In Depth - Uyea, Northmavine, Geophysical Survey
by Valerie Ward
Figure 1: Archaeology Shetland members doing geophysics at Uyea, Shetland
On the 6th of June 2026, a group of volunteers from Archaeology Shetland conducted their first geophysical survey at a site in Uyea, Northmavine (Figure 1). The target was a c.15m diameter circular feature denoted as Cuml on the First Edition National Grid series (OS) map. Cuml, short for cumulus, is a unique term used by the OS to represent prehistoric sites, often cairns of pre-Roman Age (OS 1921)(Figure 2). However, many of these Shetland cumulus sites have been re-evaluated and subsequently reclassified as Neolithic/Bronze age houses or farms or as chambered cairns. S. Jennings (2021) proposed that due to a gap in the patterns of Iron Age (IA) habitation in this area it is also possible that the previously recorded pile of stones could be the remains of an Atlantic roundhouse. The site has subsequently been removed from the landscape and is no longer visible on the ground or on satellite imagery. During the ground survey, part of a large broken trough quern was found being reused in a field
wall subdivision within metres of the target area. The proximity of the trough quern indicates likely domestic habitation in this area at some point between the Neolithic and Early Iron Age.
Electrical Resistance (ER) and Gradiometry surveys were conducted to assess if any buried remnants of the original feature remain and if so, what they may indicate. ER is ideally suited to detecting stone walls which have high resistivity and ditches or infilled areas which retain moisture better and exhibit low resistivity values. This method is, however, sensitive to rainfall. Gradiometer survey is best at detecting hearths, pits and ditches or ferrous objects. Thermoremanent Magnetisation is associated with hearths, kilns and furnaces. This produces a strong dipolar/bipolar magnetic response c.100nT. The disposal of midden material as infill of pits and ditches, as soil enhancement or dispersed around high frequency areas of human activity produces a weak to moderate positive magnetic anomaly of 1-10nT (Schmidt et al 2015 ). Both methods measure within the top 0.5-1.0m of the ground surface and therefore can be affected by depth to bedrock and overlying drift geology ( peat, glacial till etc) variations. The bedrock geology at Uyea which is situated close to the Neolithic axe factory is composed of metamorphosed igneous rocks subsequently intruded by
Figure 2: Cuml, short for cumulus, Ordnance Survey
later igneous dykes. This type of bedrock geology is not ideal for detecting small magnetic signatures associated with archaeological features. We did indeed find it difficult to stabilise the sensors of the Gradiometer in these magnetically noisy conditions. However, from late morning to mid-afternoon we were able to survey three 20x20m grids of both ER and Gradiometer data, and they confirmed the presence of a circular/sub-circular structure at the target area (Figure 3). One of the grids of Gradiometer data was repeated with different tool acquisition settings to assess their influence on the results which subsequently showed the grids were repeatable and not affected by the change in the range parameters.
Figure 3: Circular/Sub-circular Structure
The quality of the Gradiometer data is moderate given that the device had difficulty stabilising. This resulted in vertical striping in the acquisition direction. The quality of the ER data is good although a few vertical stripes are noticeable in the area of the circular anomaly (Figure 3). The ER vertical stripes occur as a result of the probes being inserted to varying degrees which can be caused by variable soil softness, or because of a changing user. While some of this striping can be removed with processing it is best to try to mitigate this through training and practise.
Both the ER and Gradiometer data underwent processing with Snuffler® and Geoplot® respectively to merge, de-spike, de-stripe and interpolate the results prior to interpretation (Figure 3).
The interpretation of both the surveys is shown in Figure 4. The ER survey detected a high-resistance broadly circular 12-14m diameter anomaly at the estimated location of the target cumulus (er1). This anomaly is consistent with a circular stone-rich area most likely the remains of a prehistoric structure, although less likely it could also be caused by quarrying into the bedrock where the centre has thicker soil. The target area appears to be relatively magnetically quiet with only one small anomaly close to the top centre of the circular ER anomaly (m3). Given the size and location of the site and the proximity to a broken trough quern, it is suggested that the Uyea cumulus site is likely to be the remains of a Ne/BA house. This is similar to other reclassifications of Cumulus sites in the wider area. However, the possibility that it is a cairn or roundhouse is not ruled out.
In addition, learning points regarding the organisation and running of a volunteer archaeology survey have been identified and noted in the full report. The next survey would benefit from targeting a site which overlies
Figure 4: Survey Interpretation
simple Devonian sandstone and siltstone bedrock geology in areas on the east of Shetland from Lerwick, & Bressay, southwards to Sumburgh.
For further details of the acquisition, processing and interpretation at Uyea click here for a link to the Archaeology Shetland Internal Geophysical Report.
For anyone is interested in learning how to use either the Gradiometer or Earth Resistance tools, a short training pack is available here. There will be further opportunities to take part in archaeogeophysical surveys in 2027.
Kind thanks to Aberdeen University for the loan of the Gradiometer and to the Bressay History Group for the hire of the Earth Resistance Tool.
Past In Depth articles can be found in the Archive.