Everything then conspired to make this first crossing of the Thames
the chief commercial point in Britain; and, since we are considering
in particular the history of the river, it must be noted that these
conditions also made of London Bridge what we have remarked it to be,
the chief division in the whole course of the stream. This character
it still maintains, and the life of the river from the bridge to the
Nore is a totally different thing, with a different literature and a
different accompanying art, from the life of the river above bridges.

We have seen that the river when it is regarded as an avenue of access
to men for commerce or for travel is, especially in early times, and
with boats of light draught, of one piece from Lechlade to London
Bridge. There was in this section always sufficient water even in a
dry summer to float some sort of a boat. But the river, regarded as a
barrier or obstacle for human beings in their movement up and down
Britain, did not form one such united section. On the contrary, it
divided itself, as all such rivers do, into two very clearly defined
parts: there was that upper part which could be crossed at frequent
intervals by an army, that lower part in which fords are rare.

In most rivers one has nothing more to do in describing those two
sections than to show how gradually they merge into one another. In
most rivers the passage of the upper waters is perfectly easy, and as
one descends the fords get rarer and rarer, until at last they cease.

With the Thames this is not the case. The two portions of the river
are sharply divided in the vicinity of Oxford, and that for reasons
which we have already seen when we were speaking of the suitability of
its banks for habitation. The upper Thames is indeed shallow and
narrow, and there are innumerable places above Oxford where it could