Geology matters 3 – working stone
By Dr Martha Johnson
Certain rocks have always been collected on archaeological sites – stone tools and implements that are classified as “Worked Stone”.
They are meticulously assessed for aspects of work, how they were made and for evidence of wear, how they were used. They are not always assessed as rock, having their petrologic properties recorded and their [possible] source locations identified.
For the propose of this research project, all Worked Stone finds recovered between 2013 and 2015 underwent the same macro petrologic assessment as the Foreign Stone finds. This would not only answer “what type of rock?” and “rock from where?” but also allow comparative analysis between the rock in the Foreign Stone and Worked Stone categories, thus answering a third question – were Worked Stone and Foreign Stone similar or different?
The comparison revealed that all of the Foreign Stone finds were examples of rock known to occur in Orkney and only eight of the 656 Worked Stone finds, subject to petrological assessment, were of a rock not found in Orkney.
Exposure to heat
The rock identified in the Worked Stone finds, though similar to that in the Foreign Stone finds, occurred at different rates (sedimentary vs igneous) and water wear and exposure to heat also differed.
This implies the Foreign Stone rocks were not just “potential” Worked Stones but brought to the site for reasons other than toolmaking.
Knowledge and choices
Crystalline igneous rocks have different physical properties than clastic sedimentary rocks. The interlocking nature of the crystals produce rocks that are usually harder and more tenacious than clastic rocks. These rocks are often recovered as hammer stones, maceheads, axes and grinding stones.
Igneous rocks also are very resistant to degrading when exposed to heat and this might explain the large percentage of heated igneous Foreign Stone finds – these rocks were possibly used for radiant heating or as a heated surface on which to cook.
Clastic sedimentary rocks, by being deposited as sediment on sea floors and later compressed into rocks, often can be cleaved – separated – along the bedding planes of deposition. This property makes them well-suited for quarrying for building stones and grinding into flat or thin tools.
We do not find Neolithic drystone walling of quarried granite – without the use of metal implements, granite cannot be readily shaped into blocks for walling.
Nor do we recover axes of siltstone as their Neolithic prototypes would have been destroyed during their initial use. Trial and error with the available rock shaped the choices made in matching the innate rock properties to the desired uses.
This knowledge made the trips to Hoy or Deerness for basalt or walks to storm beaches to gather water-rounded pieces of Stromness granite necessary. Rhyolite from Stromness and flint from south-east facing beaches was gathered to flake into blades and points. Though grey and tan sandstones are available very near the Ness, it was “important” to journey several miles afield to bring red sandstone to the site.
Why rock? It was, after all, the New STONE Age.
Recovering, identifying and analysing the chunks and bits of rock they brought into the Ness of Brodgar demonstrates their presence at the Ness are not accidental.
Comparing the Foreign Stone rock to the Worked Stone rock identified the differences in the two assemblages. Though often considered a group islands of with a monotonous series of sedimentary rocks, the Neolithic peoples of Orkney would choose to differ with that interpretation.
The people of the Ness demonstrated, through the rock choices identified at the Ness, that they knew what rock they wanted to use for a specific purpose and where to go to get it.
Rocks mattered to the Neolithic Orcadians.