Geology of Kent

Silurian and Devonian

Rocks of this age are encountered at great depth in boreholes but are not exposed at the surface.

Carboniferous

The Kent Coalfield is a concealed coalfield working the Coal Measures at depth. These rocks are nowhere exposed at the surface but considerable detail is known from boreholes and underground workings for coal. A number of coal seams were worked in the past from the upper, middle and lower Coal Measures. Fourteen seams are named sequentially downwards as Kent No 1, Kent no 2 etc. The Lower Coal Measures lie unconformably on Carboniferous Limestone.

Triassic and Jurassic

Rocks of this age are encountered in boreholes but are not exposed at the surface. The Jurassic succession detected reflects that exposed at the surface further west in southern England.

Cretaceous

The oldest rocks to appear at the surface in Kent are the mudstones, limestones, siltstones and sandstones of the Wealden Group which underpin the wooded landscape of The Weald. These are in turn overlain by the sandstones of the Lower Greensand Group, the Upper Greenstone Formation and the mudstone of the Gault. The Grey Chalk Subgroup and finally the White Chalk Subgroup overlie these and form the North Downs which run from the famous White Cliffs at Dover west-northwestwards above Ashford and Maidstone before turning slightly south of west at the Medway gap towards Sevenoaks. The succession can be listed with modern stratigraphic terminology thus:

Note that the lowermost three units were formerly known by the names 'Ashdown Beds', 'Wadhurst Clay' and 'Tunbridge Wells Sand' and together constituted the 'Hastings Beds', a name no longer formally recognised by geologists but nevertheless common in the literature. The old name 'Wealden Series' includes the 'Hastings Beds' and overlying 'Weald Clay Formation'.

Palaeogene

Unconformably overlying the Chalk are a succession of sands and clays of Palaeocene and Eocene age. The Thanet Formation is the oldest of these and occupies an area from the coast near Sandwich east through Canterbury to Sittingbourne, Rochester and, intermittently, to Dartford. North of this outcrop, and stratigraphically above it, is a thinner band of pebbly, shelly sands and clays ascribed to the Lambeth Group. North again and following the Thames Estuary coast is the Eocene age silty and sandy clay of the London Clay Formation. It underlies Herne Bay, Whitstable and the Isle of Sheppey. The youngest Palaeogene strata is a patch of late Eocene sand, silt and clay around Eastchurch and Minster on the Isle of Sheppey. The succession in Sheppey can be listed with modern stratigraphic terminology thus:

Neogene

The Miocene/Pliocene age Lenham Formation occurs in a few small pockets around the village of Lenham between Ashford and Maidstone.

Structure

Kent straddles the northern limb of a regional scale upfold of strata known as the Weald-Artois Anticline which extends westwards as far as Hampshire and east across the English Channel into northern France. The fold arises from the continuing Alpine orogeny and results in the general northward dip of the rock strata in most of Kent.

Quaternary

There are extensive spreads of coastal and estuarine alluvium on the Thames Estuary coast, in the low ground surrounding the Isle of Thanet and across Romney Marsh. Alluvium also floors the valleys of the River Medway and Stour. 'Clay-with-flints' is a commonly occurring deposit across the North Downs which is a product of periglacial conditions during the last ice age. Loessic deposists, of aeolian origin i.e. windblown, are common in east Kent, especially on the northern slopes of the Downs where they are known as brickearths. Landslips are known along the north coast of the Isle of Sheppey, the coast near Reculver and either side of Folkestone and indeed the former coastline west of Folkestone overlooking Romney Marsh.