THE NEWBIGGING ESKER SYSTEM, LANARKSHIRE, SOUTHERN SCOTLAND: A MODEL FOR TUNNEL, SUBAQUEOUS FAN AND SUPRAGLACIAL ESKER SEDIMENTATION

The Newbigging esker system, southern Scotland, displays an ordered upcurrent morphological transition through three spatially distinctive landform assemblages: single linear ridges, interrupted and terminated by shallow fans along their length; a series of multiple sub-parallel ridges with intervening and laterally adjacent fans; and a large and complex, anastomising multi-ridge structure. Within the assemblages, three major depositional environments can be identified. The first formed in ice-marginal subglacial tunnels by rapid deposition of coarse grained, predominantly boulder gravel during cessation of high-magnitude discharge events. The second formed in ice-front subaqueous fans systems by flow expansion beyond subglacial tunnel exits and consist of stacked, overlapping wedges of upward-fining cycles of parallel-laminated into rippled sands. The third shows polyepisodic sedimentation and identifies a depositional transition during a single stage of ice-front still-stand from coarse subglacial tunnel deposition, outwards into a subaqueous fan fronting the tunnel portal, then upwards into a supraglacial sandur deposited in an ice-walled trough created by tunnel unroofing. The overall sedimentary architecture is consistent with the progressive infilling of a large lake basin by successive shifts in sediment input source and associated depositional environment as the ice margin retreated during the Late Devensian glacial stage.