Geoscience Reference
In-Depth Information
The Marshwood (
c4
) and Powerstock (
c5
) vales are drained southwards by the
Char and Brit rivers and are surrounded by typical flat-topped hills and ridges capped
by Cretaceous Upper Greensand, as in the Blackdowns. Gentle folding in this area
has elevated the rocks into a dome structure (called a
pericline
), bringing soft Jurassic
mudstones to the surface in the centre of the dome and leaving younger, less easily
eroded Cretaceous sediments around the edges. Erosion has had the greatest impact on
the mudstones, so flat and low-lying topography has formed within the vales, which
are surrounded by ridges and hills made of the tougher Cretaceous sediments.
Golden Cap, 6 km east of Lyme Regis, is the highest cliff on the South Coast
and owes its name to the yellow colour of the Upper Greensand that forms its crest.
The permeable Greensand sits on top of impermeable Gault and Lias clays, and land-
slides are therefore common along this stretch of coastline. Rainwater percolates read-
ily through the Greensand but cannot penetrate the clays so easily, resulting in a build-
up of fluid pressure at the interface between the two layers. This pressure works to
destabilise the cliff face and, combined with erosion of the cliff base by waves, can
result in large and dangerous landslides as the Greensand slides over the underlying
Jurassic clays.
Landscape D: The Yeovil Scarplands
The northward-draining Yeovil Scarplands are underlain entirely by bedrock of Jur-
assic age, so the pattern of hills and valleys is not easily explained by the simplified
geological map in Figure 100. Moreover, the Jurassic bedrock here is extremely vari-
able, not only between its layers but also within a single layer, from one place to the
next. Discrete layers of resistant sandstones and limestones, picked out by erosion in
one area, often die out laterally to be replaced by more recessive mudstones and clays.
The diversity of the Jurassic rocks is due to highly variable local environments in Jur-
assic times, when the sediments were accumulating. For example, the Ham Hill Stone
is a famous calcareous building stone much used in Dorset, but it is only found at a
small number of locations, such as at Ham Hill (
d1
). This is because the stone origin-
ally formed in a ≈5 km wide Jurassic tidal channel, bounded on either side by islands
of less calcareous Bridport/Yeovil sand.
Another reason why the pattern of hills and valleys in this Landscape is unclear
is that the bedrock has been cut by large numbers of faults that were active after the
Jurassic sediments had been deposited. It is difficult to see any simple pattern in the
fault directions: groups of faults trend roughly north-south, but east-west faults are also
important. The faults influence the scenery because they sometimes juxtapose very dif-
ferent bedrock materials at the surface, and these differences are quickly picked out by
erosion to form a complex landscape.
