Deciphering the geomorphic and sedimentary record of the last Irish Ice Sheet in NW Donegal, Ireland: implications for glacial dynamics and decay configurations
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The last Irish Ice Sheet has a long history of investigation, but its characteristics and behaviour remain elusive. A comprehensive reconstruction of the last Irish Ice Sheet of NW Donegal is presented in this thesis. It incorporates the evidence-based approach whereby the character and distribution of glacial deposits and landforms in the study area have been used to interpret the nature and behaviour of the ice sheet that created them. A glacial landform mapping programme, conducted from satellite imagery, digital elevation models, aerial photographs and field mapping has yielded a series of new glacial maps for NW Donegal. These show that a variety of subenvironments coexisted beneath the ice sheet, from areal scour in the Rosses to selective linear erosion in the Derryveagh Mountains to deposition along the Northern Coastline. A flowset approach was adopted where the landform data were summarised with spatial, temporal and glaciodynamic information extracted. The glacial sediments of NW Donegal, subdivided into four physiographic zones, were comprehensively described, analysed and interpreted based on the results of an extensive field investigation incorporating electrical resistivity surveying and clay mineralogy analysis. A four phase reconstruction is proposed: Phase I involves flow from a dominant dispersal centre in the Bluestack Mountains, from which ice moved northwards and northwestwards, undeflected by topographic relief, to some distance offshore; Phase II is dominated by bedform migration and reorganisation driven by funnelled flow and increased ice flux into the opened sea loughs of Mulroy Bay and Lough Swilly; Phase III is characterised by a strong topographical control on ice flow, resulting in the fragmentation of the ice sheet into a number of separate and semi-independent ice lobes and valley glaciers. Substantial glacial lakes formed in the interlobate areas as the ice margins uncoupled with periods of ice margin stabilisation demarcated by small discontinuous ice-contact moraines; and Phase IV involves the ice downwasting with thin dead ice aprons developing hummocky, chaotic, drowned kettle-hole terrain now characteristic of the study area. A first attempt in Ireland to apply a combination of terrestrial Cosmogenic Nuclide Surface Exposure dating (10Be) and Schmidt Hammer relative-age dating is also presented in this thesis and is used to constrain the timing of ice sheet activity in NW Donegal.
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