Exploring West Sussex geology
with David Bone
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Stratigraphy (the rock layers)

The rocks of West Sussex are relatively young in geological terms but there is a wonderful variety to be discovered. Many exposures and quarries are designated as Sites of Special Scientific Interest (SSSI) or Regionally Important Geological Sites (RIGS) - more details here. This gives them some legal protection and helps to preserve the sites for future study. Publications on West Sussex geology can be found here.


We start with the Cretaceous age clays of the Wealden Group, around 125 million years old. These were deposited in a vast low-lying flood plain and contain beds of sandstone (Horsham stone) and freshwater limestone with fossil snail shells (Sussex Marble). The best sites are outside of Sussex, where the clay is used for brick making.


About 120 million years ago, sea level rose across the old land surface and a series of shallow marine sediments collectively known as the Lower Greensand Group were deposited. The sequence starts with the poorly exposed Atherfield Clay, but the subsequent Hythe Formation (used as building stone), Sandgate & Folkestone Formations (used as building sand) form thick deposits of sands and sandstones.


Increasing water depth around 110 million years ago led to deposition of marine clays (Gault Formation) and sandstones (Upper Greensand Formation), known as the Selborne Group. The Gault Clay has been used for brick-making whilst the Upper Greensand (locally known as malmstone) has been used for building stone. Starting around 100 million years ago, a calcareous ooze began to accumulate on the sea floor. Later to become consolidated into the Chalk, the ooze is mostly made of the microscopic remains of planktonic algae that thrived in the surface waters of the sea. Flint is formed from silica derived from other organism and sponges that also flourished at this time.


The youngest chalk is not preserved in Sussex, having been eroded during uplift of the Wealden anticline. Deposition next continues with Palaeogene age sediments (around 55 to 43 million years old) that lie beneath the West Sussex coastal plain. Following the Reading Formation, much of which has been shown to be fossil soils, marine conditions returned with deposition clays and sands of the Thames Group (Harwich & London Clay Formations) and the Bracklesham Group.


Deposition ended with renewed uplift of the Wealden Anticline and it isn’t until around 500,000 years ago in the Quaternary that any further sediments were deposited. Sussex escaped glaciation during the Quaternary ice ages but temperatures were low enough for permafrost to form. Sea level fell by up to 140 metres during the coldest periods but high sea level during interglacial times saw the formation of raised beaches, whilst deep river valleys became filled with sediment. Our varied and interesting landscape is very much the legacy of the ice ages.


Vegetation now obscures most of the underlying geology and it is only quarries, road cuttings or coastal erosion that permit us a glimpse of the ancient past. Today, access is forbidden to many quarries so the study of building stones provides an alternative method of understanding local geology (but collecting not permitted!).