Geology - Key to the Prosperity of Box Hill and The Mole Valley

The diverse rocks of Box Hill and the Mole Valley have contributed greatly to the prosperity of the area. Beginning a review at the base of the rock sequence, the Weald Clay has provided bricks, as the several brick works south of Dorking testify. Within the Weald Clay horizons of siderite iron ore, iron carbonate (CaCO3), were mined from Roman times onwards, peaking in the 16th and 17th centuries. Thin horizons of Paludina Limestone, cleverly marketed as Purbeck Marble, have been quarried for ornamental building stone.

Above the Weald Clay the Lower Greensand has been important for many reasons. The hard ‘ragstone’ horizons have been a popular building material. The softer sands of the Folkestone Beds at the top of the Lower Greensand have been widely used as building sand. In 1931 the Dorking Advertiser whimsically reported that the town was 167,900 tons lighter than in 1930; that being the weight of sand removed from the town’s quarries in the previous year. The purer sands were also used in glass manufacture. The ‘carstone’ within the Folkestone sands has been used, occasionally, as a source of iron ore, more widely as a building material, and, when ‘galleted’ into walls, as an effective witch repellent. The horizons of degraded volcanic ash within the Lower Greensand are termed ‘fuller’s earth’. (Fulling is the process that extracts lanolin from sheep’s wool, a necessary precursor to weaving). But fuller’s earth is a particular type of clay that has several other industrial uses. It has been quarried at several places along the Holmesdale to the east of the Mole Valley.

Dorking has been a major centre of poultry farming, as testified still by its eponymous 5-clawed breed of chicken. Poultry demand well-drained ground, and plenty of grit to aid digestion. The sandy soils of the Surrey Hills provide both. The Lower Greensand in general, and the Folkestone Beds in particular, are a major aquifer, albeit declining at an alarming rate, partly due to climate change, and partly due to excessive abstraction by the inhabitants of Mole Valley. The aquifer of the Lower Greensand is not only very pure, but also produces ‘chalybeate’ (iron-rich) water. For this reason it has been particularly suited for the cultivation of watercress (Rorippa nasturtium-aquaticum, to give it its full and delightful scientific name), believed to be a healthy source of iron. Watercress is not currently grown in the Mole Valley, but the Tillingbourne, as it flows off Leith Hill through Gomshall to join the Wey at Guildford, has hosted watercress beds for hundreds of years. Likewise the greensand waters between Guildford and Alton have also fed watercress beds since time immemorial.

Above the Lower Greensand the soft impermeable Gault clay has provided rich cattle pasture on the floodplains of the Mole and Pipp Brook. Further east along the Holmesdale the Gault Clay has been used for bricks. Its fossils, where concentrated in horizons, have been a source of phosphate fertiliser.

Above the Gault Clay lies the Upper Greensand. It is thin and of little economic importance in the Mole Valley, except in so far as it washes down and lightens the heavy Gault clay. But further east, at Godstone and Reigate, the Upper Greensand has been extensively mined in the past. The well-cemented variety, ‘firestone’, was used for refractory bricks. The soft variety, ‘hearthstone’, was used as a domestic abrasive .

Above the thin Upper Greensand is the thick sequence of Chalk of the North Downs through which the Mole has cut its valley on its way to the sea. Chalk is an important source of lime, for building mortar. It is also a fertilizer for the heavy soils of the Gault and London clays and a neutraliser of the acid soil of the Lower Greensand. Thus chalk quarries are a common feature on both the northern and southern limits of the Chalk. They are particularly spectacular along the escarpment of the North Downs on both sides of the Mole Valley. The quarries at Betchworth and Brockham are notable examples, being visible across the Weald for many kilometres. For the most part the Chalk is too soft to serve as a building stone. There are, however, several thin bands of hard well-cemented Chalk that have been quarried for building materials over the years. In the Mole Valley the most important of these is the Chalk Rock at the base of the Upper Chalk. The Chalk Rock was mined for building stone in the Westhumble caves. Galleries were excavated to pursue this stratum deep into the hillside. These caves are now a famously protected bat roost . The nodular bands of flint within the Chalk have been of great economic importance, first to provide tools for Stone Age hunters and cave wives, later in flintlock guns, and as a building material. Many houses in the Mole Valley are built of undressed flint nodules, some of the grandest of flint knapped into cubes.

Chalk is an important aquifer in the Mole Valley, across the London Basin and indeed across much of southern England. Chalk has a porosity of some 40% and can therefore store large amounts of water. The pores are extremely small, however, and permeability low (Porosity is a measure of the storage capacity of a rock. Permeability is a measure of the ability of fluid to flow through it). Chalk is commonly extensively fractured. Fractures impart an extremely high permeability to the Chalk and allow fluid to move in and out of the small pores within the Chalk rock itself. It is for this reason that chalk is such an effective aquifer. It is these same properties that make chalk eminently for viticulture. Vines need a modicum of moisture, but do not like to have their roots waterlogged. Chalk slopes are well drained, due to the permeability imparted by the fractures, but water is stored in the micro-porosity between the fractures. Vine roots may penetrate the fractures and easily access moisture in the water-saturated rock between the fractures. This is why the Champagne of France is a major centre of viticulture, and why vineyards occur on the Chalk slopes of the Mole Valley at Denbies, which was planted on the recommendation of a notorious local geologist.

Beyond the Chalk lies the lowlands of the London Clay, the main economic use of which has been for bricks.

To complete this review of the economic importance of the rocks of the Mole Valley it is necessary to describe its subterranean wealth. The earlier account of the uplift and erosion of the Wealden anticline was somewhat simplified. The Wealden basin subsided due to a series of west-east faults in the Earth’s crust. The uplift of the Wealden anticline occurred by the reversed movements of these same faults. This inversion formed a series of small anticlines adjacent to the faults. The North Downs is one of these complex faulted alignments, and a series of small oil and gas fields are trapped in anticlines on its southern side. From east to west these include the Palmers Wood oil field at Oxted, the Brockham oil field in the Mole Valley, and the Albury gas field further west.

This review shows how the diverse rocks of the Mole Valley have contributed to its past and present prosperity. This prosperity is based not only on the rocks themselves, but also on their contained fluids, their superficial soils, erosion and resultant beautiful scenery.

South to North geological cross-section through the Mole Valley to show the economic resources provided by each rock type. Note that vineyards have flourished at various times on sunny south-facing slopes. Thorncroft Manor and part of Denbies vineyards grow on raised Ice Age river gravels of the Mole.

Images from the Box Hill & Mole Valley Book of Geology by R C Selley. 2006. (C) The Friends of Box Hill.