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Geology Site Account

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Hallsford Bridge Clay Pit (site of), HIGH ONGAR , Epping Forest District, TL562024, Historical site only

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Site category: London Clay, Claygate or Bagshot Beds

Site name: Hallsford Bridge Clay Pit (site of)

Grid reference: TL 562 024

Brief description of site:

Former pit in London Clay (often referred to as High Ongar Clay Pit) which produced a large number of fossils in the 1970s and 1980s, particularly fossil crabs and nautiloids which were usually complete and in a beautiful state of preservation.

The overburden at the pit was a substantial thickness of boulder clay (glacial till) which yielded erratic rocks and derived Jurassic fossils.



A large claypit in the London Clay was formerly worked by Leca (Aggregates) ARC Ltd. at Hallsford Bridge, Mill Lane, Ongar. The company manufactured lightweight expanded clay aggregate in a factory adjacent to the works. The large pit produced a remarkable number of beautiful fossils during its working life until the factory closed and the pit backfilled in 1994.

The pit was visited several times in the 1970s by the Tertiary Research Group. Daniels (1971) provides an informative account of the works, describing the fossils found, particularly logs of fossilised wood and how they got to be on the floor of the London Clay Sea. A list of fossils can be found in Cooper (1974). The stratigraphy of the London Clay at Ongar Pit is discussed in King (1981).

The pit was also visited on several occasions by the Essex Rock and Mineral Society and fossils found are now in several private collections. The pit was most famous for exquisitely preserved fossil crabs in phosphatic nodules, particularly the species Xanthopsis leachi.

On top of the London Clay was a thickness of boulder clay, or till, which was laid down across central and northern Essex about 450,000 years ago by an ice sheet during the Anglian glaciation, the most severe cold period of the whole of the Ice Age. This material had to be removed before the clay could be extracted. The till contained derived rocks and fossils from further north, brought here by the ice, particularly Jurassic fossils.

Bridgland (1999) refers to an exposure of gravel here which was identified as deposited by northward-flowing south bank tributary of the early Thames when it flowed across north Essex prior to the Anglian glaciation.

High Ongar Clay Pit in 1978. London Clay is being excavated. A pile of excavated boulder clay (till) can be seen in the distance. Photo © British Geological Survey (P212178).


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Reference: Daniels 1971, King 1981 (p.48), Cooper 1974, Bridgland 1999 (p.146), Saward 2015.

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