Rapid thinning, disintegration and fragmentation of the Welsh Ice Cap during the last deglaciation.

During the Last Glacial Maximum (LGM; 27.5-23.3 ka), the British Irish Ice Sheet (BIIS) and Welsh Ice Cap (WIC) jostled for position around the margins of North Wales, documented by complex moraines that demonstrate the push and pull interchange between ice margins. The relative timing of thinning and retreat of the WIC thereafter has important implications for our understanding of these complex ice-marginal systems. The deglacial landscape documents numerous moraine successions in the valleys, as well as in the cirques. Until now many cirque moraines have been assumed to be Younger Dryas in age, although this has rarely been tested using cosmogenic exposure dating. Here, we present a suite of thirty-two new Cl-36 exposure ages from glacially transported boulders that span the length of Dee Valley, Northeast Wales. We interpret the timing of lateral retreat and vertical thinning with samples that span from the palaeo-ice centre in the Arenig mountains, to the proposed ice margin at the Welsh/English borderland. We also test the validity of hypothesised Younger Dryas cirque glaciers by dating boulder samples directly atop suspected inner and outer moraines. The Dee Valley has a complex cosmogenic isotope signature showing numerous pre-LGM exposure ages, reflecting the reworking of pre-exposed material down valley. Nevertheless, the majority of exposure ages indicate rapid post-LGM disintegration of ice through the Dee Valley. Across the uplands of Wales, this would have resulted in a shift to a fragmentary pattern of ice coverage. Cl-36 exposure ages from cirque moraines and high-level plateaux and ridges also confirm the upland area of Arenig Fawr (854 m)—previously suggested as a former centre of the WIC—was ice-free soon after the LGM and remained ice-free thereafter. This is in contrast to other parts of North Wales where alpine-style valley glaciers persisted until around 15 ka, with some cirques occupied by glaciers during the Younger Dryas. The implication is that the deglacial history of Wales is more complex than previously thought with significant variations through space and time. It is also evident from our study that not all cirque moraines in Northeast Wales are Younger Dryas in age, with some considerably older and reflecting earlier glacier retreat.