Geoscience Reference
In-Depth Information
Despite the complexity of much of this Landscape, some of the most spectacular
scenery is easily understood. For example, the local topography of north Dorset is due
to the presence of particular layers in the bedrock that have resisted erosion, such as the
Blue Lias limestones, the Bridport/Yeovil Sands and the Inferior Oolite. In contrast to
the Cotswold Hills to the north, many of the other Middle Jurassic layers that produce
striking topography are much less important or absent in this Landscape.
Landscape E: Blackmoor Vale and the Vale of Wardour
This Landscape also does not define itself easily in terms of its pattern of hills and val-
leys. It occupies the area of hills and vales between the main northward-draining catch-
ment of the Parrett and Brue (Landscapes
B
and
D
) and the Chalk Downs (Landscape
F
) to the south (Fig. 99). The Landscape drains largely southeastwards via the River
Stour, breaking through the main Chalk rim near Blandford Forum (
f1
). The Vale of
Wardour (
e1
) lies between the Wiltshire Downs and Cranborne Chase, while Black-
moor Vale (
e2
) is largely defined by the catchment of the River Stour before it enters
the Dorset Chalk. The landscape is of typical 'wooded clay vale' with rich pastureland
underlain by soft Jurassic clays, similar to Marshwood Vale (
c4
), described on page
137.
Although the topography of the area is complex (Fig. 105), the map pattern of the
bedrock geology and its effects on the local scenery are easier to understand. In the
Vale of Wardour (
e1
), detailed mapping of the bedrock has revealed that the margins of
the Vale are bounded by high ground underlain by Upper Greensand and Chalk, form-
ing the Wiltshire Downs to the north and Cranborne Chase to the south (Fig. 106). The
low ground of the Vale of Wardour is underlain by Jurassic clays, and this has made it
the natural route for railways and roads: the main southerly railway loop between Lon-
don and Exeter and the main A30 Salisbury-Yeovil road both run through the Vale.
More detailed examination of the Vale of Wardour bedrock allows the reconstruc-
tion of a scenically important sequence of events. Firstly, a variety of bedrock layers
was deposited during the Middle and Late Jurassic, including the Portlandian lime-
stones, which are resistant enough to produce slopes today in the east of the Area. The
deposition of these layers was followed by a general stretching of the crust across the
whole South Coast Region, resulting in normal faulting and mild folding of the bedrock
layers. The rocks were then subject to erosion as the sea flooded the Region, followed
by the deposition in the Cretaceous of the Gault clays, the Greensand and the Chalk. In
mid-Tertiary times the crust of the area was deformed under horizontal compression to
produce reverse faulting and some localised folding in the fault zones. The uplift asso-
ciated with this compression exposed the surface rocks to erosion, removing the Early
