Geophysics at The Cairns 2017

Gordon (2)Whilst the excavation at The Cairns has been up and running, myself and Gordon Higgs (a student from Sheffield University) have both been undertaking professional Placements with the project to undertake geophysical survey in the fields surrounding The Cairns.

The aims of this survey were to investigate the landscape of the Broch and to identify the precise location of the enclosure ditch around the front, (Eastern side) of the broch and to identify any evidence for additional human activity nearby.

Two different types of geophysical survey were used, resistivity and magnetometry. Resistivity survey uses an electric current to measure the resistance of the soil, from which it is possible to identify disturbance (including past human disturbance) in the soil. Resistivity is ideal for detecting ditches and stone built features, because these features will have a different resistance from the surrounding undisturbed soil. Magnetometry survey measures changes in the magnetic field which can detect heat affected and burnt features such as hearths or fuel ash middens, metal artefacts and other past disturbances.Maginterprated-page-001The results of the survey show that the landscape has been heavily ploughed in the recent past and has disturbed archaeology in places. Nevertheless, the ditch surrounding The Cairns has shown up in both types of survey. Indeed, the anomaly on the survey results showed that it appears to be a double ditch feature. These substantial ditches are passed through by a linear feature, which is a couple of metres wide and runs from the broch sown slope towards the coast. This feature appears to be an ancient ‘hollow way’, or sunken track, and could also be partly related to a lintelled passageway referred to by the antiquary the Rev. Alexander Goodfelow over a hundred years ago.

At the bottom of the field just south of The Cairns an earlier geophysical survey showed an arc in the corner of the survey. A test pit was excavated there earlier this year and identified a curving stone feature that looks like a well-built wall within an ashy soil matrix, and a fragment of a saddle quern of prehistoric type. When we extended the geophysical survey to cover more of this anomaly it showed a series of semi circular features with possible central hearths. These anomalies could be related to houses in a fairly large settlement and/or associated structures like workshops.

There may be as many as 6 or 7 of these buildings. These look very similar to Bronze Age houses seen in the World Heritage Area of Stenness/Brodgar and in other locations in Orkney and Shetland.  This exciting prospect could be tested by excavation to confirm the nature of these features.

We were hoping to find the continuation of these structures in the next field to the south, however, the remains have either been ploughed away or are smaller than expected and obscured by the field boundary itself. This lower field did show another possible circular feature and a very strange linear anomaly on the resistivity results. An igneous dyke (or possibly two) run through the lower field which was easily identifiable by our magnetometer results.  This natural geological feature is interesting in its own right as we know prehistoric communities treated volcanic dyke rock as a resource, sometimes quarrying such sources of igneous material for stone artefacts and crushed up the rock for the ‘temper’ that they mixed into the clay of ceramic pots.

There are many more discoveries in the geophysics survey undertaken and the survey work has confirmed that The Cairns broch was not an isolated feature in an empty landscape, it was part of an active, evolving landscape and it would be interesting to test some of these features to confirm if some of them are contemporary with the broch. Even though the weather seemed to be against us some days, the results from the geophysical survey have been very informative, hinting at the wider landscape and future stories for archaeologists to uncover.

Leonie Teufel, MSc. Archaeological Practice Student, UHI