Area: c 11.9 hectares, c 29.4 acres.
Reasons for Notification:
This compact site includes a former limestone quarry of outstanding geological importance, together with the steep slopes up to a hill fort which support good quality, well grazed calcareous grassland.
Chalbury Quarry provides an outstanding section through the Portland and Purbeck Beds of the ‘West Mainland’ outcrop, in which the Portland Stone occurs in a ‘chalky micrite’ (lime-mud) facies rather than the coarser carbonate facies which is characteristic of the rock sequence seen in the coastal outcrops on Portland and east of Ringstead. This difference is because the ‘West Mainland’ outcrops have been laid down in a separate depositional basin from those of the coastal area, the two basins having been separated by a submarine ridge, or ‘swell’. The sequence runs from the Portland Clay, at its base, up to the Purbeck Dirt and Cap Beds, and is richly fossiliferous. Of particular stratigraphic importance is the ammonite fauna from the basal Portland Stone. The occurrence of fossil trees, in life position, within the basal Purbeck Beds is also of considerable significance.
Tor grass Brachypodium pinnatum and sheep’s and red fescues Festuca ovine and F. rubra are the main dominant grasses of the steep hill slopes, which are grazed by sheep and cattle. This management maintains a characteristically herb-rich sward in which a large number of grasses and many of the typical and attractive ‘chalk’ plants flourish. These include kidney vetch Anthyllis vulneraria, horseshoe vetch Hippocrepis comosa,rockrose Helianthemum nummularium, clustered bellflower Campanula glomerata and the uncommon hairy rock cress Arabis hirsuta. Species typical of the western chalk are locally abundant, such as rough hawkbitLeontodon hispidus, betony Stachys officinalis, small scabious Scabiosa columbaria and devil’s-bit Succisa pratensis. The spider fauna includes several species of restricted occurrence, among them Scotargus inerraus and Xysticus erraticus.