Geology matters 1 – why rock?
By Dr Martha Johnson
Archaeology concerns itself with the study of past people and cultures. It involves the examination of the physical remains of these cultures – their materials, objects and structures.
Depending on the age and location of the site, objects of wood, bone, shell, clay, stone and/or metals are recovered and studied along with any structural evidence.
From this data, an interpretation of the people, their actions and their culture is made.
An archaeological exhibit from a Neolithic site will usually includes examples of blades and points crafted from flint, various examples of pottery, bone or stone tools, animal bone and photos or models of any structures from the site.
Today, these exhibits may also present the findings of scientific techniques such as pollen studies, pottery residue analysis, soil studies, micromorphology analysis or radiocarbon dating.
The tools, pots, bones and buildings combined with the results of the various analyses provide windows through which we can study peoples of the past and begin to understand their activities and interpret their cultural practices.
Many sites also contain a less obvious material which can serve as an indicator of the occupants’ knowledge of their surroundings and the choices they made in selection of this material – rock.
Beyond the stone tools – some magnificent, most rather humble – Neolithic sites often have a variety of loose, unworked, uninteresting rocks.
At the Ness of Brodgar these chunks and bits of “just rocks” have been recovered, assessed, identified and the collective data they produced analysed.
Investigating rock at the Ness
Between 2013 and 2016, research was conducted to petrologically assess and identify the non-tool, non-structural rock – termed “Foreign Stone” – recovered from the Ness of Brodgar excavation.
These Foreign Stone finds were to be consigned to the spoil heap as the “rocks that didn’t belong” – rocks with no apparent use or purpose. Assessment of these finds would answer the first of two research questions: the identification of the rock.
A macro petrologic assessment of colour, texture/grain size, composition, heft/specific gravity, visible minerals, nature was conducted on each rock.
From these properties, the rock was identified. The archaeological data, trench, structure and context, were also recorded for each Foreign Stone find.
An informal assessment of rock from the spoil heap in 2012 saw two non-geological aspects added to the assessment of each Foreign Stone find – evidence of water wear and evidence of exposure to heating.
The lochs on either side of the Ness do not generate enough wave action to water-wear and round rocks.
Recovering water-worn Foreign Stone would indicate deliberate transport to the Ness because the nearest “storm beaches” are near Stromness, some miles away.
A rock discoloured or degraded by fire would also indicate interaction with people. All assessment categories archaeological and petrological were recorded in a database for analysis.
To answer the second research question – rock from where? – a review was conducted of current and archival references on the petrology and geology of Orkney. This included the geology of the Ness of Brodgar isthmus and the greater West Mainland area as well as extending across the entire archipelago.
With a knowledge of the range of rocks reported in Orkney and their source locations, the rock identified in the Foreign Stone category was sourced to a location or to several possible locations.
This process included noting Foreign Stone finds not known to outcrop in Orkney.
To be continued…
- Rock matters: A geological basis for understanding the rock at the Ness of Brodgar. Dr Martha Johnson (2019)