The Uplift of The Weald and The Erosion of Box Hill and The Mole Valley

The Early Cretaceous sediments were deposited in a basin centred over what is now the Weald. They were largely derived from a landmass that lay to the north. Now it is a common feature of the Earth’s crust that what goes down, must come up, facetiously referred to as ‘yo-yo’ tectonics, or more learnedly as ‘an axis of inversion’. By the Late Cretaceous subsidence had largely ceased in the Wealden basin. The Chalk was deposited over northwest Euroland with a great uniformity of stratification. Individual beds only a few centimetres thick can be traced from the Chilterns to the south side of the Paris basin and beyond. At the end of the Cretaceous Period, however, the Earth began to move. Inversion began. The ridge of land that had extended from Wales across to Belgium began to subside, and the Wealden basin began to dome upwards. As the core of the Wealden anticline (the name given to an up-fold of strata. A down down-fold is a syncline) was eroded the Chalk was stripped off to expose deeper sand and clay strata which were eroded in turn. Rivers carried the resultant detritus draining off the flanks of the Weald. Some rivers drained south into what was to become the English Channel, others north into the newly subsiding Thames basin.

Geophantasmogram of Dorking and its surroundings showing the relationship between geology and scenery. Interbedded sedimentary strata of the Cretaceous Period dip north forming the northern limb of the Wealden anticline, the southern limb of the London Basin. Well cemented hard strata from hills, soft sediments from valleys.

The sequence of illustrations shows how the core of the Wealden anticline was initially breached with a canoe-shaped valley. Over millions of years the chalk escarpments of the North and South downs migrated north and south respectively. It is tempting to think that their present position is a snapshot of a continuous if slowly moving process. The truth is rather more complex. The Wealden basin formed by the intermittent movement of a number of faults in the Earth’s crust parallel to the basin margin. As the basin became inverted the faults began to move in the opposite direction. One such fault system extends along the foot of the North Downs from the Hog’s Back, via Ranmore Common and Box Hill eastwards. The recent (15th C.) earthquake at Reigate demonstrates that this fault is still active.

Geophantasmograms to show the uplift and erosion of the Wealden anticline. Note the River Mole draining out between Box Hill and Ranmore Common. The River Wey, to the west of the Mole, eroded westwards along the Holmesdale to capture the headwaters of the Blackwater. This is why Farnham, though a ‘gap’ town like Guildford and Dorking, lacks its own river. The truncated Blackwater is an insignificant stream rising north of the North Downs.

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