5cm layers. Everything would be dug using small tools and the exact location of every artefact we found would be recorded using the GNSS (a survey grade GPS that records to very high resolution).

We expected to find significant amounts of natural sarsen stones beneath the surface but we also hoped that we might be able to uncover buried prehistoric soil near the polissoir stone and be able to sample and analyse this using:

-        flotation which washes out the soil matrix from samples enabling the recovery of tiny plant remains which float to the surface (i.e., charred seeds and charcoal) and smaller artefacts, which are retained by a sieve along with other coarse residues,

-        micromorphology, which looks for formation processes in the soil layers e.g., rock particles deposited during polishing or trampling episodes that could relate to the use of the polissoir,

-        radio carbon dating using any well-preserved organic material (i.e., charcoal),

-        Optically Stimulated Luminescence (OSL) dating, which is a method that can date the last time that the quartz particles in the soil saw daylight,

We hoped that combining all of the above techniques would help us establish a sequence of datable events explaining how and when the polissoir was used. This story will be continued tomorrow…