ocean-going craft; and it is true of the Thames.

The estuary of the Thames plays little or no part in the very early
history of England. Invaders, when they landed, landed on the
sea-coast at the very mouth, or appear to have sailed right up into
the heart of the country.

It is, nevertheless, true that the last few miles of tidal water, in
Western Europe at least, are not to be included in this first division
of a great river.

The swish of the tide continues up beyond the broad estuary, the
sand-banks, and the marshes, and there are reaches more or less long
(rather less than twenty miles perhaps originally in the case of the
Thames, rather more perhaps originally in the case of the lower Seine)
which for the purposes of habitation are inland reaches. They have the
advantage of a current moving in either direction twice a day and yet
not the disadvantage of greatly varying levels of water. Thus one may
say of the Seine in the old days that from about Caudebec to Point de
L'Arche it enjoyed such inland tidal conditions; and of the Thames
from Greenwich to Teddington that similar advantages existed.

The true point of division which separates, so far as human history is
concerned, the lower from the upper part of such rivers is the first
bridge, and, what almost always accompanies the first bridge, the
first great town. To repeat the obvious parallel, Rouen was this point
upon the Seine; upon the Thames this point was the Bridge of London.
It is with the habitable and historic Thames Valley above the bridge
that this book has to deal, and it will later be to the reader's
purpose to consider why London Bridge crossed the stream just where it