Area: 173.46 (ha) 428.62
A Nature Conservation Review Site. In Cornwall Area of Outstanding Natural Beauty. Includes two Geological Conservation Review Sites. Boundary amended by extensions and deletions. Part owned by National Trust.
Description and Reasons for Notification:
Coverack to Porthoustock is located on the east coast of the Lizard Peninsula, the most southerly tip of mainland Britain. The site lies mainly on gabbro and includes the entire stretch of gabbro coast, with inland exposures. The coastline comprises quarries, a fossil cliff and raised beach feature and the only remaining biologically intact beach habitat in west Cornwall. Gabbro, basic igneous rock weathers to gley and stragnogley soils. In places the gabbro is overlain by a fine windblown deposit called loess. This can be up to 2 m deep and weathers to brown earths and gleyic brown earths. The combination of unusual geology, topography and mild oceanic climate has led to the development of a unique flora. The site has 4 Red Data Book (RDB) (*) and many nationally scarce species. Of particular note is the RDB species Cornish heath Erica vagans which, in the British Isles, is confined to the Lizard Peninsula and County Fermanagh, Northern Ireland. Areas of the coastline which have been quarried reveal exposures which are of geological interest.
The remote beaches at Godrevy and Leggan Coves are a rare example of beach community zonation. Where the foreshore is sandy with only scattered pebbles, a community dominated by sea sandwort Honkenya peploides occurs on or above the strandline. Other species include Babington’s orache Atriplex glabriuscula and prickly saltwort Salsola kali. This community is replaced on shingle by spear-leaved orache Atriplex prostrata with occasional sea mayweed Tripleurospermum maritimum and sea beet Beta vulgaris subsp. maritima. Further up where the beach is more stable a community of sparse red fescue Festuca rubra is found along with kidney vetch Anthyllis vulneraria, sea bindweed Calystegia soldanella and common bird’s-foot trefoil Lotus corniculatus.
The present-day cliff rises to 15 metres but is often lower. Where solid gabbro outcrops, a sparse maritime rock-crevice community occurs comprising thrift Armeria maritima subsp. maritima, rock samphire Crithmum maritimum, rock sea-spurrey Spergularia rupicola, sea aster Aster tripolium, golden samphire Inula crithmoides and buck’s-horn plantain Plantago coronopus. The nationally scarce lanceolate spleenwort Asplenium billotii is found in open crevices. Where the cliff is overlain by loess, cliff flush communities have developed, characterised by creeping bent Agrostis stolonifera, common reed Phragmites australis, fool’s water-cress Apium nodiflorum, brookweed Samolus valerandi and distant sedge Carex distans and the nationally scarce dotted sedge C. punctata in particularly wet flushes.
Maritime grassland communities are widespread but often indistinct. Typical species include cock’s-foot Dactylis glomerata, Yorkshire fog Holcus lanatus, red fescue, sea carrot Daucus carota subsp. gummifer, chamomile Chamaemelum nobile, yarrow Achillea millefolium, common knapweed Centaurea nigra, common fleabane Pulicaria dysenterica and ribwort plantain Plantago lanceolata. The prostrate Dyer’ s greenweed Genista tinctoria subsp. littoralis, English sticky eyebright Euphrasia anglica and the nationally scarce hairy bird’s-foot trefoil Lotus subbiflorus are present in grazed areas. Of particular importance is a population of the RDB Schedule 8 species pennyroyal Mentha pulegium, which is declining in Britain and confined in Cornwall to the Lizard Peninsula. The short turf community above Leggan Point provides good habitat for the nationally scarce autumn squill Scilla autumnalis and the RDB species (*) dwarf rush Juncus capitatus and twin-headed clover Trifolium bocconei are occasionally recorded here.
The vegetation of the raised beach running from Lowland Point to North Corner is a complex of maritime grassland and heathland communities. Two of the heathland communities, tall heath and mixed heath, are unique to the Lizard Peninsula in the British Isles and are widespread on this site.
