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Hambro Hill Sand Pits, RAYLEIGH, Rochford District, TQ81509194, General geological site
Site of geological interest with disused and working pits that have provided information about the underlying geology. Any further significant excavations in the area should be recorded.
Hambro Hill, just north of Rayleigh town centre, consists of the sandy clay of the Claygate Beds overlain by a thick and extensive outcrop of Bagshot Sand. There have been several sand pits here over the years providing fine exposures of Bagshot Sand – a geological formation that is very poorly exposed elsewhere in Essex.
One of the many pits working the clay and the sand here was Hambro Hill Brickworks, which apparently worked the Claygate Beds and was active between about 1899 and 1920. These pits have provided some of the most magnificent exposures of Bagshot Sand to be seen anywhere in Essex, with as much as 10 metres (30 feet) thickness of sand sometimes being visible. Visiting geologists have noted over the years that the Bagshot Sand here is exceedingly fine-grained and much more so than Bagshot Sand further west in Essex. This sand is therefore a truly marine deposit and distinctly different in character to the equivalent rocks at Bagshot in Surrey, after which the formation is named.
Towards the railway line the Bagshot Sand is overlain by iron-stained (ferruginous) gravel, which is not shown on the geological map but is ancient gravel laid down by the River Medway and similar to that which is present elsewhere on the Rayleigh Hills. An Essex Field Club visit to the area in 1925 visited several of the Hambro Hill pits and reported that the gravel was some 6 feet (2 metres) thick and contained many fragments of Lower Green-sand chert, confirming that the gravel originated in Kent. A detailed investigation of this gravel, and its erosive junction with the Bagshot Sand, was carried out by J.T. Greensmith (see French 1968).
The pioneer geologist and geographer S.W. Wooldridge (1900-1963) visited these sand pits in the early 20th century as part of his research on the geology of the Rayleigh Hills. In a paper of 1923 he describes in great detail the Bagshot Sand in the upper pit at Down Hall Brickworks, which is almost certainly a reference to the Hambro Hill pits. He describes the sand as being a pale buff colour, with lenticular bands of pale brown and lilac-coloured clay. The clay bands occurred at intervals of about 6 feet throughout the section and he attributed this to recurrent periods of heavy rainfall in the Eocene period when river-borne mud was spread widely over the sea floor.
Wooldridge also describes some extraordinary sedimentary structures in the Bagshot Sand such as ‘curled bedding’ which he attributed the sub-aquatic slippage of the newly deposited sand down a gentle marine slope. The sand was also broken up by numerous clean-cut faults, which Wooldridge could not account for considering the softness of the sediment. It may be that this was due to mass movement of the strata when it was saturated with water and frozen as ‘permafrost’ during one of the coldest periods of the Ice Age. He also described how iron-compounds had infiltrated the sand from the ferruginous gravels above and this had, in places, given the sand the consistency of a sandstone. The iron solutions had also diffused outwards from centres to form concentric layers of hardened sand, sometimes several feet in diameter. The face of the pit had cut through these ‘shells’ and wind erosion had resulted in the hardened portions standing out from the mass of the sand.
There are still some exposures of Bagshot Sand on Hambro Hill but the land is private land with no public access. It now consists of several industrial units and a wood yard.
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