Periglacial disruption and subsequent glacitectonic deformation of bedrock: an example from Anglesey, North Wales, UK Emrys Phillips1*, Jonathan R. Lee2, James B. Riding2, Rhian Kendall3, Leanne Hughes2 1. British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK 2. British Geological Survey, Kingsley Dunham Centre, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK 3. British Geological Survey, Columbus House, Greenmeadow Springs, Tongwynlais, Cardiff, CF15 7NE, UK *corresponding author telephone: +44 (0)131-667-1000, fax: +44 (0)131-668-2683, e-mail -
[email protected] Abstract The deformed metasedimentary bedrock and overlying diamictons in western Anglesey, NW Wales, record evidence of glacier-permafrost interactions during the Late Devensian (Weichselian). The locally highly brecciated New Harbour Group bedrock is directly overlain by a bedrock-rich diamicton which preserves evidence of having undergone both periglacial (brecciation, hydrofracturing) and glacitectonic deformation (thrusting, folding), and is therefore interpreted as periglacial head deposit. The diamicton locally posses a well- developed clast macrofabric which preserves the orientation of the pre-existing tectonic structures within underlying metasedimentary rocks. Both the diamicton and New Harbour Group were variably reworked during the deposition of the later Irish Sea diamicton, resulting in the detachment of bedrock rafts and formation of a pervasively deformed glacitectonite. These structural and stratigraphic relationships are used to demonstrate that a potentially extensive layer of permafrost developed across the island before it was overridden by the Irish Sea Ice Stream. These findings have important implications for the glacial history of Anglesey, indicating that the island remained relatively ice-free prior to its inundation by ice flowing southwards down the Irish Sea Basin.