Geology Site Account

Site name: Norsey Wood Nature Reserve

Grid reference: TQ 686954

Brief description of site:

Nature reserve with a varied geology. Site of geological interest with potential for geological education. Any significant excavations in the reserve should be recorded.

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Summary of geological interest

Norsey Wood is 165 acres of mixed coppice woodland close to the centre of Billericay with a varied geology which has led to a variety of different soil types. The geology is similar to other high points in south Essex and there are several aspects of this that visitors would find of interest, but they will probably be most curious about the unusually sandy and brightly coloured soil, and the remarkable abundance of what appear to be beach pebbles on the highest ground. These extremely well-rounded flint pebbles belong to an extensive outcrop of gravel, the origin of which has long been controversial.

Beneath the whole area is London Clay, a stiff, blue clay laid down in a subtropical sea some 50 million years ago during the Eocene period. Above the London Clay is the Claygate Beds, sandy clay laid down as the London Clay Sea became shallower and the coastline came nearer. The London Clay and Claygate Beds both contain fossils but because of the limited exposures none are known to have been found in Norsey Wood. The Claygate Beds, in turn, are overlain by beds of almost pure sand called the Bagshot Sand, which was laid down as this ancient sea became even shallower. This delightful, yellow sand therefore represents an ancient shoreline during the Eocene period. Like most sands, the Bagshot Sand in Essex consists almost entirely of quartz grains but there are also layers rich in minute flakes of sparkling mica and others with wisps of darker sand which could be concentrations of minerals such as zircon, tourmaline, topaz and garnet. Like most sedimentary rocks, this sand owes its origin to the destruction of other rocks and these heavy minerals are those that would typically result from the erosion of granite or gneiss. Crystals of these minerals, particularly zircon, are virtually indestructible, and they may have been reworked several times since their formation, perhaps arriving in the Bagshot Sand from the erosion of the Cretaceous sandstones of the Weald.

Intense erosion of the landscape by rivers during the Ice Age has considerably modified the Billericay landscape, carving fairly deep valleys which have exposed the London Clay and Claygate Beds in the valley floors but left outcrops of Bagshot Sand on the high ground. This pattern is typical of many of the hill tops in south Essex, most notably the Langdon Hills, but Billericay is a good example with the High Street running along a Bagshot Sand ridge. This ridge was cut through for the construction of the railway in 1887 and the resulting cutting, nearly 20 metres deep, produced one of the finest sections through the Bagshot Sand ever encountered in Essex (Whitaker 1889). From the Billericay ridge the Bagshot Sand high ground extends in an easterly direction through Norsey Wood forming a well-drained plateau which supports mainly sweet chestnut coppice and occasional colonies of heather. This is in contrast to the steep-sided valleys where the London Clay and Claygate Beds give rise to clay soils, supporting alder, ash and willow trees, with plants such a sedge, ferns and sphagnum moss. Springs are common where water draining through the Bagshot Sand is thrown out on meeting layers of clay in the Claygate Beds, producing areas of marshy ground.

Forming the very highest ground on top of the Bagshot Sand is a layer of gravel and in Norsey Wood this gravel is notable because it contains remarkably well-rounded, smooth, almost spherical pebbles. It was worked between the 1850s to the end of the First World War mainly for mending the roads and geological investigations in the early 20th century recorded the gravel in the wood to be at least 5 foot thick (Dines & Edmunds 1925). The western gravel pits are mostly built over but the eastern pits remain as hummocks and hollows. The main access path from the visitor centre into the wood is surfaced with imported limestone aggregate but further on the paths are surfaced with these delightful, native pebbles. They can also be seen in the arable field opposite the main entrance to the wood. The Norsey Wood gravel has been called the Bagshot Pebble Bed and considered to represent an ancient beach, laid down as the coastline of the London Clay Sea finally passed over Essex.

Although there is no doubt that the pebbles in the gravel were rounded and smoothed on a beach around 50 million years ago (during the Eocene period) they generate controversy amongst geologists, which can be traced back to Victorian times. The debate is about the origin of the Bagshot Pebble Bed itself, which occurs not only at Norsey Wood but also north of here at the village of Stock where the pebbles are abundant in the fields. It also occurs at Weald Country Park and possibly also at Warley. This debate appears to be only of academic interest but it is important in order to reconstruct the history of the Essex landscape. The view of many geologists is that the Eocene beach was not actually here in Billericay but somewhere to the south-west and that these pebbles were transported to this spot by a river during the Ice Age. This suggestion is based on the existence of similar gravel, formerly called pebble gravel, on the summits of the highest hills in south Essex such as the Langdon Hills. The gravel here also contains distinctive pebbles from Kent which means that it was almost certainly laid down in the early Ice Age, perhaps as much as one million years ago, by rivers flowing north from Kent. These rivers existed when the Thames flowed further to the north before it was diverted to its present course by the Anglian ice sheet.

The inevitable conclusion is that hill tops such as the Langdon Hills were once the bottoms of river valleys, indicating that an astonishing amount of erosion of the landscape has taken place since then “ in a period of probably less than a million years. But the gravel at Billericay is at a lower height above sea level, and is lacking in pebbles from Kent (Bridgland 1999). To the present writer it seems unlikely that the Norsey Wood pebbles have been laid down by an Ice Age river but were actually formed here, on an ancient beach some 50 million years ago, although they have been much disturbed by subsequent periglacial activity.

At Norsey Wood the importance of geology as the foundation of the landscape can be demonstrated and how variations in underlying rock types affect not only the soils but also the flora and fauna. The London Clay and overlying Claygate Beds are occasionally exposed in the valleys and small ravines and there are exposures of Bagshot Sand and Bagshot Pebble Bed on the paths and in the roots of fallen trees. Every pebble here has a story to tell €“ they were formed at a time shortly after the extinction of the dinosaurs when horses were the size of foxes, the seas were full of sharks and crocodiles, and Essex had a subtropical climate €“ something to think about when walking the pebbled paths of the wood.

Norsey Wood is owned by Basildon Council, and is a Local Nature Reserve. There is a visitor centre with leaflets on the reserve. If some of the reserves geology could be revealed there would be the opportunity for a geological trail guide with associated material such as the story of Norsey Woods pebbles from ancient subtropical beach via Ice Age rivers to present day nature reserve.

Some of Norsey Woods ‘beach pebbles’. Photo © G. Lucy