Ice sheet advance, dynamics, and decay configurations: evidence from west central Scotland

Abstract

A 3700-km 2 area adjacent to the Firth of Clyde, Scotland, is examined to constrain the development and dynamics of the western central sector of the last British and Irish Ice Sheet. Results from geomorphological mapping, lithostratigraphic investigations, three-dimensional geological modelling and field observations are combined to produce an empirically constrained, five-stage conceptual model of ice sheet evolution. (A) Previously published dates on interstadial organic deposits and mammalian fossils suggest that the Main Late Devensian (MLD) (MIS 2) glaciation of central Scotland began after 35 ka cal BP. During build-up, ice advanced from the western Scottish Highlands into the Clyde and Ayrshire basins. Glaciomarine muds and shelly deposits scavenged from the Firth of Clyde were redeposited widely as shelly tills and glacial rafts. Ice advance against reverse slopes generated, and subsequently overtopped, ice-marginal sediment accumulations. We hypothesise that some of these formed pre-cursor ridges which were moulded into suites of ribbed moraine during the glacial cycle. (B) Sustained stadial conditions at the Last Glacial Maximum (LGM) (c 30–25 ka cal BP) resulted in development of a major dispersal centre over the Firth of Clyde and Southern Uplands. This dispersal centre locally preserved previously formed subglacial bedforms, and fed a wide corridor of fast-flowing ice east towards the Firth of Forth. (C) Initial deglaciation promoted a substantial re-configuration of the ice surface, with enhanced westward drawdown into the outer Firth of Clyde and eastward migration of an ice divide towards the Clyde-Forth watershed. (D) Renewed ice sheet thickening over the Firth of Clyde may have accompanied growth of the Irish Ice Sheet during the Killard Point Stadial (c 17.1–15.2 ka cal BP); it was associated with limited bed modification. Subsequent ice sheet retreat was characterised by substantial meltwater production, ponding and erosion. (E) Late stages of MLD ice sheet retreat were punctuated by one or more significant ice margin oscillations. Discovery of De Geer moraines at the site of a former proglacial lake in western Ayrshire allows glacier flow at the ice margin to be approximated as ≤290 m a −1 during one such oscillation. Such velocities were probably enabled by basal sliding and shallow sediment deformation. At this stage those parts of the MLD ice sheet margin that were grounded in the Firth of Clyde were extremely vulnerable to collapse. Final disintegration of glacier ice in the Clyde basin probably occurred early in the Lateglacial Interstadial (Greenland Interstadial-1), coinciding with marine incursion to c 40 m above present day sea level.