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Glacial History The modern physical landscape and geography of North Wales mainly reflects the influence of intensive glacial erosion during the Pleistocene epoch. During the late Pleistocene glacial phase (Devensian), a large British Ice Sheet developed in response to global cooling. This ice sheet developed by coalescence of several ice centres, one of which was centred in North Wales. Welsh ice was centred on the Migneint plateau and the Arenig Mountains with outlet glaciers radiating from that centre. These outlet glaciers carved fairly extensive glacial trough valleys, some good examples are the valleys of Nant Ffrancon and Llanberis/Nant Peris (Snowdonia) and Tal-y-Llyn (Cadair Idris). Anglesey and the northern coast were overridden by ice from the Irish Sea, moving south from Scotland, Ireland and the Lake District. Its direction of movement was south-westward. At times it succeeded in penetrating the mainland for several kilometres and reaching considerable height. Deposits were dredged in frozen conditions from the floor of the Irish Sea by ice approximately 700m thick. During phases of more severe climatic conditions, Welsh ice quickly spread over the coastal plains and south Anglesey preventing the Irish Sea ice from advancing. Erratics from the mainland have been recorded on the west of Anglesey, so the coast was subject to Welsh and Irish ice at different times, thus tills of both are found. This reinforces the importance of the North Wales area for observing the interactions between two ice sources - Welsh ice and Irish Sea ice. The majority of glacial landforms found beyond the mountain areas were last formed or modified during the last glacial maximum approximately 18,000 yrs BP (Dimlington Stadial). The freshest glacial deposits and landforms plus those found in high mountain areas were last formed/modified during the last glacial even (Loch Lomond Stadial or Younger Dryas) approximately 11,000 - 10,000 yrs BP. Snowdonia is an area which once supported local glaciers during the Loch Lomond Stadial. Glaciers emerged from the cwms and were channelled down existing valleys (carved during the previous glacial phases), and in a few cases carved out new ones. The post-glacial landscape evolution has largely developed through landscape adjustments especially isostatic rebound (uplift), slope modification and coastal processes (including sea-level rise). Bibliographic Sources Addison, K. (1997) Classic Glacial Landforms of Snowdonia. Geographical
Association, Sheffield. |
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