Unravelling Younger Dryas glaciation in the Tweedsmuir Hills, Southern Uplands, Scotland

In Britain, the glacial geomorphological record has been widely utilised to infer palaeo-glacier geometries and ice dynamics with much of this work focussing on the Scottish Highlands during the Last Glacial Interglacial Transition (LGIT), in particular the Younger Dryas (YD; c. 12.9 - 11.7 ka BP). The Southern Uplands represents the largest upland area south of the Highlands but have received limited research attention over the last century. The Tweedsmuir Hills are located in the central Southern Uplands, which form an area of dissected plateau approximately 320 km2. Early research in the 1800s identified a range of glacial landforms thought to be associated with the YD. The majority of previous work has focussed on isolated valleys and ignored the potential for plateau icefield glaciation, which has significant implications for understanding of the dynamics and geometries of the YD ice masses. Recent numerical modelling experiments covering the period 38 - 10.4 ka BP (Hubbard et al., 2008 cf. E109B8 and E102b2) have predicted a significant body of ice for the Southern Uplands at the onset of and throughout the YD, which cannot be verified at present due to a lack of empirical data. This research aims to provide the first systematic mapping and climate reconstruction for the Tweedsmuir Hills. The results of air-photo interpretation and field mapping, which utilised a morphostratigraphic approach, has demonstrated a more extensive glaciation than previously mapped. This consists of two separate icefields over the southern and northern Tweedsmuir Hills which cover an area c. 45 km2 and 25 km2 respectively with Equilibrium Line Altitudes (ELAs) calculated to have ranged from c. 419 m to 634 m. For both icefields ELAs of individual outlets reflect topographic controls rather than steep precipitation gradients like those derived for other icefields in Scotland (e.g., the Monadhliath Mountains and Beinn Dearg). New radiocarbon dating of basal stratigraphies and Terrestrial Cosmogenic Nuclide Analysis (TCNA) of in situ boulders data place the icefields within the context of the YD. Landform evidence also indicates smaller valley glaciers occupied some of the south-easterly catchments until the end of the YD. All of these results differ significantly from the traditional paradigm which suggests that due to low accumulation, only restricted ice masses developed in the Tweedsmuir Hills during this time.