Tall heath is found where drainage is impeded and is characterised by Cornish heath, black bog-rush Schoenus nigricans, purple moor-grass Molinia caerulea, cross-leaved heath Erica tetralix and saw-wort Serratula tinctoria.
Well-drained soils support mixed heath comprising Cornish heath, bristle bent Agrostis curtisii, purple moor-grass, bell heather Erica cinerea, gorse Ulexeuropaeus , western gorse U. gallii, dropwort Filipendula vulgaris and wild thyme Thymus polytrichus subsp. arcticus. The hybrid violet Viola lactea 3 riviniana is also found in these stands.
The fossil cliff is degrading but rises to 70 metres in places. Outcropping gabbro tors favour subterranean clover Trifolium subterraneum, butcher’s-broom Rucusa culeatus, hairy bird’ s-foot trefoil and lanceolate spleenwort.
In waterlogged areas, particularly around The Grove and Lowland Point, willow carr dominated by rusty willow Salix cinerea subsp. oleifolia is found, with rich ground flora of tall herb and ferns. Lady-fern Athyrium filix-femina and male fern Dryopteris filix-mas occur along with broad buckler-fern Dryopteris dilatata and great horsetail Equisetum telmateia. The Grove also supports a large colony of dotted sedge.
A spindle copse Euonymus europaeus in the west of the site has a notably rich ground flora with 39 species recorded including a large population of the nationally scarce bastard balm Mellitus melissophyllum. Such areas provide both food and cover for birds and are of particular importance during the spring and autumn migrations.
The site is also of great importance to bryophytes. The Grove in particular provides an excellent habitat with both trees and boulders being clothed with bryophytes. Lateral cryphea Cryphaea heteromalla and minute pouncewort Cololejeuna minutissima are frequently found here, whilst river pocket moss Fissidens rivularis occurs on wet rocks. The two nationally scarce species purple crystalwort Riccia beyrichiana and smallest pottia Pottia davalliana (commutata) have been recorded on the coastal cliffs.
The Lizard Complex consists of a large serpentinised peridotite body, cut by later gabbros, basic dykes and granitic veins and having spatial affinities with amphibolitic rocks. Recent interpretations consider the Complex to represent the tectonically juxtaposed remnants of a disrupted ophiolite complex.
At Porthoustock the large number of dykes accounting locally for between 50 – 80 percent of exposures, cutting the gabbro has led to the suggestion that they represent the basal past of a sheeted dyke swarm. Such swarms are a characteristic feature of the lower crust in an ophiolite, and their occurrence at Porthoustock indicates it to be the highest structural level of the Lizard ophiolite to be preserved.
Geochemical studies have shown that the dykes and the enclosing host gabbro are chemically related with the dykes being more evolved than the gabbro parent. Contact relations are varied with some dykes showing sharp chilled margins while others with lobate junctions and no chilling are indicative of injection into hot gabbro. These relations indicate that the dykes were intruded episodically during the cooling of the gabbro and subsequently after its consolidation.
Late hydrothermal veins in the St Keverne gabbro here contain a variety of zeolite minerals, prehnite, pectolite and unusual crystallisations of calcite. Of the zeolites, natrolite and analcime are particularly well developed, the former occuring as prismatic crystals up to 16 cm long and the latter as typical trapezohedrons up to about 5 cm. Other zeolites present in smaller amounts are stilbite, heulandite and chabazite. Calcite and prehnite occur in several different and unusual forms. The paragenetic sequence from high temperature zeolites with low silica and low water contents (analcime, natrolite) to low temperature silica-rich hydrated zeolites (stilbite, etc.) is very clearly demonstrated, as is the alteration of zeolites to clay minerals. In addition, the gabbro exhibits many interesting features including coarse pegmatitic areas with pyroxene crystals up to 25 cm in length.
Dean Quarry is an extremely important instructional site which also contains the finest examples of zeolite minerals to be found in Britain.
* These plants are included in the Red Data Book listing of rare and endangered species.