The Great Stour Project - River and Coastal Flooding

Castle Hill, Folkestone: geology and scenery

Castle Hill (213378) is situated on the crest of the North Downs immediately to the North of Folkestone. Its name derives from a fortification known as Caesar’s Camp, a hill top stronghold formerly consisting of a bailey and surrounding ditches protected by wooden pallisades. Although its name suggests a Roman origin, it is believed to date from around 1140 and is therefore Norman in origin.

From an elevation of 140 metres, there is a splendid view over the Shuttle terminal at Cheriton, along the scarp of the North Downs and across the low Weald to the shingle promontory of Dungeness. To the south, the town of Folkestone extends to the shore of the English Channel and beyond, the white cliffs of the Cote d’Opale in France.

With the exception of the Dungeness foreland, the landscape is composed of sedimentary rocks of the Cretaceous period which lasted from 135m to 65m years BP. In the early Cretaceous, the study area was part of a large delta into which rivers deposited sands known as the Wealden Series. After some time, however, the sea advanced leading to the accumulation of Gault Clay which was in turn succeeded by a great thickness of Chalk (Fig. 1). It is known that the chalk which today stretches from Salisbury Plain in the West to the Flamborough Head in the North and underlies much of Picardy and the Paris Basin was much more extensive. Isolated outcrops are preserved as far afield as the island of Mull and County Antrim. The Lower Greensand, which forms a secondary escarpment in the western part of the Weald is not present in the east.

At the end of the Mesozoic era, the long period of Chalk deposition was halted by gentle uplift and the sea retreated from much of the area it had covered during the Upper Cretaceous. In Southern Europe, the mid-Tertiary brought the onset of violent orogenic folding leading to the creation of the Alpine fold mountain chain. Britain was subjected only to the outer ripples of the Alpine storm resulting in the development of a few open folds in the Mesozoic and Tertiary strata. The chief of these structures are the Wealden Anticline flanked by the London Basin, the Hampshire Basin and the Paris Basin. Superimposed on the broad flexures, there were local smaller folds such as the Thanet pericline which brings the chalk back to the surface between the depressions of Pegwell Bay and the Thames Estuary.

Much of the Tertiary was a period of relentless erosion and planation of the Wealden dome such that, by the Miocene period an extensive peneplain is thought to have extended over much of the area. The Eocene folding would certainly have fissured the chalk, particularly on the anticlines, and the softer rocks beneath would have afforded little resistance to marine or sub-aerial attack.

Subsequent differential erosion has produced the general form of the Weald today. The core is composed of the sandy Hastings Beds and forms the elevated ground of the Central or High Weald which culminates in Beacon Hill (240m). This upland is bounded on three sides by the Low Weald developed on the thick Weald Clay, forming the lowland below the downland scarp. The North Downs themselves are part of the northern rim of the Weald which continues via the Hampshire Downs and South Downs to the sea at Beach Head. To the north of Castle Hill the chalk strata dip to the north and is reflected in the gradual reduction in altitude and its eventual disappearance under the Ash levels. To the west and east, the line of the scarp can be followed toward the Wye Gap and the White Cliffs of Dover respectively. The reason why the Downs converge to the west is due to the overall westward thickening of the strata and the regional plunge of the anticline, at an angle of between 1 and 5 degrees, to the west.

The differential resistance of the Wealden rocks should be reflected in the shape of the coast with the more resistant forming headlands and the least resistant forming bays. At first it appears that this is not universally true since the very prominent Dungeness foreland corresponds with the clay of the Low Weald. However, the Dungeness promontory is the product of recent sedimentation in what was previously a broad bay. If this is taken into account, then the coastline does exhibit the expected characteristics. Following the coast in a southerly, then westerly, direction from the Isle of Thanet the North Foreland is succeeded by Pegwell Bay, the South Foreland, Dungeness and finally Beachy Head where the South Downs meet the sea.

For further reading see:

David K. C. Jones (1981), “South East and Southern England”, Methuen
H. H. Read and Janet Watson (1966), “Beginning Geology”, Macmillan