140 SUBTERRANEAN GEOLOGY OF SOUTH-EASTERN ENGLAND.
the South Downs thence to Beachy Head. The absence of the Chalk
in this area, known as the Weald, of course enables the borer to avoid
a start in six or eight hundred feet of that formation. And in certain
parts of the Weald district not only is it possible to avoid the Chalk,
but also the rest of the Cretaceous strata, and to begin in the upper-
most bed of the Oolitic (or Jurassic) series. The existence of this
chalkless district the Weald, is the result of the existence of an anti-
clinal axis or ridge which traverses it in an east and west direction.
This anticlinal is simply a fold of elevation in the strata, the conse-
quence of the action of subterranean forces. There can be no doubt
that at one time the Chalk extended across the area between the
North and South Downs, and the outlying patches of Tertiary beds
now resting on the Chalk here and there, both north and south of the
Weald, show that they once formed a continuous sheet above it
But as what is now south-eastern England became elevated so as to
form land, the Tertiaries forming the highest beds of the anticlinal ridge
naturally emerged first and were swept away by the waves, together
with the greater part of the Chalk covering the present Weald area.
Sub aerial agencies now came into action, and rivers taking their rise
in what must have still been slightly the highest ground, flowed
northward and southward from the centre of the Weald. The result
of the action of these various sub-aerial agencies is to be be seen in the
present configuration of the ground. If we journey from London to
Brighton we cross the Weald district in a direction nearly at right
angles to that of the anticlinal axis, and the arrangement of the rocks
becomes obvious to the careful observer. After passing Croydon we
leave the Tertiaries of the London Basin, and travel on the Chalk
which there dips northwards. Soon after emerging from the Merstham
tunnel we see, on looking backwards, the chalk escarpment (or outcrop)
of the North Downs, ranging east and west. Brickfields on the flat
ground between Merstham and Red Hill mark the outcrop of the
Gault clay from beneath the Chalk, while the sandy ridges of Red Hill
belong to the Lower Greensand which underlies the Gault. South of
Red Hill we cross another belt of flat country, the Weald clay, which
there rises from beneath the Lower Greensand of Red Hill. At Three
Bridges Station we find ourselves in undulating country consisting of
the various sandy and clayey strata known collectively as the Hastings
Sands, which underlie the Weald clay and are the lowest beds found
in this part of the Weald district. All these formations from Croydon
to a few miles south of Three Bridges dip northwards, but the Hastings