Geodiversity at Eridge Rocks
By Peter Anderton of the Sussex Biodiversity Partnership
Geodiversity is the variety of rocks, fossils, minerals, soils and landscapes which can be found and observed in any chosen area. It also includes the variety of natural processes, such as stream erosion, which modify our present day environment and which modified environments in the geological past. Geodiversity provides the foundation for habitats, ecosystems and biodiversity.
In Sussex, geodiversity is dominated by the Chalk rock which underlies the South Downs and by the older sandstones and clays which underlie the High Weald, the Low Weald and the Wealden Greensand landscapes. These older rocks were deposited during the Lower Cretaceous epoch of geological time from 145 to 100 million years ago. The sandstones at Eridge Rocks are characteristic of the middle part of the Wealden Group and were deposited about 135 to 133 million years ago.
A geological cross section showing the underground layers of rock along a profile running from Eridge Rocks to Harrison’s Rocks. The Ardingly Sandstone at Eridge Rocks and Harrison’s Rocks forms the distinctive sandrock cliffs so typical of the High Weald. Underlying it are the thinner sandstones of the Lower Tunbridge Wells Sand unit and overlying it are the Grinstead Clay and lower part of the Upper Tunbridge Wells Sand.
These impressive cliffs of Ardingly Sandstone are 10m high and form large blocks separated by vertical gaps. The massive sandstone blocks are made up of successive layers (beds) and the weaker layers have been picked out by weathering, forming notches.
Vertical gaps like this between the massive sandstone blocks are known as gulls. These follow naturally formed vertical fractures (called joints) which have been opened up partly by weathering and erosion but often by movement of the blocks themselves. This movement probably occurred during cold climate (ice age) episodes in the Pleistocene period of geological time, the last of which ended 10,000 years ago. Melting of frozen ground would have allowed blocks to move downslope.
The Ardingly Sandstone is quite soft and friable but the cliff faces are covered with a dark grey weathered crust which helps to protect the underlying sandstone from erosion. When exposed the fresh sandstone is lighter coloured.
This massive sandstone block is about 8 m high with an undercut base and large overhang. The undercutting may be due to greater erosion of a weaker layer at the base of the cliff or the sapping action of water seeping from the base of the sandstone where it meets the underlying less permeable rock. The result is to undermine the cliff face.
Bedding layers are relatively horizontal and weathering has picked out weaker layers forming notches.
The sandstone face here shows bedding layers pockmarked by honeycomb weathering. This forms by salt weathering when saline water percolating through the sandstone evaporates near the surface and forms salt crystals which can grow and break the rock to form small pits. As it does not penetrate the weathering crust the honeycomb weathering here is probably a relic of former climatic conditions during the Pleistocene period.
This cliff face about 10m high has rounded upper surfaces covered with polygonal cracks. These may have formed under cold climate conditions due to alternate freezing and thawing. Alternatively they may result from cracking of a thin mineralised crust. The face beneath the overhang is covered with honeycomb weathering pits.
Rocks provide a valuable window into past environments and biodiversity. This sandstone block exposes a set of trough like or dipping bedding patterns, which show that it was deposited a braided stream environment. The Wealden rocks were deposited in nonmarine environments and this interpretation is supported by the fossil evidence. Characteristic fossils range from freshwater gastropods and fish to reptiles and dinosaurs.