the highest point in the Mendips, rising to 325 m. The summit is
underlain by sandstone and conglomerate of the Portishead Formation,
which outcrops in the core of the Blackdown Pericline. The hard sandstone
forms the higher ground because it is immune from dissolution and
resistant to erosion, and gives rise to acidic, wet, often peaty
soils. The curious hummocks, tumps and straight lines on the summit
are the remains of a World War II bombing decoy. They were designed
to draw enemy bombs by mimicking potential targets in Bristol such
as railway stations and marshalling yards.
The acidic soils on the sandstone support dwarf shrub heath, dominated
by mature heather and typical heathland grasses. A few small mires
are also present, providing suitably wet and acidic conditions for
sphagnum mosses and a host of other wetland plants. Dense bracken
has become established on the northern side of the hill. The area
supports many heathland insects and birds, many of them nationally
rare and/or declining.
On the south side of Blackdown, the GB Gruffy Field Nature Reserve
contains several different habitats including damp, species-rich
neutral-acidic pasture, rocky limestone crags, unimproved limestone
pasture and lead-rich spoil heaps, the latter supporting the nationally
scarce alpine penny-cress. This gives rise to a wide variety of plant
and animal species.
Several small streams drain off the sandstone on the south side of
the hill, and across the Avon Group mudstone outcrop before disappearing
underground on reaching the Black Rock Limestone. Two stream sink
underground in the GB Gruffy Field Nature Reserve. The larger stream
reappears in GB Cave. Accessible only to cavers, this cave is
almost 2 km long and descends to a depth of 135 m. The main passage
includes one of the largest chambers in any Mendip cave with many
superb stalactites and stalagmites.
Nearby, a large, fenced depression marks the site of a major sinkhole
that collapsed following heavy rain in 1968. The sides of the depression
are formed of the fine-grained wind-blown loess, which covers much
of the reserve. The other stream sinks nearby and flows into Charterhouse
Cave, a smaller cave discovered in 1984. The region is also pockmarked
with old mine shafts and spoil tips forming 'gruffy ground'.