Geoscience Reference
In-Depth Information
ers of the Middle and Late Jurassic, particularly the limestones of the Corallian, Forest
Marble, Inferior Oolite and Fuller's Earth Rock, and the sandstones of the Yeovil
Formation. The pattern is complicated, partly by the lack of continuity of some of these
resistant layers (reflecting lateral changes in Jurassic environments of deposition), but
also due to local faulting (Figs 105 and 106).
Landscape F: The Wiltshire and Dorset Downs and Cranborne Chase
In the north of Area 4, this Chalk landscape includes part of the Wiltshire Downs (
f2
),
forming the northern margin of the Vale of Wardour. Compared with the large area
of Chalk Downs that makes up Salisbury Plain further to the northeast (see Area 10),
these Chalk hills are high and deeply dissected, particularly along the indented western
margin of the Chalk, where Brimsdown Hill (
f3
) reaches 284 m in elevation. Large val-
leys here generally tend to have a valley floor consisting of mass-flow deposits, called
head, and river deposits, called
alluvium
, which usually consist of mass-flow deposits
that have been eroded, locally transported and deposited by streams.
More centrally within Area 4, the Dorset Downs (
f4
) are the westernmost chalk
uplands in England. They form the northern and western edges of the Hampshire Bas-
in, which extends towards the east across Areas 5 and 6. The Chalk layer slopes gently
southeastwards from the escarpments around the northern and western edges of the
basin towards its centre, which is filled by sediments of early Tertiary age (see Land-
scape
G
). Although Chalk slopes also bound the southern edge of the basin, they are
much narrower because the Chalk layer here dips much more steeply. In fact, it is al-
most vertical in some localities, for example the narrow Chalk ridge of the Purbeck
Hills (see Landscape
I
). This difference between the northern and southern structures
is due to the mid-Tertiary compressional movements discussed above.
