146 SUBTERRANEAN GEOLOGY OF SOUTH-EASTERN ENGLAND.
Somersetshire, where the ground is covered by Secondary rocks,
any coal-basins that exist will be found northward of the sub-
terranean prolongation of the Mendip ridge.
Looking from Somersetshire eastward, the probable presence of
the anticlinal ridge below the Secondary rocks is shown in the existence
of the long narrow valley known as the Vale of Pewsey, towards the
western end of which the town of Devizes stands. This vale consists
of Upper Greensand, and trends in an easterly direction for about 18
miles from Devizes, being bounded on the north by the Chalk
escarpment of Marlborough Downs, on the south by that of Salisbury
Plain, The continuity of the Chalk has here been destroyed through
the presence of an anticlinal fold which has elevated the Upper
Greensand along the line of the Vale of Pewsey and has led to the
disappearance of the Chalk there. The continuation of this anticlinal
eastward is marked by the appearance of two isolated patches of Upper
Greensand in the middle of the Chalk, the more westerly being about
four miles south of Hungerford, the other six or seven miles south of
Newbury. Proceeding eastward in the direction indicated by these
Upper Greensand inliers we come to the north-western end of the
Weald district. Mr. Topley4 points out that in the Weald the chief
anticlinal lines are well marked close to but south of the Hog's back
(as the escarpment of the North Downs between Farnham and Guild-
ford is called) and between Tunbridge and Tunbridge Wells, the
line at the latter place trending somewhat south of east towards the
northern end of Romney Marsh. ("Geology of the Weald," pl. 2.)
The existence of a special line of upheaval at the north-west corner
of the Weald is marked by the extremely high dip of the Chalk
between Farnham and Guildford, which greatly narrows the width of
its outcrop there; and by making the dip slope almost as steep as
the outcrop or escarpment side of the ridge has caused it to receive
the name of Hogsback. And about a mile south of Guildford, at
Pease Marsh, there is an inlying patch of Weald clay in the midst of
the Lower Greensand, which has the same significance as the patches
of Upper Greensand in the midst of Chalk south of Hungerford and
Newbury. It may be well to add that Mr. Topley (Weald memoir)
calls attention to the fact that some of the apparent upheaval in the
Weald district is due to the local thickening of the underlying beds.
We may now turn from the evidence afforded by geological struc-
ture outside South-eastern England and surface indications within
4 Geological Survey Memoir "On the Geology of the Weald."