The reason that the Thames, like so many other European boundaries, has come late into the process of demarcation, and the reason that its use as a limit is more apparent in civilised than in uncivilised times, is simply the fact that limits and boundaries themselves are never of great exactitude save in times of comparatively high civilisation. It is when a complex system of law and a far-reaching power of execution are present in a country that the necessity for precise delimitation arises. In the barbaric period of England there was no such necessity. Doubtless the men of Berkshire and the men of Oxfordshire felt themselves to be in general divided by the stream; but had we documents to hand (which, of course, we have not) it might be possible to show that exceptional tracts, such as the isolated Hill of Witham (which is much more influenced by Oxford than by Abingdon), was treated as the land of Oxfordshire men in early times, or was perhaps a territory in dispute; and something of the same sort may have existed in the connection of Caversham with Reading. In this old age of our civilisation the exactitude of the boundary which the Thames establishes is apparent in various survivals. Islands now joined to the one bank and indistinguishable from the rest of the shore are still annexed to the farther shore. Such a patch is to be found at Streatley, geographically in Berkshire, legally in Oxford; there is another opposite Staines, which Middlesex claims from Surrey. In all, half-a-dozen or more such anomalous frontiers mark the course of the old river. One arrested in process of formation may be seen at Pentonhook. A boundary--that is, an obstacle to travel--has this further feature, that the point at which it is crossed--that is, the point at which the obstacle is surmounted--is certain to become a point of strategic and often of commercial importance. So it is with the passes over mountains and with the narrows of the sea, and so it is with fords and bridges over rivers. So it is with the Thames. The energies both of travel and of war are driven towards and confined in such spots. Fortresses arise and towns which they may defend. Depots of goods are formed, the coining and the change of money are established, secure meeting places for speculation are founded. Such passages over the Thames were of two sorts: there are first the original fords, numerous and primeval; next the crossing places of the great roads.