flank of Mendip between Cheddar and Wells forms an imposing
scarp when viewed from the south. The underlying Carboniferous
Limestone here has been thrust up into a series of stacked 'sheets'
of rock along two major thrust faults; the 'South-western Overthrust'
and the Ebbor Thrust. The small Carboniferous Limestone inliers
of Nyland Hill and Lodge Hill, rising up from the Somerset Levels
to the south, represent the eroded remnants of another thrust sheet
of limestone just poking through the cover of younger Triassic
and Jurassic rocks.
Much of the lower slopes of the Mendip scarp are underlain by a drape
of Triassic Dolomitic Conglomerate, which partially buries the Carboniferous
Aerial view of Draycott and Westbury (click to enlarge view).
Known locally as Draycott Marble, the conglomerate was quarried at
several places in Draycott and Westbury-sub-Mendip, and can be seen
in many of the houses and walls in the area.
The Carboniferous Limestone outcrops in many places, but the Burrington
Oolite is particularly well exposed in the Draycott Sleights nature
reserve where the rocks have been folded into a small down-fold or
syncline. The Upper Carboniferous Quartzitic Sandstone and
the Coal Measures (mostly sandstone and shale) can be seen in the
Deer Leap and Ramspit Nature Reserves. However, most of the Coal
Measures is cut out by the Ebbor Thrust, which runs through
the reserve. Along this fault, the Black Rock Limestone has been
thrust over the top of the younger Quartzitic Sandstone. The outcrop
of the sandstone is marked by a sliver of wet boggy ground, and a
small stream which sinks underground in the overlying limestone along
the line of the thrust fault.
The hill above Draycott is known as Draycott Sleights. Now a nature
reserve, Draycott Sleights contains a variety of habitats including
several types of herb-rich limestone grassland with scattered scrub,
rocky limestone crags, small cliffs and rock exposures. The Rodney
Stoke National Nature Reserve encompasses broad-leaved woodland,
scrub and limestone grassland and is a good example of a typical
Mendip ash wood.
In 1969, blasting in Westbury Quarry exposed a massive sediment-filled
cave, at least 70 m long and 30 m high. Excavations by the Natural
History Museum in the 1970s discovered abundant small mammal bones
and in places the remains of the extinct cave bear Ursus deningeri.
The deposits were laid down in a Middle Pleistocene interglacial
period about 620 000 years ago.
A belt of twelve large closed basins punctuate the southern rim of
the Mendip plateau between Cheddar and Ebbor Gorge. These hollows
are formed by the dissolution of the limestone. During the last glaciation
underground drainage was impeded by permafrost and these depressions
became lakes. Their floors are covered with glacial windblown loess,
and often pockmarked by sinkholes where the loess has subsided into
cavities in the underlying limestone.
Once of the best developed basins is at Brimble Pit, near Westbury-sub-Mendip
where the former lake bed contains a small pond and nine sinkholes.
Another occurs in the Somerset Wildlife Trust's Middledown Reserve,
and is marked by two large sinkholes. Two other closed basins can
be seen; the road from Draycott to Cheddar Head crosses one at Bristol
Plain Farm, and the other, Cross Swallet, can be seen from the Priddy–Westbury
Topographical map of closed depressions (click to enlarge view).