4.3 Discrete magnetic features

Given the known archaeology, many of the small magnetic anomalies are likely to represent pits, hearths, large postholes or similar cut features, though again these are difficult to distinguish from the background geological activity. It is probable that some of these relate to post buildings, although none can definitively be identified as such. As the great majority of postholes identified during the excavations were substantially less wide than the distance between the traverses (0.5m) and, given the limited duration of occupation, unlikely to have been backfilled by material with a particularly strong magnetic contrast, it can be expected that even under ideal conditions only a fraction of all surviving postholes would be noted. Hence we should not expect this survey to reveal a full system of postholes relating to post buildings. Likewise, it is worth noting that a number of survey choices (including traverse separation and antenna wavelength/frequency), geological conditions (e.g. the presence of discontinuities or 'clutter' in the glacial till) and archaeological conditions (particularly the lack of significant contrast between sub-surface interfaces of, say, small cut features with organic fill) influence the effective discriminating power of the GPR and interpretation is consequently less straightforward.

However, two strong positive anomalies with a negative 'halo' can be identified in the gradiometry just inside the west gate (at coordinates [-90m, 20m]), which correspond to the location of a set of principal posts from the largest excavated pre-fortress building. The strong magnetic signature is presumably created by material dumped in the holes after excavations, possibly ferrous material or stones and burnt clay from the nearby central hearth of the building, which was demolished in the last stages of the excavation.