2013:024 - Ballynagalliagh, Derry

At Ballynagalliagh the CAF commenced a project to evaluate different forms of archaeological evaluation and testing. An area of approximately 40acres, which is zoned for a charitable housing development is, over the course of a number of years, to be tested using a number of different evaluation types, trench densities, layout patterns, informed by geophysics in some trenches and without the assistance of geophysical survey in others to provide a number of datasets which may be compared against each other to compare methods of uncovering archaeological remains from a green field site.. To this end the Ballynagalliagh site has been subdivided into a grid of 50m squares. Over the course of the next few years up to 75 squares will be evaluated, with the first ten squares tested in 2012.
The presence of archaeological remains at the site is not known but given the size of the development area and the presence of Early Medieval and prehistoric remains in the fields around the development site it seems very likely that there are some remains of different periods on the site.
In advance of the project a geophysical survey of the site was carried out (Gimson 2012) which has identified a considerable number of geophysical anomalies which might be archaeological features. This survey used two geophysical survey methods, magnetometry and earth conductivity.
There are several questions which can be addressed by this project: What percentage of features detected by geophysical survey are ground “proved” by either test trenching or topsoil stripping? What percentage of archaeological features found by either test trenching or topsoil stripping are not detected by geophysical survey? Can geophysical survey be used to improve the ability of trial trenching at a density of 10% to find sites compared with “blind” trial trenching at the same density? Can geophysical survey be used to improve the ability of trial trenching at a density of 5% to find Early Medieval or prehistoric sites? Is combined trial trenching and geophysical survey as effective at finding archaeological features as monitoring topsoil stripping on its own?
To answer these questions three main types of testing strategy are envisaged: 22 squares, spread throughout the development are going to be tested at a sampling interval of 10%, without the benefit of the geophysical survey to place the squares but using, in rotation, one of three grid patterns shown by Hey and Lacey (2001) to be the most effective. Twenty one squares are going to be tested at a sampling interval of 10% but with the location of the trenches decided with the assistance of the geophysical survey data. Twenty two squares are going to be tested at a sampling interval of 5% with the location of the trenches decided with the assistance of the geophysical survey data.
In 2012 ten 50m by 50m squares, containing within them 46 evaluation trenches, were excavated. This initial area was not thought likely to contain archaeological features from the results of the geophysical survey and no archaeological features were uncovered although a number of linear features, which matched linear anomalies on the geophysical survey, were found. These are probably relatively recent field boundaries removed in the later 19th or 20th century to make larger fields for modern agriculture.
At the end of the test trench evaluation it is proposed that the topsoil stripping by the developer, in advance of actual construction, is monitored to see if any archaeological features, not identified by the geophysical survey and test trenching are uncovered
References
Hey, G. and Lacey, M. 2001 Evaluation of Archaeological Decision-making Processes and Sampling Strategies, Oxford Archaeological Unit, unpublished report carried out for Kent County Council with EU funding.
Gimson, H. 2012-08-01 Proposed Development Galliagh, Londonderry, Archaeological Geophysical Survey, Earthsound Geophysics, unpublished report carried out for Dingles Builders (NI) Ltd.
Cormac McSparron, Centre for Archaeological Fieldwork, School of Geography, Archaeology and Palaeoecology, Queen’s University Belfast.
Editor’s note: Although excavated during 2012, the report on this site arrived too late for inclusion in the bulletin of that year.