Geology Site Account

Site name: Hornchurch Dell

Grid reference: TQ 543868

Brief description of site:

Hornchurch Railway Cutting is well known to geologists as the most southerly place in England that glacial boulder clay, or till, has been discovered, therefore marking the maximum southerly extent of the ice sheet during the whole of the Ice Age. Glacial till has also been found in The Dell, which makes this site of geological interest with potential for further research. The geology revealed by any excavations in The Dell should be recorded.

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Summary of geological interest

Just south of St. Andrew's Church is a wide, deep hollow with steep, wooded sides that was formerly a gravel pit but is now completely overgrown. The hollow is known as The Dell and it is of some antiquity as there are records that in the 18th century the then grassy banks formed a perfect amphitheatre and accommodated thousands of spectators for sporting events.

Hornchurch is well known to geologists as the most southerly place in England that glacial boulder clay, or till, has been discovered, therefore marking the maximum southerly extent of the ice sheet during the whole of the Ice Age. It was also the place where boulder clay was found to be overlain by gravels of the lower Thames (Orsett Heath Gravel) providing evidence that the glaciation of the North London area (about 450,000 years ago) occurred before the deposition of the oldest terrace gravel of the modern Thames. In other words it demonstrated that the Thames was diverted to its present course after the arrival of this ice sheet. This discovery was made when the railway cutting north of the church was excavated for the new railway line in 1892 (see site record for Hornchurch Railway Cutting).

In the late 1970s chalky boulder clay was also discovered when the electricity substation was built in the northern part of The Dell. Geologists therefore carried out an exploratory investigation in October 1981 to try to expose a similar geological succession to that found in the railway cutting. It was hoped that a full succession of London Clay, boulder clay and Thames gravel could be permanently exposed at the southern end of The Dell (at TQ 5440 8675) but unfortunately it was found that the boulder clay had died out and the gravel here directly overlies the London Clay.

It is interesting to note that, although there is no permanent exposure here, the finding of boulder clay during the construction of the substation technically makes The Dell the maximum southerly extent of the ice sheet during the whole of the Ice Age, not the railway cutting.