Subglacial bedform geomorphology of the Irish Ice Sheet reveals major configuration changes during growth and decay

The belated realisation that ribbed. (Rogen) moraines form such an integral part of Irish geomorphology, and the piecemeal approach to previous drumlin mapping, is probably responsible for the highly contrasting views of palaeoflow patterns of the Irish Ice Sheet. Using a high resolution (25 m) digital elevation model we present morphological maps of a large part (100 x 100 km) of the so-called 'Drumlin Belt' of north central Ireland. The landforms comprise mostly ribbed moraine much larger than found elsewhere (up to 16 km in length), which in places are superimposed on each other. Contrary to most prior assessments we find the bedform record to contain numerous and overlapping episodes of bed formation (ribbed moraine, drumlins and crag-and-tails) that provide a palimpsest record of changing flow geometries. These demonstrate an ice sheet with a centre of mass and flow geometry that changed during growth and decay. Using distinctive flow patterns and relative age relationships between them we reconstruct ice sheet evolution into four phases during a single glacial cycle. in phase 1 (early in the glacial cycle), Scottish and local ice coalesced to form a northeast-centred Irish Ice Sheet. As it grew its centre of mass migrated southwards, culminating in a major N-S divide positioned down the east of Ireland (phase 2, ca. Last Glacial Maximum). During retreat, the centre of mass migrated at least 120 km northwards and became established in northwest Ireland and at this point a dramatic bedforming event produced one of the world's largest and most contiguous ribbed moraine fields (phase 3). Final deglaciation is thought to be by fragmentation into many topographically controlled minor ice-caps (phase 4). Rather than any dramatic or unexpected behaviour, the reconstructed phases indicate a relatively predictable pattern of ice sheet growth and decay with changes in centres of mass, and does not require major readvances or ice-stream events. Copyright (C) 2001 John Wiley & Sons, Ltd.