It is a thin unit, generally less than 2 m thick, but shows a northward increase up to greater than 10 m locally.
Lithologically it consists of a mixture of sands, silts and clays, locally with shells and lignite.
The unit is typically about 2 m thick but reaches a maximum of 7 m. It was deposited in an inner shelf marine environment.
Unit A consists of tuffaceous clayey silts and silty clays, with many layers of tephra.
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 or from the laying of sewers across Blackheath.
[9] 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.