The river Thames ran through this marsh. It was then much broader than at present, because there were no banks or quays to keep it within limits: at high tide it overflowed the whole of the marsh and lay in an immense lake, bounded on the north by this low cliff of clay, and on the south by the rising ground of what we now call the Surrey Hills, which begin between Kennington and Clapham, as is shown by the name of Clapham Rise.