'Wealden rivers' north of the Thames: a provenance study based on gravel clast analysis

Knowledge of the Thames drainage system prior to the diversion of that rive r by the Anglian glaciation is reviewed, with particular emphasis placed on evidence for the courses of south-bank tributary streams. These were exten ded versions of modern Wealden rivers, such as the Blackwater, Mole and Med way, which formerly flowed northwards across areas that are now drained sou thwards to the modern Thames. Their former courses are recorded by belts of gravel rich in Wealden rocks, particularly Greensand chert. Their input in to the main river can also be seen in changes in the composition of the ear ly Thames gravels, in which confluences with south-bank tributaries resulte d in the boosting of the Greensand chert component. This information has be en gathered over the past three decades or so by systematic clast-lithologi cal analysis of the various fluviatile gravels, a technique that has enable d their composition to be quantified, their provenance to be determined and the river responsible for their deposition to be identified.