hey run north and south, cutting in some cases directly
through the escarpments; on the north, for instance, the Wye, the Mole,
the Darenth, the Medway, and the Stour; and on the south the Arun, the
Addur, the Ouse, and the Cuckmere.

[Illustration: Fig. 39.--Map of the Weald of Kent.]

They do not run in faults or cracks, and it is clear that they could not
have excavated their present valleys under circumstances such as now
exist. They carry us back indeed to a time when the Greensand and Chalk
were continued across the Weald in a great dome, as shown by the dotted
lines in Fig. 38. They then ran down the slope of the dome, and as the
Chalk and Greensand gradually weathered back, a process still in
operation, the rivers deepened and deepened their valleys, and thus were
enabled to keep their original course.

Other evidence in support of this view is afforded by the presence of
gravel beds in some places at the very top of the Chalk escarpment--beds
which were doubtless deposited when, what is now the summit of a hill,
was part of a continuous slope.

The course of the Thames offers us a somewhat similar instance. It rises
on the Oolites near Cirencester, and cuts through the escarpment of the
Chalk between Wallingford and Reading. The cutting through the Chalk has
evidently been effected by the river itself. But this could not have
happened under existing conditions. We must remember, however, that the
Chalk escarpment is gradually moving eastwards. The Chalk escarpments
indeed are everywhere, though of course only slowly, crumbling away.
Between Farnham and Guildford the Chalk is reduced to a narrow ridge
known as the Hog's Back. In the same way no doubt the area of the Chalk
formerly extended much further west than it does at present, and,
indeed, there can be little doubt, somewhat further west than the source
of the Thames, almost to the valley of the Severn. At that time the
Thames took its origin in a Chalk spring. Gradually, however, the Chalk
was worn away by the action of weather, and especially of rain. The
river maintained its course while gradually excavating, and sinking
deeper and deeper into, the Chalk. At present the river meets the Chalk
escarpment near Wallingford, but the escarpment itself is still
gradually retreating eastward.

So, again, the Elbe cuts right across the Erz-Gebirge, the Rhine through
the mountains between Bingen and Coblenz, the Potomac, the Susquehannah,
and the Delaware thr