Geoscience Reference
In-Depth Information
patches of gravel and alluvial terraces forming a surface blanket, representing episodes
in the much younger Quaternary development of the landscape.
Landscape H: The Weymouth Lowlands and the Isle of Portland
The Weymouth Lowlands stretch along the coast from Burton Bradstock (
h1
) to Wey-
mouth, bounded to the north by the Chalk ridge that forms the edge of the Dorset
Downs. The bedrock of the area is Jurassic in age and consists of east-west trending
limestone ridges interspersed with clay vales.
The Weymouth Lowlands have been eroded downwards to reveal a very broad
upward fold in the bedrock - the Weymouth Anticline - which has an east-west trend-
ing axis and gentle slopes, steepest in the north where the slope reaches 10 degrees.
The oldest layers, exposed by erosion in the central part of the anticline, are of Middle
Jurassic age, but a full succession of younger Jurassic strata is also visible. Many faults
have been discovered in these strata by drilling and seismic work aimed at the detailed
exploration of possible oil-bearing reservoirs. Most of the faults are normal faults, sug-
gesting that they formed during a period of extension or stretching of the crust, though
the gentle folding of the Weymouth Anticline implies a later period of crustal compres-
sion. Some 5 km east of Weymouth, a tighter upfold (the Poxwell Anticline) runs east
to west, parallel to but inland from the coast, and contains Early and Late Cretaceous
layers to both north and south. Inclinations of up to 65 degrees have been measured on
the very steep north slope of this fold, contrasting with the gentle southern slope, which
confirms that the overturning movement during this late compressional phase was in
the same northward direction as occurred in other late movements in this area.
The Isle of Portland (
h3
) is a limestone plateau which slopes southwards, from
high northern cliffs to barely 10 m above the sea. This slope and the layers visible in
the northern cliffs of Portland are part of the southern limb of the Weymouth Anticline.
The Isle is famous for its bedrock, the Portland Stone, a shelly, often oolitic, limestone
that has been widely used as an ornamental stone in important British building projects
such as St Paul's Cathedral in London. The Portland Stone has been quarried since Ro-
man times and the Isle is extensively scarred by old quarries and spoil heaps.
