Part 05:  Sudden catastrophic Climate Change



By late Cenozoic times, the UK was pretty much where it is now in relation to the Poles and the Equator. But there was one last sting in the tail. A mere 1.6 million years ago, Earth went from its stable warm climate into a series of violent climatic oscillations - the ice-ages and intervening warm periods of the Quaternary. 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.



The Quaternary has featured at least four intensely cold periods during which large parts of upland Britain were covered by ice sheets for long periods. Although Anglesey was probably overrun by ice on these occasions, only evidence from the last major glacial phase - the Late Devensian - is known. 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 Angelsey] 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: Stewart Campbell.


Geology of Anglesey: a journey through time