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Geology Site Account


Hadleigh Great Wood, HADLEIGH, Castle Point District, TQ820875, General geological site

 
 
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Hadleigh Great Wood

Site of geological interest with disused pits that have provided information about the underlying geology. Any further significant excavations in the area should be recorded.

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Site description

Hadleigh Great Wood (sometimes called Belfairs Nature Reserve) is the main survivor of a group of ancient woodlands on the southern slopes of the Rayleigh Hills. It lies on the south-western side of the valley of the Prittle Brook, which forms the north-eastern boundary. From the highest point of 64 metres (210 feet) above sea level in the western corner the reserve gently slopes down to 42 metres (140 feet) in the eastern corner. The Prittle Brook itself originally possessed many meanders but unfortunately most of these were cut off by straightening the brook some time after the 1920s but the dry beds of some of these former meanders are still visible.

The geology of the wood is similar to other woods in this part of Essex and the fauna and flora of the wood varies with the underlying geology. However, it is not always easy to identify the various rock layers as exposures are rare and the boundaries between the beds are indistinct due to a covering of ‘head’ (see below).

Beneath the entire area is a thickness of about 130 metres (430 feet) London Clay which was deposited on the floor of a subtropical sea some 50 million years ago. London Clay is exposed in the bed of the Prittle Brook and in places can be seen large fragments of septarian nodules which are characteristic calcareous concretions with cracks lined with calcite that occur frequently in the clay. The London Clay is overlain by a sandy clay called the Claygate Beds, and this is overlain in turn by Bagshot Sand, a yellow/brown sand deposited close to a shoreline. The boundary between the Claygate Beds and the Bagshot Sand is hard to trace on the ground but in other woods it is often marked by a change in the angle of the slope and by a line of springs. The Bagshot Sand is, however, often seen thrown out of animal burrows in the north-west of the wood where it consists of medium to fine grained sand, in places stained by iron. The whole sequence of rocks is still horizontal but has been uplifted and eroded over millions of years. As the ground surface of the wood slopes down to the valley the London Clay is therefore exposed at the valley bottom, the Claygate Beds in the centre of the wood, and the Bagshot Sand caps the highest ground. Hadleigh Great Wood therefore tells the story of a gradual shallowing of the London Clay sea as you walk uphill from east to west.

In the south of the wood there are wide, shallow former gravel pits but the origin of this gravel is unclear. It is too thin to be marked on the geological map but is probably ancient Medway gravel that has migrated downhill to this spot from the high ground at Hadleigh during the coldest periods of the Ice Age by the process of ‘solifluction’. Much of the surface geology of the wood is, in fact, composed of a thin deposit called ‘head’ which consists of a mixture of clays and sands from the underlying beds that have been reworked and redeposited by ‘freeze-thaw’ action during this time. Also present is a large amount of fine, wind-blown sediment called loess, which blankets a large part of south-east Essex and was deposited as a wind-blown sediment, again during the Ice Age.

Hadleigh Great Wood and Belfairs Wood are situated either side of the boundary between the districts of Castle Point and Southend but both Woods are owned and managed by Southend Borough Council. An excellent account of the geology of Hadleigh Great Wood by Jerry Bowdrey can be found in the booklet Hadleigh Great Wood: The Wildlife and History of Belfairs Nature Reserve, published by the South Essex Natural History Society (Spooner and Bowdrey 1988).


 

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Reference: Spooner & Bowdrey 1988 (p. 1-4).

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