Geoscience Reference
In-Depth Information
FIG
120.
The Needles (Fig. 116,
b1
) and Alum Bay, western Isle of Wight. (Copyright re-
served Cambridge University Collection of Air Photographs)
North of the central belt and its stepfold, the Isle of Wight is underlain by soft-
weathering early Tertiary materials that have been dissected by local stream and river
erosion. The multicoloured cliffs of Alum Bay (Fig. 120) are formed of steeply tilted
(locally vertical) early Tertiary sediments that have been folded along with the Chalk.
The colours of the sandstones and mudstones in the cliffs are exceptionally bright and
varied, and collections of multicoloured sand samples in transparent tubes are carried
home from Alum Bay by holidaying families every year. White sands are made of pure
quartz, whose grains lack the thin coating of iron oxide that colours the Bay's red to
yellow sands. Green sands have been coloured by the iron-bearing mineral
glauconite
,
preserved when reducing conditions are generated by organic material in the sediment.
Dark layers of
lignite
(a coal-like material) were formed by the burial of ancient veget-
ation. These striking bands of colour were formed during deposition, when the layers
were in their original near-horizontal orientation, before they were tilted to near-vertic-
al by folding and faulting as the stepfold was created.
South of the Isle of Wight central belt, the bedrock consists of Early Cretaceous
Wealden strata, which form the core of the southern Isle of Wight Anticline or upfold.
The elevated area near the south coast is a remnant of Chalk grassland lying above the
Wealden. The southern face of the coast (
b2
), between St Catherine's Point and Dun-
nose, has cliffs that tower above terraces stepping down to the shore. This coastal strip
is one of the largest areas of land-slipping in western Europe, because overlying ma-
terials have slipped down the coastal slopes created by storm erosion, lubricated by
underlying muddy layers after prolonged periods of rain.
