The Harwich Formation is a

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

found in the

London Basin The London Basin is an elongated, roughly triangular sedimentary basin approximately long which underlies London and a large area of south east England, south eastern East Anglia and the adjacent North Sea. The basin formed as a result of compr ...

of southeastern

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. It is

Ypresian In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

(early

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

) in age. It lies unconformably on the

Lambeth Group The Lambeth Group is a stratigraphic group, a set of geological rock strata in the London and Hampshire basins of southern England. It comprises a complex of vertically and laterally varying gravels, sands, silts and clays deposited between 56 ...

over most of its extent, but may overlie either the

Thanet Formation The Thanet Formation is a geological formation found in the London Basin of southeastern England.Chalk Group The Chalk Group (often just called the Chalk) is the lithostratigraphic unit (a certain number of rock strata) which contains the Upper Cretaceous limestone succession in southern and eastern England. The same or similar rock sequences occur ac ...

to the south. It is overlain by the London Clay Formation.

Subdivision

The formation is subdivided into several members.

Swanscombe Member

The Swanscombe Member is the youngest member recognised within the Harwich Formation. It is a thin unit, generally less than 2 m thick, but shows a northward increase up to greater than 10 m locally. It is glauconitic, generally markedly so. Lithologically it consists of a mixture of sands, silts and clays, locally with shells and

lignite Lignite (derived from Latin ''lignum'' meaning 'wood'), often referred to as brown coal, is a soft, brown, combustible sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35% and is considered the lowest ...

. It typically has a gravelly base. It was deposited in a mid to inner shelf marine environment.

Tilehurst Member

The Tilehurst Member is regarded as a local variant of the Oldhaven Member by the BGS but as a separate unit by other workers. It is found to the west and northwest of London. It consists of glauconitic silt, sandy silt and sandy clayey silt, with sand layers in the form of thin laminae. The unit is typically about 2 m thick but reaches a maximum of 7 m. It was deposited in an inner shelf marine environment.

Harwich/Wrabness Member

The Harwich Member is regarded as obsolete by the BGS, being replaced by the Wrabness Member. The Wrabness Member is subdivided into a lower Unit A and an upper Unit B. Unit A consists of tuffaceous clayey silts and silty clays, with many layers of

tephra Tephra is fragmental material produced by a Volcano, volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, ...

. The upper boundary with Unit B is an apparent

disconformity An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

. Unit B consists of fine-grained sand with layers of clay, affected by bioturbation. The Wrabness Member reaches a maximum thickness of 24 m.

Oldhaven Member

The Oldhaven Member is dominantly a fine-grained glauconitic sand with cross-bedding, lamination and evidence of bioturbation.

Blackheath Member

The Blackheath Member is a distinctive unit that has been proposed to be raised to formation status.

Hales Clay/Orwell Member

The Hales Clay Member is regarded as obsolete by the BGS, being replaced with the Orwell Member. The Orwell Member is itself subdivided into three parts, known, from the base, as units A, B and C. Unit A consists of up to 1 m of fine-grained glauconitic sands with black gravel and some fossil fragments at the base. Unit B conformably overlies Unit A, consisting of up to 1.75 m of bioturbated silty sands and sandy clayey silts, locally with broken shells and dark clay laminae. Unit C, up to 2.5 m in thickness, consists of tuffaceous sandy silt with fine sand laminae.

Use

Gravels and sands of the Blackheath member were at one time extensively quarried across Blackheath. Of the numerous quarries, only two remain relatively untouched, with the remainder being infilled either with rubble from bomb-damaged areas in the

Second World War World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

or from the laying of sewers across Blackheath. On the northeast side of the Heath, Vanbrugh Pits exposes the typical black flint pebbles in the sides and on the base of the old quarry. On the southwestern edge of the Heath, Eliot Pits were quarried down to the base of the Harwich Formation, the top of the underlying Lambeth Group.

References

{{Reflist Geology of London Paleogene England Geologic formations of England