Erratic boulders in Sussex

1 October 2013
Geology

This story was in 'Down to Earth' 84, August 2013, a geological magazine that organises a lot of field trips, in Europe and further afield, and has pieces by local geologists in various parts of the UK. There have been lots of erratic boulders found along the south coast between Portsmouth and Brighton. They include granites, syenites, schists, slates, vein quartz, quartzites, sandstones, and limestones, and some of them can weigh up to ten tonnes. These erratics are mainly lodged in Pleistocene sands and gravels, which form the West Sussex coastal plain, having an origin in the Ice Ages. It is thought many of them were deposited around 200,000 years ago at a time of high sea level, in order to account for their presence above modern sea level. The peculiar thing is that Sussex was never glaciated – so an alternative process to movement of the boulders by glaciers had to be sought, in order to explain their presence. The clue is found in the origin of the boulders – on the French side of the Channel, on the Isle of Wight, and in the Channel Islands – possibly even gouged out of the Channel itself. The commonly accepted process is ice rafting – boulders carried along in floating sea ice (ice bergs) driven by wind and eventually running aground. It is essential therefore that the Channel existed in a previous interglacial period. If the Channel had not existed geologists would have a problem in explaining the presence of erratic boulders where they are currently found. The consensus view is that the ice rafting occurred during the onset of a cold glacial period – before sea level fell as water was being locked up in glaciers and ice sheets. Hence, we have a situation where geology is being dictated by the theory of Ice Ages. Is there ever a period when there is ice rafting and high sea levels? In a catastrophist model that might be so but in a uniformitarian model it is rather stretched. Because high sea levels are supposed to be a feature of interglacials, and low sea levels a feature of ice sheet formation, they are suggesting the boulders occurred, not at the close of a glacial period but shortly before one, when sea levels were high. This is peculiar as the ice rafting suggests temperatures were already very low – and a lot of sea water had become ice (so why had sea levels not fallen).

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