The Devensian Lateglacial and early Flandrian stratigraphy of Southern Snowdonia, North Wales.

Abstract

The Devensian Lateglacial and early Flandrian stratigraphy of southern Snowdonia, North Wales has been examined using pollen analytical, radiocarbon dating and geomorphological mapping techniques. These techniques were used to reconstruct the effects and timing of environmental changes on the Lateglacial and early Flandrian vegetation and landscape. Detailed geomorphological mapping enabled the reconstruction of former "cwm" glaciers and snowpatches of inferred Loch Lomond (Younger Dryas) Stadial age. These former glaciers were small and extremely localised, their distribution being influenced by altitude, aspect and snowdrift A local equilibrium fim line of 600 m OD was estimated from former glaciers on Cader Idris and used to infer a mean July temperature of 8.5 ' C at sea-level during the Stadial. Pollen analyses of three mire sites located outside the former ice limits and three upland lake sites from inside the limits helped to corroborate a Loch Lomond (Younger Dryas) Stadial age for the "cwm" glaciation. Logistical problems of sampling upland lake sites were overcome by the innovative use of sub-aqua divers. An exceptionally thick sequence of Lateglacial and early Flandrian organic-rich sediments was discovered at Llyn Gweman and provided a particularly high degree of stratigraphic resolution for radiocarbon dating vegetational changes and other environmental events. Late Devensian ice was shown to have abandoned the Llyn Gwernan site before ca.13,200 BP at which time the landscape was rapidly colonised by a pioneer vegetation in which Rumex was prominent. Juniper spread into the area at ca.13,000 BP and was succeeded by the expansion of birch woodland at ca.12,100 BP. Gradual climatic deterioration was inferred from this time onward, culminating in the revertence to a tundra-like environment during the Loch Lomond (Younger Dryas) Stadial, hereby dated to between ca.11,200 BP and 10,000 BP. The climate improved at around 10,000 BP and vegetational succession progressed from herb-rich grassland, through expansion phases for juniper, willow, birch and eventually hazel at ca.9,100 BP.