Geology Site Account

Summary

Site of Special Scientific Interest designated for the importance of its geology. The St.Osyth Gravel Pit SSSI is a triangular overgrown hollow which has been saved from landfilling but unfortunately no sand and gravel is now visible. This is an important site for providing evidence of the diversion of the Thames

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Site description

About 450,000 years ago a catastrophic change affected the Thames causing it to alter its course and adopt the route we know today. The gravel at St. Osyth pit is of two types: the Lower St. Osyth Gravel, which dates from just before the Thames was diverted, and above this is the Upper St. Osyth Gravel which was laid down after the Thames had disappeared from the area. This is clearly indicated by the stone content of the Lower Gravel which is typical of that deposited by the early Thames (the Kesgrave Sands and Gravels). The Upper Gravel, on the other hand, has fewer of the exotic stone types of the early Thames, such as quartz and quartzite, and more Rhaxella chert, brought in by meltwater streams from the nearby Anglian ice sheet.

The deposits reveal just how rapid the diversion was as the Thames suddenly ceased to flow through central Essex as a result of being blocked upstream in Hertfordshire and west Essex by the Anglian ice sheet. Gravel of the same age is present in the cliff at Holland-on-Sea. St. Osyth lies upstream from the confluence with the Thames and Medway rivers whereas Holland lies within the area of the confluence.