Essex Rock & Mineral Society Field Trip
Hadleigh Castle Country Park.
Permanent geological section created on 22nd April 2007
by members of the Essex Rock and Mineral Society.
With the permission of the manager, a permanent two metre high section through the Bagshot Sand has been created at Hadleigh Castle Country Park. The site is at the top of the steep, wooded, south-facing slope of the spur of land south of Sandpit Hill (this spur was once known as West Hill) at approximately TQ 8018 8603. The work was carried out in about three hours by six members of the Essex Rock and Mineral Society (ERMS), kindly assisted by the country park manager, Andrew Woodhouse.
The Bagshot Sand is a yellow to orange brown fine-grained sand which was laid down in a shallow subtropical sea around 50 million years ago. It caps the high ground in many places in central and southern Essex. These are isolated remnants of a continuous deposit up to 25 metres (80 feet) thick that must originally have covered the whole county but has been largely removed by erosion. Some years ago it was possible to see Bagshot Sand in numerous Essex sand pits but these have all been infilled or are overgrown and apart from of the occasional pile of sand thrown out from rabbit burrows and the sandy soil on footpaths (for example at High Beach in Epping Forest) there are now no other known exposures of Bagshot Sand in Essex.
Hadleigh Castle Country Park was chosen as a site for a permanent section because the steeply sloping ground meant that it could be easily dug and also because nearby was the site of the British Geological Survey (BGS) Hadleigh borehole which meant that the stratigraphy of the Bagshot Sand in the area was well known. The BGS borehole was at an altitude of 70 metres (230 feet) above sea level near the top of Sandpit Hill (TQ 8002 8654) which is about 500 metres north of the ERMS section. The borehole was sunk in 1973 and revealed the complete geological succession in the Hadleigh area for the first time, passing through 10 metres (33 feet) of Bagshot Sand, 17 metres (55 feet) of Claygate Beds, 132 metres (433 feet) of London Clay, 12 metres (40 feet) of Oldhaven and Woolwich Beds before terminating in the Thanet Sand at a depth of 176 metres (576 feet).
Following completion of the work, ERMS member Roger Hewitt revisited the site two days later and produced a very detailed description (a graphic log) of the strata visible in the section, describing the colours by using the codes in the geologists’ standard Rock Colour Chart. Comparison was made with the BGS Hadleigh borehole, the graphic log of which is printed in full in the 1986 BGS memoir ‘Geology of the country around Southend and Foulness’ (pages 70-77). Despite the shallow depth of the ERMS section (about 2 metres), Roger demonstrated that it was possible to match the stratigraphy with the borehole record, showing that 1.35 metres of the strata in the section could be precisely correlated with the strata from 8.35 metres to 9.70 metres depth in the borehole, indicating that the strata dips south by ten metres or so between the borehole and the section. This correlation was possible because the Bagshot Sand here is variable; as you move up the vertical section, i.e. forward in geological time, the colour of the sand subtly changes and there are alternating layers of sand and silt, some with very fine laminations. Other layers in the sand have clearly been disturbed by burrowing organisms such as crustaceans which lived on this ancient sub-tropical sea floor. 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 that are known to occur in the Bagshot Sand. Like other 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. However, 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 Sussex Weald.
Photograph included to give one an idea of the fine detail observable. The laminations are very approximately in the order of 2 or 3 millimetres. These last two photograph if anything should remind one to take a ruler to put a scale to the detail.
Comparison with the borehole log also revealed that, fortuitously, the boundary between the Bagshot Sand and the underlying Claygate Beds occurs just above the base of the ERMS section which makes the section of much greater educational value. The Claygate Beds are best described as sandy clays that occur between the London Clay and the Bagshot Sand. The increasing amount of sand in the Claygate Beds indicates a gradual shallowing of the London Clay Sea and the position of the upper and lower boundaries of the Claygate Beds has therefore been controversial. The sediment being deposited on the subtropical sea floor about 50 million years ago became sandier as the sea retreated until it eventually became a vast complex of river deltas that spread across the whole of what is now Essex.
The heavy rain in the three months that has elapsed since the work was done has caused the bottom two parts of the section to slump but the upper part remains as a vertical sand face a metre or so high. It is hoped that it can be periodically cleaned and perhaps even extended and deepened on a future ERMS visit. Hadleigh Castle Country Park offers some of the best views in Essex and is only a short walk from Hadleigh Castle. It is best visited after a period of dry weather when the remarkably fine sand can be best appreciated. The main car park and toilets can be found at the bottom of Chapel Lane (Grid reference TQ 799868) which is a turning off the A13 at Hadleigh.
Gerald
Lucy
July
2007
Further reading:
Bristow, C. R., Ellison, R. A. and Wood, C. J. 1980. The Claygate Beds of Essex. Proceedings of the Geologists’ Association. Volume 91. Pages 261-278.
King, C. 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper No. 6.
Lake, R. D., Ellison, R. A., Henson, M. R. and Conway, B. W. 1986. Geology of the country around Southend and Foulness. British Geological Survey Memoir. HMSO.