Kent (including Medway)
Kent lies on the northern side of a large and eroded dome of rocks known as the Wealden anticline. This structure, and the rocks comprising it, strongly influence the local landscape and give rise to the North Downs.
Kent is dominated by a nearly complete sequence of Cretaceous rocks, deposited within and on the edge of a shallow sub-tropical sea. The rocks are tilted to the north, with the consequence that increasingly younger rocks are exposed the further north you travel with the youngest Tertiary rocks exposed along the north Kent coast.
During the Ice Age of the last two million years, no glaciers reached Kent, but the area was under the influence of very cold tundra-like conditions at the southern edge of the ice sheets. Processes of erosion and deposition during this period have contributed significantly to the formation of the present Kent landscape.
Cretaceous
The Lower Cretaceous sequence begins with a series of sands and clays comprising the Ashdown Sand, Wadhurst Clay, Tunbridge Wells Sand and the Weald Clay (together forming the Hastings Beds), which form the core of the Wealden anticline. These sediments were all deposited in a fresh to brackish-water embayment (which, apart from its north coast, covered most of Kent) opening out into marine areas to the north (into East Anglia) and south (into France).
The sands were deposited on a broad zone of braided river floodplains and channels, while the clays were deposited in quieter-water, shallow lagoons and bays. The Wadhurst Clay contains several marsh soil beds towards its base and top indicating that the embayment at this time was very shallow.
Rocks of the marine Lower Greensand overlie the non-marine Weald Clay. Marine conditions were established as the connection of the embayment with the sea became stronger and sediments were deposited in a variety of shallow marine and nearshore environments. Initially, offshore muds (now shales and mudstones) of the Atherfield Clay were deposited followed by shallow marine sands of the Hythe, Sandgate and Folkestone Beds.
The Hythe Beds comprise alternating layers of sandy limestone and calcareous sandstone ('rag and hassock'). The Folkestone Beds are predominantly made up of sandstone deposited in a strongly tidal nearshore environment.
A continued rise in sea level led to formation of a larger and deeper sea and deposition of the Gault Clay and Upper Greensand. These rocks are of the same age, with the mudstones of the Gault predominating in east Kent and the sandstones of the Upper Greensand occurring mainly to the west of Sevenoaks. The Gault contains phosphatic nodules in discrete bands and has a rich marine fauna with abundant ammonites, bivalves and gastropods.
A major phase of land subsidence marked the beginning of the Upper Cretaceous, leading to deposition of the Chalk. The chalk is a white pure limestone. It contains thin beds of marl and nodules of flint, either scattered or in bands. The Chalk Downs surround the Weald, with the North Downs extending from Farnham (Surrey) to Dover. The cliffs of Kent provide excellent sections through the chalk but the rocks are often more accessible in the many inland pits and quarries.
Tertiary (comprising Neogene and Palaeogene)
At the end of the Cretaceous there was a major lowering of sea level and The Weald of Kent became a low-relief land area with a shallow sea to the north. Deposition of the Thanet Sand in this sea marked the beginning of the Tertiary Period (65-2 million years ago).
In north-east Kent, the Thanet Sand is a marine sandy clay that contains the remains of bivalves, gastropods and sharks teeth. In north-west Kent, the deposits are more brackish and sandy, and fossils are rare. The shallow marine, estuarine and lagoonal sands and clays of the Upnor and Woolwich Beds were deposited after the Thanet Sand and in some locations shell beds packed with bivalves and gastropods occur.
After these marginal marine conditions, a rise in sea level around 50 million years ago led to the deposition of the sands of the Oldhaven Beds and the London Clay. The London Clay is widespread across north Kent and is a uniform marine clay, which rests with a sharp contact on the underlying Woolwich Beds. Bands of harder, cemented clay, known as cementstones within the Clay, have yielded a diverse range of marine fossils including starfish, crabs, lobsters, fish, turtles, bivalves and gastropods.
Quaternary
Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases). The ice sheets did not reach as far south as Kent, but the influence of the alternating cold and warm phases can still be seen in the landscape.
