Geoscience Reference
In-Depth Information
The structure of the Isle of Purbeck area is well illustrated by a cross-section
through St Alban's Head (
i4
), as shown in Figure 109. Further west, Lulworth Cove
(
i1
) is one of the most famous features of this stretch of coast, demonstrating the con-
trol of the coastal geometry by the folding of the bedrock (Fig. 110). The outer coastal
wall of the Cove is formed by steeply layered, erosion-resistant Portland limestone,
which has been breached by wave action during storms. The breach probably first oc-
curred at the mouth of a former stream flowing into the sea where the entrance to the
Cove is now, allowing waves to erode the softer sediments behind the limestone barri-
er. The Cove was then excavated in relatively weak Wealden mudstone, and has now
cut back far enough to expose the tougher Late Cretaceous Chalk as a steep-sided cliff
at the back of the bay. The remarkably regular, circular form of the Cove seems to be
a response to the diffraction (or bending) of the crests of storm waves as they enter the
mouth of the Cove, fanning out and causing longshore transport of the available debris.
FIG
112.
Cliffs at Kimmeridge Bay (Fig. 99,
i3
) showing the typical bedrock layering, of-
ten cut by faults. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Just west of Lulworth Cove, Stair Hole displays a beautiful example of a small
fold called the Lulworth Crumple.
About 2 km west of Lulworth Cove is Durdle Door, another famous feature of the
Jurassic Coast. In this case, a steeply sloping limb of the Purbeck Stepfold (or
mono-
