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 other inlets from the lower river were flooded.  Yet it is remarkable that in this alluvium, more disturbed and dug than any other in Europe, little or nothing of human relics, of boats, or of piles has been discovered, and this absence of testimony also points to the remoteness of date from which we should reckon the human control of the river.

Here, as in many other conjectures concerning early history or pre-history, one is convinced of that safe rule which, in Europe at least, bids us never exaggerate the changes achieved by the last few centuries or the contrast between recorded and unrecorded things.

The tendency of most modern history in this country has been to exaggerate such changes and such contrasts.  In the greater part of modern popular history care is taken to emphasise the difference between the Middle and Dark Ages and the last few centuries.  The forests of England are represented as impassable, or nearly so; the numbers of the population are grossly underestimated; the towns which have had a continuous municipal existence of 1500 years are represented as villages.

The same spirit would tend to make of the Thames Valley in the Dark and Middle Ages a very different landscape from that which we see to-day.  The floods were indeed more common and the passage of the river somewhat more difficult; cultivation did not everywhere approach the banks as it does now; and in two or three spots where there has been a great development of modern building, notably at Reading, and, of course, in London, the banks have been artificially strengthened.  But with these exceptions it may be confidently asserted that no belt of densely inhabited landscape in England has changed so little in its natural features as the Thames Valley.