The use of integrated sedimentological and geophysical methods in drumlin research - a case study of Pigeon Point, Clew Bay, Northwest Ireland

Despite significant advances over the past decades, our understanding of drumlin formation and associated ice-bed processes is still incomplete. In this paper, we present the integrated use of geomorphological, sedimentological and geophysical techniques as a powerful means to force a breakthrough towards solving the drumlin enigma. We report on investigations of the anatomy of the Pigeon Point drumlin, Clew Bay, Ireland.We found that the bulk of the landform, which displays a classical drumlin shape, consists of silty-clayey diamicton showing evidence of deformation, hydrofracturing and comminution. The unit is interpreted as a sub-glacial traction till/comminution till. The thin unit overlying this basal till consists of silty-sandy diamicton, and is interpreted as a para-glacially modified melt-out till. The partly cemented third unit consists of stratified, massive to graded sands and gravels. Its contact with the sub-glacial traction till consists of a series of concave shapes, which suggests that it was deposited in meltwater channels that flowed in sub-glacial cavities and that cut laterally into the drumlin. We propose that highs in the undulating rockhead relief, as shown in the seismic profile, have provided nuclei which initiated drumlin formation. This idea is supported by the observation of local detached bedrock slabs that grade upwards into a comminution till. In the long profile, very high normalized induced polarization (IP) values form a wedge-shape, which is interpreted as a set of conjugate thrusts, or a ‘pop-up’ structure. The structure is positioned directly above one of the undulations in the bedrock, suggesting a direct relationship. The high values are thought to reflect the presence of pre-existing clays, which were sheared into the till, thus forming linings in the thrust features. It is concluded that glacitectonic processes, notably differential bedrock weathering and thrusting, have played a key role in the formation of this drumlin