Geology of Wolstonbury Hill

Approaching the South Downs from the north along the London to Brighton road (A23) the impressive line of the South Downs rises more than 200 metres from the low lying clay vales. As the A23 cuts into the Downs, Wolstonbury Hill is at the forefront of the Downs, stepped forward from the westward continuation of the line from NewtimberHill to Steyning. The A23 road follows the natural route with the lowest gradient where the older Gault clay and Grey Chalk occupy the ground leading around Wolstonbury past Star House and Newtimber Limeworks to the low col at Pyecombe (Figure 1)

hill

Figure 1. Map of the Wolstonbury Hill area showing the relatively low lying col at Pyecombe separating Wolstonbury Hill from Newtimber Hill and providing an easy route for the A23 to enter the Downs. The col and low lying ground to the northwest is formed along the line of the Pyecombe

Anticline (see Figure 2).
geology map of the Wolstonbury and Newtimber

Figure 2. Simplified geology map of the Wolstonbury and Newtimber Hill area showing the distribution of Chalk formations and showing how the Pyecombe Anticline brings older and softer rocks to the surface creating the separation (col) between the two hills. Compare the ground contours shown on Figure 1 with the distribution of formations in Figure 2. Map based on British Geological Survey Sheet 318/333 Brighton and Worthing, 2006. Since 1992 the Chalk of England has been completely remapped using the new stratigraphy (Figure 3) developed in Sussex and Dorset (Mortimore, 1986, 1997; Bristow et al., 1997). Hence the geographical names for many of the formations are based on Sussex field sections

Including quarries around Lewes (New Pit and Lewes chalks) and the coast sections from Eastbourne (Holywell) to Seaford and Newhaven.  Other names come from Dorset (West Melbury and Zig Zag Hill), and Hampshire (Culver and Portsdown). Each of the formations has a distinctive impact on the geomorphology of the Downs across England and France and this is well displayed at Wolstonbury.

Broadly, the Chalk strata making the South Downs dip gently southwards into the English Channel. This gentle dip creates the ideal scenario for the formation of escarpments with scarp slopes and dip slopes. Wolstonbury is part of the main or Primary Chalk Escarpment. Secondary and tertiary escarpments are created by younger formations to the south. These escarpments and intervening dip slopes reflect the different properties of each formation (e.g. Bristow et al., 1997).

At the foot of the Primary Escarpment beneath Wolstonbury is a thin Upper Greensand Formation. The contact between the Upper Greensand and the Chalk is frequently a spring line (e.g. at Fulking) and this is reflected in the presence of wells at Star Cottage and The Warren along the base of Wolstonbury. From the bench created by the Upper Greensand, along which early transport routes developed, the first Chalk formation is rich in clay (West Melbury Marly Chalk) and weathers out with a concave slope. The harder, higher density Holywell Nodular Chalk Formation forms a convex slope, and in places an extended bench or platform in the lower slopes. A conspicuous concave slope in the main part of the escarpment face is formed in the New Pit Chalk Formation which also produces a creamy white and ‘slabby’ brash where ploughed in contrast to the rough nodular brash of the Holywell and Lewes Chalk formations. The harder Lewes Nodular Chalk formation forms a convex slope at the summit of the Primary Escarpment and caps the highest hills including Wolstonbury.

In the South Downs the first regular flint bands occur in the Lewes Nodular Chalk Formation and these flint bands are exposed in the old scrapings on the western side towards the summit of Wolstonbury. These are some of the best developed flints in the English Chalk and correlate with the Grimes Graves flints of Norfolk where they were mined in the Neolithic. Other old Chalk pits around Wolstonbury still expose Chalk representing the different formations. Only one working pit remains, Newtimber Limeworks and this exposes an excellent section in parts of the Grey Chalk and a nearly complete exposure of the Holywell Chalk, including the Plenus Marls and Melbourn Rock.

References
Bristow, R., Mortimore, R.N. and Wood, C.J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists’ Association, 108, 293–315. Mortimore, R.N. 1986b. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the  Geologists’ Association, 97, 97-139.
MORTIMORE, R.N. The Chalk of Sussex and Kent. Geologists’ Association Field Guide No.57. 139pp. MORTIMORE, R.N., WOOD, C.J. & GALLOIS, R.W. 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series, No. 23. Joint Nature Conservation Committee, Peterborough. 558pp

rd stratigraphyFigure 3. Standard stratigraphy for the Chalk of southern England showing the range of Chalk involved in the formation of Wolstonbury.


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