Summary of site:
A great deal is inferred in this area of around 16km˛ from a single, limited outcrop and local landforms. Some caution is therefore needed with this interpretation.
The sole outcrop, in Legamaghery Pit, is 1.5m high and 8m long but it is packed with information and interest. It consists of pebble gravels, medium grained sands and clays in ill-defined beds, 30-60cm thick. The sandy clay to pure clay beds have been deformed by glacial activity and show a range of structures, such as undulating folds 40cm high by 50cm wide; small scale (in the centimetre range) normal and reverse faults; larger listric faults (with the fault plane curving into the bedding plane below) in the order of 1m long with a displacement of 60cm, stretching of fine grained laminations into boudinage structures (where the beds are pulled apart into parallel, sausage-like shapes), compression of soft sediments (particularly in clay-rich beds), distortion of soft beds due to vertical loading by other sediments, clay layers rolling around more solid materials in the opposite direction to the main movement, and zigzag folding (when a second small-scale fold fabric is superimposed on an earlier one, refolding it into the zigzag pattern). Such a range of deformation in so confined a space is uncommon and indicates the influence of moving ice after deposition.
The sediments settled into a standing water body, a glacial lake of some kind. From the alignment of the deformation structures it is clear that the ice retreated, was arrested, probably by a phase of climatic deterioration, and again began to advance, almost due south. The ice does not appear to have substantially overridden the sediments but ‘pushed’ them along a wide zone of contact. They were not frozen during this process and the large volume of contained water trapped below the many clay beds was able to migrate through the sediment body, contributing to the many styles of deformation present in the Legamaghery Pit. Confirmation of a southerly re-advance can be seen in thrusted moraines 2km to the north of Lackagh.
Bearing in mind the caution required by such limited exposure, the general story here is of outwash sediments, moraines and eskers standing on bedrock contained within the upper Glennamuck Valley. They cross the valley south-south-west to north-north-east and are considerably incised, initially by meltwater and now by modern drainage.