ever, is not continuous,
as the hills are breached by a succession of valleys, forming gaps
through which high-roads and railways converge upon London. The rivers
flowing through these gaps run northward, and, except in the extreme
east, are members of the Thames basin. These breaching valleys, which
are characteristic of the South Downs also, "carry us back to a time
when the greensand and chalk were continued across, or almost across,
the Weald in a great dome." The rivers "then ran down the slopes of the
dome, and as the chalk and greensand gradually weathered back ...
deepened and deepened their valleys, and thus were enabled to keep their
original course."[1] The western termination of the North Downs is the
Hog's Back, a narrow ridge, little more than a quarter of a mile broad
at the summit, sloping sharply north and south, and reaching 489 ft. in
height. At the west end a depression occurs where the rivers Wey and
Blackwater closely approach each other; and it is thought that the Wey
has beheaded the Blackwater, which formerly flowed through the gap. In
this depression lies Farnham, the first of a series of towns which have
grown up at these natural gateways through the hills. The Wey, flowing
south of the Hog's Back, breaches the Downs at its eastern extremity,
the town of Guildford standing at this point. The next gap is that of
the Mole, in which Dorking lies. Between Guildford and Dorking the main
line of the Downs reaches a height of 712 ft., but a lateral depression,
followed by the railway between these towns, marks off on the south a
loftier range of lower greensand, in which Leith Hill, famous as a
view-point, is 965 ft. in height. East of the Mole the northward slope
of the Downs is deeply cut by narrow valleys, and the depression above
Redhill may have been traversed by a stream subsequently beheaded by the
Mole. A height of 868 ft. is attained east of Caterham. The next river
to break through the main line is the Darent, but here another lateral
depression, watered by the headstreams of that river, marks off the
Ragstone Ridge, south of Sevenoaks, reaching 800 ft. The lateral
depression is continued along the valleys of streams tributary to the
Medway, so that nearly as far as Ashford the Downs consist of two
parallel ranges; but the Medway itself breaches both, Maidstone lying in
the gap. The elevation now begins to decrease, and 682 ft. is the
extreme height east of the Medway. The direction, hitherto