With the definition of exact frontiers for the English counties,
however, a process whose origin can hardly antedate the Norman
Conquest by many years, the Thames at once becomes of the utmost
importance as a boundary.

Its higher and hardly navigable streams are not so used. The upper
Thames and its little tributaries for some ten miles from its source
are not only indifferent to county boundaries, but run through a
territory which has been singularly indefinite in the past. For
instance, the parish of Kemble, wherein the first waters now appear,
has been counted now in Gloucester, now in Wilts. But when these ten
miles are run, just after Castle Eaton Bridge, and not quite half way
between that bridge and the old royal palace at Kempsford, the Thames
becomes the line of division between two counties, and from there to
the sea it never loses its character of a boundary.

It is a tribute to the great place of the river in history that there
is no other watercourse in England nor any other natural division of
which this is so universally true.

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;