between Ealing and Kew, where Kew Bridge now stands; and, as we shall
see, the Thames was passable at many other places. But the special
character of the passage at Wallingford lay in the fact that it was a
ford upon which one could always depend. Below Wallingford the
crossings were either only to be effected in very dry seasons or,
though normally usable, might be interrupted by rain.

It is at Wallingford, therefore, that the main lowest passage of the
Thames was effected, and it was through Wallingford that Berkshire
communicated with the Chilterns. Wallingford is, then, the second
point of division upon the Thames when one is regarding that river as
a defence or a boundary. Below Wallingford there was perhaps a regular
crossing at Pangbourne; there was certainly a ford of great importance
between Streatley and Goring; and all the way down the river at
intervals were difficult but practicable passages--notably at Cowey
Stakes between the Surrey and the Middlesex shore, a place which is
the traditional crossing of Cæsar. The water here in normal weather
was, however, as much as five feet deep, and this ford well
illustrates the difficulties of all the lower crossings of the Thames.

The effect of the river as a barrier must, of course, have largely
depended upon the level to which the waters rose in early times. It is
exceedingly difficult to get any evidence upon this--first, because
however far you go back in English history some sort of control seems
always to have been imposed upon the river; and secondly, because the
early overflows have left little permanent effect.

As an example of the antiquity of the regulation of the Thames we have
the embankment round the Isle of Dogs, which is Roman or pre-Roman in
its origin, like the sea-wall of the Wash, which defends the Fenland;