Geology Site Account

Site name: Chigborough Lakes Nature Reserve

Grid reference: TL 871 088

Brief description of site:

Former gravel pits now an Essex Wildlife Trust nature reserve. Minor exposures of Blackwater terrace gravel dating from the end of the last glacial period in the unrestored western pit. The reserve is accessible during opening hours (access and car park off Chigborough Road).

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Summary of geological interest

Chigborough Lakes Nature Reserve is a 46 acre reserve managed by Essex Wildlife Trust. It consists of a number of worked out flooded gravel pits to the north of the Blackwater Estuary. The reserve provides exposures of terrace gravel from the River Blackwater dating from the last glacial stage (the Devensian) about 20,000 years ago although there has been extensive growth of vegetation since the gravel working ceased and little of the gravel is now visible.

The Blackwater gravel in this area has produced some remarkable fossils dating from this time “ one of the coldest periods of the Ice Age. In the early 1980s, just 500 metres west of here at the former Lofts Farm Pit, a cold-climate mammalian fauna was discovered with bones of reindeer, woolly mammoth, woolly rhinoceros, hyaena, bison and wolf. The finds included the tusk of a mammoth. As a result of this discovery the site of Lofts Farm Pit is now designated as a geological Site of Special Scientific Interest (SSSI).

Unfortunately Lofts Farm Pit is now completely restored and is just a lake with no visible geology. However, the western pit at Chigborough Lakes Nature Reserve remains unrestored and therefore provides a glimpse into this period of our past - a time when an Ice Sheet covered a large part of Britain, the North Sea did not exist and the River Blackwater was a tributary of the Thames-Medway River.

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Scientific interest and site importance

The present River Blackwater reaches the sea east of Maldon but mapping of the North Sea floor has revealed that at the time these Blackwater gravels were deposited it flowed into the now submerged valley of the Thames-Medway, about 20 kilometres south of Clacton. At that time sea level was so low the Thames-Medway River then flowed out across the southern North Sea basin to join the Rhine. River terraces are formed by a river successively cutting down which means that each terrace is at a lower level than the previous one. The terraces of the rivers Thames and Thames-Medway that are of Devensian (last glacial) age are now below sea level and so the only place where gravels of this age can be found are in its tributaries such as the Blackwater. The ice sheet at this time came no further south than what is now the Norfolk coast.

In this area the Blackwater terrace sand and gravel varies in thickness from 0.6 metres to over 3 metres (nearby boreholes show a maximum original thickness of 5.8 metres) although much of it on the reserve has now been quarried away leaving mounds of unwanted material. The gravel lies directly on a bedrock of London Clay and former sections in the quarry revealed that the junction between the gravel and the clay is undulating and very irregular, possibly due to initial scouring of the surface by the river or by subsequent freeze-thaw action on the ground (permafrost).

Associated with the gravel in this area are irregular patches of brickearth up to 2.1 metres in thickness that may represent overbank deposits. An overbank deposit is a fine-grained sediment deposited from suspension by floodwaters on the former floodplain of the river. A large mound of this pale-coloured brickearth can be seen by the main footpath on the south side of the reserve.

The nearby Lofts Farm Pit (TL 866 092) also worked the Blackwater terrace gravel and yielded an impressive assemblage of large mammal remains in the early 1980s as well as smaller vertebrates and even insects and pollen. These fossils were found in organic sediments interbedded with the gravel and indicate a cold climate. The site where the fossils were found is now a geological SSSI to ensure that the possibility remains for further research if necessary. The lake at Lofts Farm Pit can be seen by crossing Scraley Road and taking the public footpath. No fossils are known to have been found in the terrace gravels at Chigborough Lakes Reserve but the exposures of gravel, and the proximity of the SSSI, provide potential for interpretation and education.

Devensian age Blackwater terrace gravel overlying London Clay at Drapers Farm in 1968. The photograph was taken at TL 875 085, which is only just south of Chigborough Lakes Nature Reserve. (Geological hammer for scale). Photo © British Geological Survey (P210817).