Geology Site Account

Site name: Crown House Borehole (site of)

Grid reference: TL 5054 4288

Brief description of site:

Site of borehole that in 1894 revealed the existence of the Cam-Stort buried "tunnel' valley.

See Newport Grammar School borehole for further explanation.

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Details

This borehole at Great Chesterford is one of several that have revealed the presence of a remarkable buried valley in north-east Essex. This steep-sided valley is cut into the Chalk beneath Great Chesterford and is completely filled with glacial gravel and sand with no sign of it on the surface. It was carved by gravel-laden meltwater under tremendous pressure beneath an ice sheet.

The borehole was sunk in 1894 at the Crown House, north of the church. It penetrated to 48 metres (156 feet) before reaching the chalk bedrock.

Tunnel Valleys

In parts of East Anglia boreholes have revealed deep, steep-sided valleys cut into the chalk bedrock and now completely filled with glacial sand and gravel and often hidden by a covering of boulder clay. Known as buried valleys or tunnel valleys these remarkable natural features were formed beneath the Anglian ice sheet when it covered north Essex 450,000 years ago and were the main drainage routes for meltwater. To carve such deep channels the gravel-laden water must have been under tremendous pressure (due to the weight of the ice above) and the irregular surface of the channel floors indicate that in some places the water was even forced uphill.

In Essex the best example of a buried valley is the Cam-Stort Buried Valley which is present from north of the border with Cambridgesire south as far as Bishops Stortford and at Newport it is over 100 metres (300 feet) deep. It was first recognised by William Whitaker in his paper On a deep channel of drift in the valley of the Cam, Essex which was published in 1890 in the Essex Naturalist.

The deepest buried valley, however, is beneath the Stour valley on the northern border of Essex. In the vicinity of Cavendish the base of this valley is well below sea level at the remarkable depth of 143 metres (470 feet).