Geology Site Account

Summary

Historical site but there is some potential for geological education. Danbury Common is a public open space with a varied geology. Any significant excavations in the area should be recorded.

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Site description

The village of Galleywood occupies the top and the east-facing slopes of a ridge of high ground that extends from Billericay, through Stock, to Galleywood. This ridge has a varied geology that has had an effect on the diversity of trees and other plants present, creating a number of different habitats.

The foundation of the whole area is London Clay, laid down under a sub-tropical sea about 50 million years ago, which is exposed in the valleys of the Wid to the west and the Sandon Brook to the east. As a result of the shallowing of the London Clay Sea, the London Clay passes up into a sandy clay called the Claygate Beds, which makes up the bulk of the ridge. In places the Claygate Beds have been worked to make bricks - an old brickyard on Galleywood Common (TL 704 024) originally exposing 3 metres (10 feet) of yellow clayey sand. The base of the Claygate Beds is marked by a line of weak springs in places, for example south west of Galleywood, where water draining through the sandy clay seeps out on meeting the London Clay below.

The Claygate Beds are overlain by Bagshot Sand, a fine yellow sand laid down as the sea became even shallower. Bagshot Sand caps the highest ground at Galleywood - around the church and at the southern end of Galleywood Common - which gives rise to sandy soils and the development of heathland. On the top of the Bagshot Sand is a layer of gravel containing remarkably well-rounded pebbles, called the Bagshot Pebble Bed, which is thought to be all that is left of an ancient beach, laid down as the coastline of the London Clay Sea finally passed over Essex. The pebble bed was worked for gravel in the 19th century on the north part of the common at TL 703 029 and there are many pits and depressions in the area.

During the middle of the Ice Age, perhaps about 500,000 years ago, the River Thames flowed through here, depositing extensive layers of sand and gravel in the Chelmsford area, known as Kesgrave Sands and Gravels. About 450,000 years ago, the Anglian Ice Sheet spread south into Essex, diverting the Thames to its present course, and laying down a distinctive clay known as boulder clay or till. An extensive sheet of this material covers the north and east-facing slopes of the Galleywood Ridge and extends beneath much of Galleywood. Boulder clay contains rocks brought here by the ice sheet from much further north, even as far away as Scotland, and so the gardens of Galleywood could turn up any number of strange and curious rocks and fossils.

The most important geological site in the area was Galleywood Gravel Pit (TL 7055 0385) which, in 1934, revealed 1.2 metres (4 feet) of London Clay at the base overlain by up to 7.6 metres (25 feet) of Kesgrave (Thames) Gravel, capped by up to 3 metres (10 feet) of boulder clay. This was a fascinating window into the geological history of Galleywood from sub-tropical seas to Ice Age glaciers. The pit appears to have been where the childrens playground now is, south of Osprey Way, but no evidence of it seems to have survived the subsequent redevelopment of the area.