Wealden district - North Downs
The North Downs are a range of hills extending from mid Surrey eastwards through northern and eastern Kent. They are bounded to the south by a steep slope but fall away gently to the north. This area of downland is formed by the Chalk, which is composed mainly of a very fine-grained white or pale grey limestone. The Chalk often contains nodules of flint, a very hard form of silica. It is best seen in the cliffs near Dover (Plate P209962) and Margate. The Chalk is generally about 250 m in thickness in this area, reaching about 280 m in north-east Kent. The Chalk is the most important aquifer, or source of underground water, in southern England. Most of the water flow in the Chalk is not through the pore spaces between the grains of the rock but along fine fractures within it. These fractures are both horizontal and vertical and connect together to make pathways for water to flow through. Because the Chalk is composed of calcium carbonate which can be slowly dissolved by groundwater, the fractures become wider over long periods of time leading to the quite rapid flow of water through some parts of the Chalk.
In the generally low-lying ground along the northern Kent coast, the Chalk is overlain by younger sedimentary bedrock layers comprising silt and fine-grained sand, beneath clay (Palaeogene sediments). Up to 200 m of these layers are present in places along this coast.
The Chalk is underlain by a layer of mudstone called the Gault Clay, which is about 40 to 50 m in thickness. This comes to the surface from beneath the Chalk at the foot of the North Downs. Other layers forming the younger sedimentary bedrock in the Weald (Cretaceous, Jurassic and Triassic sediments) also extend northwards under the Chalk of the North Downs, but these layers rapidly thin out in this area. This reflects the fundamental difference in the structure beneath the North Downs compared with that of the remainder of the region.
Beneath the younger sedimentary bedrock layers there is a down-fold or trough beneath east Kent that contains a coal-bearing sequence of sandstones and mudstones, similar to those found in the South Wales coalfield, and probably reaching more than 1000 m in thickness. The coalfield rocks (Coal Measures) are underlain by several hundred metres of hard Carboniferous Limestone beneath which lie basement rocks. As noted above most of the North Downs overlie the southern edge of the London Platform. However, the depth to the basement increases southwards rapidly falling to several kilometres depth in the Weald, and in places along the southern edge of the North Downs it is already more than 1 km below ground level.
These basement rocks are generally harder and denser than all the overlying sedimentary bedrock and they are also more strongly folded, in places tilting at steep angles. While the rocks themselves are less porous than the younger sediments they are cut through by fractures that do contain groundwater and include geological faults, where the rocks on each side of a fracture have moved relative to one another. These rocks are mainly between 500 to 360 million years old and comprise grey and red mudstones, siltstones and sandstones.