ss 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.

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 exception; 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 centuries an
importance certainly due to geographical causes alone. Two principal
events of English history--the crossing of the Thames by the Conqueror
and the successful challenge of Henry II. to Stephen--depend upon the
site of this crossing. Long before their time it had been of capital
importance to the Saxon kings, so early as Offa and so late as Alfred.
If the bridges built at Abingdon in the fifteenth century had not
gradually deflected the western road, Wallingford might still count
the fourteen churches and the large population which it possessed for
so many centuries.

Apart from Wallingford, however, the fords, as fords, did little to
build up towns or to determine the topography of English history. Of
more importance were the crossings of the great _roads_.

When one remembers that the south of England was originally by far the
wealthiest part of the country, and when one considers the shape of
Ireland, it is evident that certain main tracks would lead from north
to south, and that most or all of these would be compelled to cross
the Thames Valley. We find four such primeval ways.

One from the Straits of Dov