Geology Site Account
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Hockley Woods, , Rochford District, TQ832918, Potential Local Geological Site
Site of geological interest with potential for promoting geology. The geology revealed by any significant excavations in the park should be recorded.
Hockley Woods is one of the largest continuous stretches of native woodland in the east of England. The foundations of the woods is London Clay which occurs in the floor of a valley at the eastern end. Above this is the sandy clay of the Claygate Beds, and as the ground rises towards the west the Claygate Beds pass up into fine yellow sand of the Bagshot Beds which forms the high ground. There are many fallen trees and their upturned root plates provide glimpses of the underlying geology.
The high ground of Hockley Woods is a continuation of the Rayleigh Hills and, like Hadleigh and Thundersley, the highest points are capped with patches of ancient gravel which were deposited by a northward-flowing Medway river over 700,000 years ago. The gravel at Hockley Woods, known as Oakwood Gravel, is shown on the geological map as capping the high ground in two places on the southern boundary of the woods but it also appears to be present on the sloping ground lead-ing away from the main car park. Oakwood Gravel is a relatively well preserved fragment of one of the highest and oldest terraces of the Medway, part of a left-bank terrace system of a valley now submerged beneath the North Sea (Bridgland 2003). As a result of the considerable erosion that has taken place during the Ice Age these and other isolated patches of gravel on the top of the Rayleigh Hills are all that remains of these ancient river terraces. The pebbles in the gravel are mainly flint but there is also a proportion of distinctive rocks from the Weald of Kent that proves its Medway origin.
The main paths through the woods have been paved with imported grey limestone aggregate but on the other paths, such as on the southern boundary, the pebbles can often be seen underfoot. Most pebbles are extremely well-rounded indicating that they must have been part of a beach in the distant past, probably during the Palaeocene period some 50 million years ago. It is impossible to exactly recon-struct the surface geology of Essex at the time the River Medway flowed through here but its tributaries must have brought these smooth, almost spherical, pebbles here from the west, where these older Palaeocene rocks are present at the surface.
During the intense cold of the most recent glaciation the ground was permanently frozen (permafrost) with only the top layer thawing during each brief summer. This layer of saturated soil and rock was able to slowly flow downhill bringing some of the pebbles and sand down into the valleys.
The pebbles of Hockley Woods, like the pebbles of Thorndon Country Park in Brentwood, therefore have a long history. From their origin as flint nodules in the Chalk, some 80 million years ago, they are now on their way downhill, via streams and rivers, to their third resting place - the floor of the North Sea.
Hockley Woods is owned by Rochford District Council and is accessible at all times. The main entrance and car park is situated off the B1013 at Hockley.
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Reference: Bridgland 2003
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