Geology Site Account

Site name: Glemsford Picnic Site (Essex side of the border)

Grid reference: TL 8319 4640

Brief description of site:

Picnic site only. Site of a buried 'tunnel valley' formed during the Ice Age. No evidence on the surface but an opportunity to inform the public through signboards.

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Summary of geological interest

The Stour valley in the vicinity of Glemsford is situated above one of the most remarkable geological features in East Anglia. Here beneath your feet is a deep steep-sided valley cut into the chalk bedrock and formed by a river flowing beneath an ice sheet 450,000 years ago. There is no sign of it on the surface because it is now completely filled with glacial sand and gravel.

This buried tunnel valley was one of the main drainage routes for meltwater from the Anglian Ice Sheet, which covered most of Britain, and the whole of East Anglia, during the coldest part of the Ice Age. At its maximum extent the ice sheet was thousands of feet thick and these sub-glacial rivers contained vast quantities of water under tremendous pressure (due to the weight of the ice above). Laden with rocks and gravel, they were capable of carving deep channels in the bedrock.

This buried valley can be traced from Thurlow, north of Haverhill, south to Wixoe where it turns eastwards beneath the present main valley and plunges to depths well below sea level at Cavendish and here at Glemsford. At this spot the floor of the buried valley is 143 metres (470 feet) beneath your feet. This depth was established at the Glemsford borehole nearby and means that the buried valley floor (the Chalk bedrock) is over 100 metres (330 feet) below sea level.

The Glemsford Picnic Site is on the Essex side of the Suffolk border. It provides an excellent opportunity to inform visitors about this remarkable geological feature. It is next to the flooded gravel pits known as Glemsford Pits Nature Reserve (Glemsford Pits SSSI) (not accessible to the public). An interpretive board is recommended for the picnic site.

Glemsford Picnic Site has a deep secret. Photo © G. Lucy