Geology Site Account
Cudmore Grove Cliffs and Foreshore (part of Colne Estuary SSSI), EAST MERSEA, Colchester District, TM068146, Site of Special Scientific Interest
Site name: Cudmore Grove Cliffs and Foreshore (part of Colne Estuary SSSI)
Grid reference: TM 068146
Brief description of site:
The cliffs at Cudmore Grove Country Park provide superb exposures of gravels laid down by the Thames-Medway River during a glacial period 300,000 years ago. Organic sediments at beach level sometimes yield fossils, including fossil wood and mammal bones, that indicate that they were deposited during an interglacial period. Also exposed on the foreshore are deposits from a more recent interglacial period, the Ipswichian interglacial (120,000 years old), and known as the 'hippo site' due to the presence of hippopotamus bones.
Cudmore Grove Country Park, at the eastern end of Mersea Island, is one of the most important geological sites in Essex. Ice Age deposits were first discovered here in 1906 when fossil shells were discovered in a well in the floor of a gravel pit. The site of this pit is probably the overgrown hollow now known as ‘The Dell’ near the entrance to the Country Park. In those days the cliffs were stable and covered with vegetation but erosion by the sea in recent decades has created the excellent coastal exposures that now exist. In the 5 to 6 metre high cliffs can be seen layers of sand and gravel laid down by the combined Thames-Medway River 300,000 years ago as it flowed north-east towards Clacton and out across what is now the North Sea to become a tributary of the Rhine.
Although the rapidly eroding cliffs are of geological interest the scientific importance of Cudmore Grove did not become apparent until 1979 when Essex amateur geologists Bill George and Stephen Vincent discovered mammal bones and teeth in hollows on the London Clay foreshore. Although they found only a small number of these fossils in a restricted area they comprise a distinctive fauna that includes, straight-tusked elephant, bison, giant deer and hippopotamus which has led to this part of Cudmore Grove being called the ‘Hippopotamus Site’. The fauna appears to be identical to that previously found beneath the beach at the so called ‘Restaurant Site’ about 1.5 kilometres south-west of Cudmore Grove and is characteristic of the Ipswichian Interglacial Stage about 120,000 years ago. Due to a lower sea level the Thames at this time was flowing below its present level and so these sediments must be associated with an early course of the River Blackwater.
The prime interest of this site, however, is a large channel-like depression in the London Clay at the base of the cliffs and exposed on the foreshore that was discovered just three years later in 1982. The depression is a former river channel associated with the Thames-Medway River or one of its tributaries and contains layers of gravel, silt and clay with numerous bones and shells. It is thought to have been infilled some 300,000 years ago during the ‘Purfleet’ interglacial stage (Marine Isotope Stage 9). Known as the Cudmore Grove Channel it contains a great diversity of fossils making it one of the richest Ice Age sites in Britain. A layer of clay in the channel was packed with shell fragments and contained the bones of beaver, macaque monkey, wolf and bear and a large number of small animals such as rodents, bats, birds, frogs, toads and fish. In fact the fauna of small vertebrates at this site is the richest ever found in Britain, providing a comprehensive picture of the wildlife in eastern Essex 300,000 years ago. The channel has also provided information about the vegetation of the area at this time due to the presence in the deposit of pollen and fragments of wood including two fossilised tree trunks that were found to be protruding from the beach. At the very base of the channel a worked flint flake in fresh condition was found which indicates that humans, probably an early species of Neanderthal, were also living here at this time
Cudmore Grove therefore provides not only a fine exposure of sand and gravel laid down by the long-lost Thames-Medway River but also important fossiliferous river channel deposits from two distinctly different interglacial periods. The Country Park is owned by Essex County Council and is open from dawn until dusk. There is a car park and an information room.
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Reference: Dalton 1908, Bridgland 1994 (p. 347-357), Bridgland, Allen & Haggart 1995 (p. 255-276).
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