Glacially deformed bedrock at Wylfa Head, Anglesey, North Wales

Abstract

AbstractWidespread glacitectonically deformed bedrock was exposed in open trenches during site investigation at Wylfa, Anglesey. Bedrock consists of well—jointed low grade metasediments, with foliation dipping northwards. Drift thickness varies from 0–24m, with a drumlinised surface topography on the southern margin of the site. Bedrock merges upwards through a sheared zone at least 2m thick into locally derived lodgement till. A dark coloured calcareous tili overlies the local till in places. The deformed bedrock zone shows overturning, compressive folding and shearing over a heavily striated plane of decollement, compression being directed towards the south. Large detached slabs of bedrock are present in the basal till, and in one borehole a 1.1m high void 5.1m below rockhead was filled with well-sorted glacial sand. Glacial deformation of bedrock is discussed in the context of rockhead geometry, bedrock structure, basal water pressures, and effective stresses beneath the glacier.