Exploring West Sussex geology
with David Bone
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The Quaternary deposits of West Sussex form a fragmentary sequence, often poorly exposed. They represent periods of high and low sea level as the climate fluctuated through the ‘ice ages’. Best known is the raised beach sequence where ‘Boxgrove Man’ was found. Dating back 500,000 years, the site is no longer accessible or well exposed. The sediments can still be seen at a few sites, such as here at Slindon, where beach sediments are overlain by cold-period gravels washed off the hills.

Associated with one period of raised sea level are erratic boulders of granites and other exotic rocks that are believed to have come from Brittany, the Channel Islands or south-west England. The photograph shows one in Chichester Harbour. Probably carried along in floating ice, these rocks are up to 10 tonnes in weight, but the mechanism that brought them here is unknown. I believe they may have been carried here by a tsunami that ripped boulder-laden ice off the coastlines to the south and west.

Sea level fell dramatically during each ‘ice age’, only to return with the onset of climatic warming (as sea level is doing today). River channels gradually became choked with mud, although their deeply incised channels can still be traced further offshore.

Alluvium-filled channels can be seen at various places along the coast, such as here at Earnley. The mud often contains brackish-water shells, roots and branches of trees, and occasional animal bones. Older deposits contain bones of elephant and rhinoceros.

Our downland landscape owes much to weathering processes during the Quaternary. Tundra-like conditions during the cold periods would have exposed the soft rocks of West Sussex to severe weathering. Snow-melt would have added to rainfall in carving the extensive network of the now dry river valleys that dissect the South Downs. The West Sussex coastal plain was created by raised sea levels and it is now blanketed by the most recent cold-period deposits that were washed off the Downs.