Mendips header
 Overview maps
 Locality areas
  Cheddar Gorge
 Burrington Combe
 Shipham & Rowberrow
 Crook Peak & Axbridge
 Banwell to Churchill
 Harptree & Smitham Hill
 Draycott & Westbury-sub
 Wookey Hole & Ebbor
 Great Elm & Vallis Vale
 Mells & the Wadbury Valley
 The Vobster area
 The Whatley area
 Torr Works & Asham Wood
 Beacon Hill
 Stoke St Michael & Oakhill
 Holwell & Nunney
 Shepton Mallet & Maesbury
 Gurney Slade & Emborough
 The Nettlebridge valley
 Rocks of Mendips
 Geological timescale
 Ancient environments
 Geological structure
 Minerals and mines
  Minerals and mines
 Industrial archaeology
  Stone as a resource
 Employment & the economy
 Quarrying & geodiversity
 Quarrying & the environment
 History of quarrying
 Caves and karst
 How caves form
 Dry valleys and gorges
 Dolines and sinkholes
 Mendip caves
 Going caving
  Flora and fauna
 Typical Mendip habitats
 Special Mendip habitats
 Horseshoe bats
 Appendix of names
 Biodiversity of western
 Biodiversity of eastern
 External links
 Detailed site information
  Coal mining
  Mendip quarry companies
  East Mendip quarries
 Biodiversity of eastern
  West Mendip quarries
 Biodiversity of western
 Site map
Blackdown is 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 summit of Blackdown, at 325 m the highest point on the Mendip Hills.

Flora and fauna
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.

A collapse sinkhole above GB Cave.
  Wet heathland developed on the Portishead Formation sandstone on Blackdown.

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'.
  Bat Passage in GB Cave.


goto the British Geological Survey home page