Geoscience Reference
In-Depth Information
cline
- Fig. 109) is exposed in the Portland Stone, where it has been eroded into a spec-
tacular doorframe-like arch.
Just east of Lulworth Cove is a famous locality where a 'fossil forest' may be
seen. It is on the coastal footpath just within the boundary of a large military firing
range, so access is not permitted at certain times. Tree trunks of large conifers and ferns
that flourished in the Late Jurassic were preserved here when the Jurassic sea level rose
and the trunks became surrounded by mats of primitive plant material, preventing their
decay.
The steep bedrock layers of Chalk and limestone that cause the spectacular
scenery of Durdle Door and Lulworth Cove continue eastwards along the coast, before
striking inland to form the Purbeck hills. Along the coast, they are replaced by increas-
ingly flat-lying layers of clay and limestone. Land-slipping is common in this area,
where the cliffs have permeable, resistant tops overlying soft, easily eroded Kimme-
ridge Clay.
Kimmeridge Bay (
i3
; Fig. 112) is much visited by scientists. This is partly because
its name has been adopted for one of the worldwide subdivisions of Jurassic time, but
it is also of interest because it is the original locality that gave its name to the Kimme-
ridge Clay, a layer that provides a high proportion of the oil wealth of the North Sea.
The grey mudstones exposed in Kimmeridge Bay are rich in fine-grained organic ma-
terial generally referred to as
kerogen
. If this material has been buried deeply enough,
and subjected to high temperatures, for long enough, the kerogen becomes altered, re-
leasing oil that can migrate along and through any permeable layers in the bedrock.
The oil can either seep out at the surface or become trapped by folds, faults or loss of
permeability to form a natural reservoir, which may then be exploited as an oil field.
The contribution of the Late Jurassic Kimmeridge Clay to the economy of Britain has
been enormous.
AREA 5: HAMPSHIRE AND THE ISLE OF WIGHT
This area includes the striking coastal scenery of the Isle of Wight, as well as the sea-
side cities of Bournemouth and Portsmouth. Southampton sits in the centre of the area
within the low-lying Hampshire Basin, while to the north Salisbury and Winchester lie
within Chalk hills (Figs 113 and 114).
The bedrock succession of this Area records a variety of episodes starting in the
Late Jurassic, through the Cretaceous and into the early Tertiary, representing 150 mil-
lion years of Earth history. Although the total thickness of the bedrock layers shown in
Figure 115 is around 2 km, this figure is misleading because it does not represent sub-
sidence of the Earth's surface by this amount at any one place. In fact, some of the lay-
