Area: 21.4 hectares, 52.9 acres.
The site is within the Dorset Area of Outstanding Natural Beauty. Part-owned by the National Trust. Site area increased since last revision.
Reasons for Notification:
Blackdown is situated at an altitude of about 200 metres near the western limit of the Bagshot Beds. It is an extremely important geological site and the plant communities which have developed on the acid podzolic soils give the area additional biological interest.
This is a key locality for study of the Tertiary Period of geological time in Britain, between 35 and 55 million years ago. The importance of the site is twofold. Firstly, the rocks within the site provide evidence of a type of sedimentation unique in rocks of this age in southern England, namely sedimentation in a powerful alluvial river system, with features suggesting sediment deposition in a “braided river” environment – a network of continually-changing river channels. Secondly, the pebbles and rock fragments which comprise the gravels provide important information on the history of development of this part of southern England at this time, and an analysis of their composition and source enables geologists to determine the evolution of the region. In particular, it provides evidence that some movement along the Ridgeway Fault took place in Eocene times.
Parts of the site are dominated by mature ling Calluna vulgaris with bell heather Erica cinerea, the mosses: red stemmed feather moss Pleurozium schreberi, heath plait moss Hypnum jutlandicum, broom fork moss Dicranum scoparium and heath star moss Campylopus introflexus and several lichens of the genus Cladonia including C. portentosa, C. rangiformis, C. coccifera, C. floekiana and C. crispata. Bilberry Vaccinium myrtillus, a local plant in Dorset, is scattered and in places forms dense patches. Bracken Pteridium aquilinum which dominates other areas is locally associated with broad buckler fern Dryopteris dilatata, bramble Rubus fruticosus, wood sage Teucrium scorodonia, wavy hair-grass Deschampsia flexuosa, bristle bent Agrostis curtisii and in damper areas, purple moor-grass Molinia caerulea. Western gorse Ulex gallii is the dominant scrub species but common gorse Ulex europaeus also occurs and there are several hawthorn Crataegus monogyna, including some sizeable old specimens which support abundant epiphytic mosses and lichens. One part of the site was badly burned in 1976 and has suffered considerable erosion, whilst regenerating ling, currently supports a plant community with a high content of wavy hair-grass and the mosses juniper hair cap Polytrichum juniperinum and bristly hair cap P. piliferum.