that distance, for
Port itself is 500 yards in length from east to west. We can be certain,
however, that so far as tradition goes we need not look more than a mile
below Port for the ford, nor less than say half a mile from its last
houses.

Fortunately, we have other convergent indications which can guide us with
greater precision.

We must remember that, apart from the bringing of merchandise over to the
neighbourhood of Port, the ford, which may, and most probably did, exist
before Port became of any importance, led all the central traffic of the
Vimieux country (which is the district on the left bank of the Somme)
towards the Straits of Dover and their principal port at Boulogne.

Now, the way from the right bank of the Somme to Boulogne is interrupted
by several streams, much the most marshy and broad of which is the Authie.
The Romans bridged the Authie at _Ad Pontes_ in the course of their great
Trunk Road to Britain, and any way which led from the lowest ford over the
Somme to Boulogne would have to join that great Trunk Road before or at
the bridge if it were to take advantage, as commerce would have to do, of
that sole passage of the very difficult and marshy Authie valley which can
nowhere be crossed save upon a causeway. I have in a former page remarked
upon the importance of Ad Pontes (the modern Ponches), and pointed out
that it gives the whole county its name of Ponthieu. We must expect,
therefore, any direct commercial way northward from the ford to make
directly for Ponches. To strike the great Trunk Road higher up would be to
go out of one's way; to strike it lower down would be to strike the Authie
Valley at an impassable point.

When an ancient way has disappeared, certain indications of its track,
especially as that track may be presumed to be direct, survive, and among
these are wayside tombs, parish boundaries, and mills or other places
which, for the conveyance of heavy merchandise, are placed near such a
road if possible. All these three kinds of indications are available in
this particular case. The medieval mill which was so important a monopoly
of the medieval community was not built in the most natural place for it,
on the summit of the hill just above Port, but some thousand yards and
more away down the river bank, and over against it is a group of tombs.
Moreover, between the two runs the long north-western boundary of the
parish or commune of Port which is prolonged in the boundary o