on the northern flank of the Beacon Hill Pericline, Stoke
St Michael and Oakhill lie on the narrow plateau of steeply dipping
Carboniferous rocks sandwiched between the higher ground formed by
Devonian and Silurian rocks to the south and the younger Coal Measures
exposed in the Nettlebridge valley. The limestone outcrop is pockmarked
by several disused quarries and hosts several quite extensive cave
Aerial view of Stoke St Michael (click to enlarge view).
Much of the Carboniferous Limestone sequence has been exposed by
quarrying. The Vallis Limestone is exposed in the privately owned
Stoke Lane Quarry near Stoke St Michael. Further down the valley
is the dormant Cooks Wood Quarry, which offers an outstanding section
through the upper part of the Carboniferous Clifton Down Limestone
and the overlying Oxwich Head Limestone. The limestone here dips
at about 80° to the north. The Oxwich Head Limestone consists
of a series of sandy limestones with thin beds of soft mudstone
and some coal that form prominent recesses in the cliff face. The
coal developed during temporary periods of low sea level during
The Clifton Down and Oxwich Head Limestone is exposed in the disused
Fairy Cave Quarry, through which runs the Withybrook Fault.
The upper part of the Oxwich Head Limestone also outcrops in Whitehole
Quarry. A prominent bedding plane at the rear of the quarry is covered
in hollows which have been interpreted as Carboniferous 'tree boles',
developed at a time of low sea level when trees grew on the emergent
limestone surface. These hollows were formed by rain water flowing
down the tree trunk and dissolving the limestone around its base.
The area contains several cave systems formed where streams draining
the sandstone slopes of Beacon Hill sink underground on reaching
the Carboniferous Limestone. Some of these stream sinks can be followed
underground. The largest of these is Stoke Lane Slocker. Here the
stream can be followed by cavers for almost 2 km towards the resurgence
at St Dunstan's Well in the Nettlebridge valley. This spring is the
major resurgence for most of the stream sinks (or 'slockers') in
the area, and has a mean flow of about 150 litres per second. Nearby,
Fairy Cave Quarry has intersected a network of caves feeding this
spring. Approximately 4.5 km of cave passage has been explored, of
which about 0.8 km has been removed by quarrying.
Several other springs rise from the Carboniferous Limestone. Three
small springs, partially fed by stream sinks around Oakhill emerge in Ashwick
Grove, while to the east, Whitehole Spring in Leigh Wood is a good
example of a tufaceous spring.
|| The water here is saturated
with calcium carbonate which is precipitated as tufa, a very porous,
pale grey rock which forms a series of stepped dams known as gours.
The contrasting rock types form a variety of different habitats in
the area including an area of species-rich unimproved limestone
meadow limestone near Stoke St Michael.
The Quartzitic Sandstone outcrop gives rise to acidic, nutrient-poor
soils giving rise to an association of plant species rare on Mendip.