Geology Site Account

Site name: Hadleigh Park

Hadleigh Park has landscape features of geological interest with disused pits that have provided information about the underlying geology. A disused brick pit in the park (TQ 7998 8641) provides a publicly-accessible section through the Bagshot Sand and is the best exposure of this formation in Essex. The section is provided with an interpretive board giving information about the geology of the park and the environment 50 million years ago when these sands were deposited. A full colour geological trail guide to the park has been produced by GeoEssex and is available from the park office.

Since 2012 the park has been used as a mountain bike centre with bike trails criss-crossing the site.

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Site description

Situated a only a short distance west of the castle, Hadleigh Castle Country Park offers some of the best views in Essex. The hilly landscape of the park consists of London Clay overlain by the sandy clay of the Claygate Beds which in turn passes up into Bagshot Sand on the highest ground. These rocks were laid down when Essex as submerged beneath a subtropical sea about 50 million years ago.

From the car park a track leads downhill past a large fishing lake on the left which is part of the country park but not open to the public. It has a good vertical section through the Bagshot Sand (TQ 7992 8676) which can be visited by appointment. The lake was originally a pit serving an extensive brickworks that was in existence during the first half of the twentieth century and some of the buildings of the former brickworks can still be seen. The steep slopes of the old pit are much disturbed by badgers which have a liking for these sandy rocks.

From here the path continues south to the main part of the country park which is a valley flanked by the steep slopes of Round Hill to the west and Sandpit Hill to the east. Both Round Hill and Sandpit Hill are capped by Bagshot Sand. From here the path continues down to grazing marsh, sea wall, and a narrow strip of saltmarsh alongside Benfleet Creek.There are meanders and oxbow lakes in Benfleet Creek.

The steep ground hereabouts is prone to extensive landslips. This is particularly evident on the east side of Sandpit Hill (on land owned by the Salvation Army) where successive rotational slips have created a series of sloping terraces separated by small cliffs or scarps. The scarps provide glimpses of the underlying geology with sticky clay at the bottom of the hill and sand at the top, providing a good illustration of the shallowing of the London Clay Sea. The mud laid down on the subtropical sea floor became more and more sandy until it eventually consisted entirely of the fine yellow sand we now call Bagshot Sand, which was probably laid down across most of Essex in a great complex of river deltas.

In 1973 a cored borehole was drilled by the British Geological Survey in order to study the complete geological succession in the Hadleigh area for the first time. The site chosen was near the top of Sandpit Hill (TQ 8002 8654) at an altitude of 70 metres (230 feet) above sea level so that the maximum depth of strata could be examined. After passing through 10 metres (33 feet) of Bagshot Sand and 17 metres (55 feet) of Claygate Beds the borehole penetrated the full thickness of London Clay which here was 132 metres (433 feet) thick. Below the London Clay the borehole continued through 12 metres (40 feet) of Oldhaven and Woolwich Beds (which yielded numerous fossil shells) before terminating in the Thanet Sand at a depth of 176 metres (576 feet). Beneath the Thanet Sand is the Chalk which forms the foundations of the London Basin. The borehole results are recorded in minute detail in the 1986 geological survey memoir for the Southend district. Of particular interest was the discovery of a septarian nodule at the base of the London Clay at a depth of 159 metres which contained particles suspected to have originally been volcanic ash. Bands of volcanic ash, probably from Scottish volcanoes, are common at the base of the London Clay near Harwich but this was the first time it had been identified in this part of Essex.

The best exposure of Bagshot Sand in the park is situated in an old brickworks pit on the side of Sandpit Hill and is publicly accessible by a gate in the fence (TQ 7998 8641). It is an excellent vertical section of yellow sand capped by dark earth with some pebbles. The section is dug into a series of delta and near-coastal sands. These are of egg- timer quality, being of very well sorted, 0.1mm, angular quartz grains. The soil at the top of the section contains flint pebbles and chert fragments. These are derived by soil creep from gravel beds higher up in Hadleigh. The chert is from the Lower Greensand of Kent; it was brought to this area by the ancestral river Medway.

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 subtropical 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.

The brickworks associated with this pit was in operation at the end of the nineteenth century. This brickworks and others on Salvation Army land were linked by a tramway to a small wharf on the Thames, and the embankment where the tramway crossed the main railway line can still be seen.

Section through the Bagshot Sand on the edge of the fishing lake (photo: Jeff Saward).