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. 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
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. The upper boundary with Unit B is an apparent
disconformity. 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==