Deposits from this time typically include gravels deposited in river floodplains, which form terraces at various heights on the valley sides, and head. Head comprises angular pieces of rock and soil derived locally from the extensive frost shattering of rocks and the subsequent movement of this material down valley slopes. Large areas of clay-with-flints, derived from the weathering of material overlying the present-day chalk, occur across the North Downs and wind-blown, fine-grained sediment known as loess is also found across much of north Kent.
On the coast, Dungeness, the largest shingle structure in the United Kingdom has developed over the past 5000 years. Shingle is aligned in sub-parallel ridges of differing ages with the oldest ridges furthest from the coast. Behind, Romney Marsh is reclaimed saltmarsh protected by the shingle of the ness.
Geological highlights:
- The most impressive large-scale feature of the scenery of Kent is the manner in which the domed form of the Wealden anticline has been picked out by erosion of the variably resistant Cretaceous rocks. The central Weald (south Kent) is composed of the sandstones and clays of the Hastings Beds forming an area of deeply dissected high ground. This passes northwards into a broad zone of lower-lying land occupied by the softer shales of the Weald Clay, which in turn gives way to the north into the Lower Greensand escarpment. The Chalk North Downs encircle the Lower Greensand hills, from which they are separated, by a narrow belt of low lying land marked by the softer Folkestone Beds and Gault Clay.
- Building stone was formerly obtained from shelly limestones in the Weald Clay. Bricks of various grades and qualities are manufactured in large quantities from many of the clay units, particularly the Weald Clay.
- When fresh, the Greensand rocks commonly have a green colour due to the presence of the mineral glauconite, hence the name 'Greensand'.
- Major landslips are common along inland escarpments and parts of the coast. A large slip can be observed at Folkestone Warren, where massive chalk has slipped on underlying Gault Clay. Here twelve major landslips have occurred since 1765, the most notable being in 1915 when the coastal railway line was displaced.
- River sands and gravels associated with the Thames were deposited during a warm period of the Ice Age (probably the Hoxnian, some 400,000 years ago) in North Kent. One site at Swanscombe, near Dartford, is famous for the discovery here of the remains of a human skull and many human artefacts such as hand axes. The interpretation of the skull remains has caused controversy since their discovery in 1935. The general consensus is that the skull belonged to early Neanderthal man who flourished during the last glaciation. The Swanscombe site has also yielded a superb fossil mammal and bird fauna, including species such as lion, straight-tusked elephant, 2 extinct species of rhino and a macaque amongst others.
- The cliffs and foreshore between Warden and Minster on the north coast of the Isle of Sheppey provide extensive exposures through the London Clay and represent one of the best-known fossil collecting sites in Britain. The section has been the focus of scientific study since the 18th century and, over this time, has yielded a considerable number of fossil animals and plants. The articulated remains of crabs and lobsters occur at some levels and fossil insects have also been found. The remains of fossil plants (seeds and fruit) also occur, with tropical lianas and coconut being present. The site is, perhaps, most famous for the fossil fish that are found here, with more than 160 species recorded, including many species of shark. Large numbers of fossil turtles, crocodiles, snakes and birds have also been found making this the most important Tertiary fossil site in Britain.
- There has been a small coal mining operation in east Kent since the end of the 19th Century. The coal occurs in Upper Carboniferous rocks and represents a continuation, under the English Channel, of the more extensive coalfield of northeast France and northwest Belgium. These rocks do not outcrop at the surface but occur at depth. Coal was originally discovered in 1896 at the foot of Shakespeare Cliff, Dover, but its extraction was found to be uneconomic at this location. Later exploration and the sinking of shafts between Dover and Canterbury led to the opening of a number of collieries (e.g. Snowdon, Tilmanstone). However, Kent Coal is friable (crumbly) and unsuitable for domestic use; extensive works also had to be employed to contain water flows in the mines. The commercial exploitation of the coalfield lasted less than a century, with Betteshanger the last of the collieries closing in 1989.
Local sites
The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.
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