Geoscience Reference
In-Depth Information
made full use of these attractive stones, and their town centres are filled with beautiful
historic buildings.
At Wootton Bassett (
b4
), south of the M4 motorway and just southwest of Swin-
don, the clays of the Wiltshire Clay Vale region have produced a geological phenomen-
on that is extremely rare in Britain - a series of mud-springs. The springs are situated in
a small wood called Templar's Firs, and were first described as three domed 'blisters'
some 10 m long by 5 m wide by 1 m high. The skin of the blisters was formed by
matted vegetation and contained a core of liquid mud which oozed out of any fissure
in the skin and into a nearby brook. The blisters have since been burst by inquisitive
visitors, but the springs themselves remain. Surveys have shown that the vents contain
liquid mud down to a depth of at least 6 m, and that the volume of the underlying mud
chamber is much larger than originally anticipated: local farmers have stories of cattle
drowning in the springs and, in 1990, in an attempt to block up the main spring for
safety reasons, a local contractor dumped 100 tonnes of quarry rubble into the main
vent. Within half an hour the stone had disappeared without a trace and the displaced
mud spilled out over the surrounding countryside, blocking a nearby stream. The area
has since been fenced off and warning signs posted.
The exact cause of the Wootton Bassett springs is controversial, but they appear
to result from a combination of local bedrock structure and particular rock types. The
springs emerge through the Late Jurassic Ampthill Clay Formation. Underlying this
clay is a layer of permeable Corallian Limestone, which acts as an aquifer in which
water migrates to the lowest part of a local downfold, where it escapes upwards when
the water pressure is sufficient. When the water emerges at the surface it moves the
surrounding muddy material upwards with it.
Further east, the Midvale Ridge forms a southern limit to the Oxfordshire Clay
Vale. It can be traced from southwest to northeast, running from Chippenham, through
Swindon and on towards Oxford. The ridge is formed mainly of sandy Corallian Lime-
stone of early Late Jurassic age, although locally there are also patches of latest Late
Jurassic Portlandian sandstones and limestones. These hills are in places capped by a
layer of Early Cretaceous Lower Greensand, which has prevented the weathering and
erosion of the limestone below, forming prominent sandy knolls. Broadly speaking, the
ridge has a tabular profile and, in contrast to the surrounding clay vales, the soils are
sandy, light and free-draining, home to scattered woodlands interspersed with sandy
pastures.
The Midvale Ridge has a long history of human settlement, and there are a num-
ber of important Roman sites located on prominent areas of higher ground. More re-
cently, the small villages and hamlets typical of this region have been built upon spurs
and subsidiary low ridges, using locally occurring Corallian Limestone as the main
