The Thames
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 impor- tance. 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.
Of the original fords we have already drawn up a list. Few have, merely as fords, proved to be of strategic or commercial value. Oxford may have been an early ex- ception ; and the difficult passage at Abingdon founded a great monastery but no military post : the rise of each was connected, as was Reading (which had no ford), with the junction of a tributary. Wallingford alone, in its character of the last easy and practicable ford down the river, had for
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