It has been conjectured that there was a Roman ford or ferry at the east end of Little Wittenham Wood, where it touches the river.  The conjecture is ill supported.  No track leads to this spot from the south, and close by is an undoubted ford where now stands Shillingford Bridge.

Below this again there was no crossing until one got to Wallingford; and here we reach a point of the greatest importance in the history of the Thames and of England.

Wallingford was not the lowest point at which the Thames could ever be crossed.  So far was this from being the case that the tidal Thames could be crossed in several places on the ebb, notably at the passage 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 Caesar.  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; 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.