The Geomorphology and Quaternary History of Anglesey
For about the last 2.6 million years, Britain and Ireland have been near their present latitude where they have experienced the numerous profound climatic shifts of the Quaternary Period. Although many glacial phases interspersed with warmer interglacial interludes occurred during this time, the last major ice-sheet glaciation of Anglesey about 25,000 years ago, not surprisingly, left a dominant imprint. A powerful Irish Sea ice stream (ISIS) submerged the island beneath a kilometre of ice. Fed by glaciers in Scotland, Ireland and the Lake District, this immense body of ice traversed the island broadly NE to SW, dredging huge quantities of sand and rock from the seafloor and then plastering them across the island. Telltale evidence of the ice’s passage includes the famous drumlin swarms, ‘trains’ of erratics, spreads of boulder clay and meltwater sands and gravels, and striated and sculpted bedrock surfaces. Although the ice had wasted from the island by about 18 ka, it was not until 5,200 years ago that rising post-glacial sea levels, brought about by continued climatic warming, created the Isle of Anglesey.
Anglesey contains an exceptional range of glacial geomorphological features and sedimentary deposits. Although the gross morphology of Anglesey’s land surface is pre-glacial in origin, its landscape was modified considerably during the Pleistocene glaciations and the island is now dominated by an erosion surface cut across the older rocks.
During that glaciation, around 20,000 years ago, Anglesey was completely submerged by ice. Two ice sheets from different sources were involved. The Snowdonian mountains were the source of ice streams that moved broadly northwards towards Anglesey, while the massive Irish Sea ice sheet, fed by glaciers from Scotland, Ireland and Cumbria, moved onto the island from the north. The Irish Sea ice stream was dominant, and travelled north-east to south-west across the island, broadly in keeping with its NE-SW-trending, structurally controlled rock ridges. The Welsh and Irish Sea ice streams met in the region of the present-day Menai Strait and produced a confluent south-westward flow.
Deposits from the Irish Sea Glacier tend to contain a wide range of rock types from its diverse source areas and from the varied geology of the seafloor traversed. A red colouration is common, being derived partly from Permian and Triassic rocks offshore such that it used to be known as the Red Northern Drift. The origin of these glacial sediments is shown by the granites from Ailsa Craig [ Firth of Forth], Shap [Lake District], sandstone & coal fragments [Lancashire] and basalt & flint [Antrim]. Deposits from the Welsh Ice sheet [very rare on Anglesey] reflect the geology of its source areas, with a high proportion of Cambrian slates and mudstones, varied Ordovician igneous materials and a blue-grey colouration. Their virtual absence shows the power of the Irish Sea Glacier which displaced them along the coast of the mainland.
Above: till – deposited by the Irish Sea ice-sheet, about 20,000 years ago – is now being reworked by the sea at Beaumaris – part of the seemingly endless cycle of sedimentary erosion and deposition. Photo John Conway.
watch Impact on GeoMon coast due to climate change presentation from Dr John Conway