flank of Mendip between Banwell
and Dolebury is formed by a narrow discontinuous ridge of Carboniferous
Limestone. This forms the steeply dipping northern limb of the Blackdown
Pericline. The limestone ridge was initially formed during Triassic
times and then buried beneath the Triassic Dolomitic Conglomerate
and Mercia Mudstone infilling the Lox Yeo valley. Only in the last
few million years has erosion exhumed the hard limestone beneath,
in effect recreating the former Triassic landscape.
Aerial view of Banwell to Churchill (click to enlarge view).
The Carboniferous Limestone is well exposed in quarries alongside
the A38 at Churchill Batch. Here, the graffiti covered slabs of Burrington
Oolite has been so severely folded that rocks have been overturned,
and strata appear to dip to the south.
||The transition from overturned
rock to vertically bedded and then upright, northerly dipping strata
can be seen in the cliffs further south along the A38. The fossiliferous
Black Rock Limestone outcrops in the many crags and scree slopes
on the south side of Dolebury Warren which is dominated by the Iron
Age hill fort. The rocks here are very steeply dipping or even vertical.
and potato stones
Evidence for old ochre workings can be seen in many places along
the margins of the ridge between Banwell and Dolebury, particularly
around Sandford. In places, small pits, spoil heaps and adits can
be seen and specimens of ochre and quartz crystals, or 'Bristol Diamonds'
can be found on the old spoil heaps. These quartz geodes, known locally
as 'potato stones' are formed by the replacement of halite nodules
by quartz. The halite (rock salt) nodules were originally deposited
in ephemeral salt lakes during the Triassic,
and evidence of these salt and gypsum deposits can still be found
in the Mercia Mudstone.
At the western end of Banwell Hill are two caves; Banwell Bone Cave
and Banwell Stalactite Cave. Miners broke into Banwell Stalactite
Cave in 1757, but the entrance collapsed and the cave was largely
forgotten about. In 1824, an attempt to reopen the Stalactite Cave
led to the discovery of Banwell Bone Cave. This cave consisted
of a large chamber almost filled with sediment and containing many
thousands of bones. The bones includes many cold climate species
such as reindeer, brown bear, wolverine and arctic fox dating from
the early part of the last glaciation about 80 000 years ago. The
Bishop of Bath and Wells, George Henry Law, who owned the land,
took the bones as wonderful evidence for Noah's Flood. Both caves
are on private land but are occasionally open to the public. On
Sandford Hill are several small caves and mine adits including
Mangle Hole which intercepts the underground river feeding the
springs in Banwell.
At the eastern end of Sandford Hill is the disused Sandford
Quarry, which is developed mostly in the Burrington Oolite. In
1884, the quarry was a few small pits on the south side of the hill,
but over the next 100 years expanded considerably into the western
side of the hill. The quarry was closed in the 1990s as part of an
arrangement to extend Whatley Quarry near Frome and parts of the
site are now used for climbing and abseiling.
The nature reserve at Dolebury Warren is owned by the National Trust
and managed by the Avon Wildlife Trust. This is an area of contrasting
flower-rich limestone grassland and heathland, developed on acid
loessic soils. Butterflies and insects are plentiful, especially
on south-facing slopes where the conditions tend to be warmer.