strates 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;
and at Ealing, Staines, Abingdon, and twenty other places we have
sites probably pre-historic, and certainly at the beginnings of
history, which could never have been inhabited if the neighbouring
fields had not been drained or protected. The regularity of the stream
has therefore been somewhat artificial throughout all the centuries of
recorded history, and the banks have had ample time to acquire
consistency.

It is certain, of course, that works of planting, of draining, or of
embankment, which required continuous energy, skill, and capital,
decayed after the coming of the Saxon pirates, and were not undertaken
again with full vigour until after the Norman Conquest. Even to-day
the work is not quite complete, though every year sees its
improvement: we are still unable to prevent regularly recurrent floods
in the flats round Oxford and below the gorge of the Chilterns; but
for the purpose of this argument the chief fact to be noted is that no
serious interruption to the approach of the river seems to have
existed in historic times.

In pre-historic times many stretches of the river must have afforded
great difficulties of approach. The mouths of the Ock, the Coln, the
Kennet, the Mole, and the Wandle must each have been surrounded by a
marsh; all the plain between Oxford and the Hinkseys must have been
partially flooded, as must the upper reaches between Lechlade and
Witham (on one side or the other of the stream as it winds from the
southern to the northern rises of land), and as must also have been
the long stretch of the right bank below Reading. The highest spring
tides may have been felt as high up the stream as Staines, and both
the character of the surface and the contour lines permit one to
conjecture that the valley of the Wandle and several