Southern England 130,000,000 years ago would
not be very recognisable today. The area now known as the Isle of Wight
belonged to an extensive costal floodplain that was still physically part of
mainland Europe. It was still possible to reach North Africa overland and land
bridges may still have made North America accessible. There were hills and
uplands to the north-east where London would be one day and beyond these a
shallow sea. In the west and south-west mountainous uplands spread from
Cornubia ( Cornwall ) into Armorica ( France ), further west was the widening
seaway that eventually would become the Atlantic Ocean.
The Lower Cretacceous Wessex
and Vectis formations represent the oldest exposed
rocks on the Isle of Wight, the oldest beds of the Wessex Formation are
believed to be around 130,000,000 years old, this places them in the
Barremian age. The overlaying Vectis Formation,
the top of which may be Aptian age, is slightly
younger at about 125,000,000 years. The combined maximum exposed thickness of
the two formations is in the region of 250 metres, this may represent
deposition over a period of some 15,000,000 years.
The clays and sandstones of the Wessex Formation were
deposited by a series of mandering river systems, the predominant variagated
clays gain their mottling as a result of repeated wetting and drying in a
seasonally semi-arid environment, and also from ancient microbal activity in
the soil. Occasional grey clay bands packed with plant and sometimes vertebrate
remains are known as plant debris beds. these are belived to be the result of
rivers swollen with monsoonal rain bursting their banks and depositing whatever
detritus the floodwaters accumulated in a sheet of muddy sediment. Some of
these beds can be several metres thick, and may provide circumstantial evidence
as to what the ancient may have been like, material from the plant debris beds
is often preserved as fusain ( burned plant material ) probably representative
of forest fires common during periods of sustained drought when temperatures
are high and plant material is dried to the point of kindling.
It is believed that during the Lower Cretaceous the landmass
that incorporated the Isle of Wight was situated closer to the equator than
today, and the average global temperature was higher. Plants from the Wessex
and Vectis Formations support this in that some of them have growth rings
preserved which indicate that the plants growth often slowed or stopped
altogether for prolonged periods of time, this is suggestive of sustained
periods of drought and is reminiscent of some modern tropical areas where it
may not rain for years.
The Vectis Formation is
composed mainly of grey clays interspaced by bands of shelly limestone, in
comparison to the multi-hued Wessex Formation it looks rather monotonous. This
is misleading, as the Vectis Formation records the gradual transition of one
environment to that of a totally different nature. The clays and limestones of
the Vectis Formation were deposited in shallow near-shore lakes or lagoons,
fossils in the lower part of the formation indicate a low salinity
fresh/brackish water community. Occasional limestone slabs containing the
footprints of large vertebrates and horiszons showing mud-cracks and dried
ripple marks suggest that sustained dry periods were still occuring.Towards the
top of the formation true marine fossils are abundant, these tell us that
gradually rising sea-levels were beginning to inundate and overwhelm the
coastal flood-plains.
G. Hulman August 2001
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