National Grid Reference: SH 210845

Site Area: 399.4 ha


This site is of special interest for its geological and biological features, including heathland and maritime grassland communities, coastal cliffs and ledges, its assemblages of vascular plants and birds, invertebrates and its solid geology. The site lies on the north west corner of Holy Island and includes the most westerly point on Anglesey. Holyhead lies immediately to the east.

The cliffs around South Stack lighthouse display some of the most magnificent exposures of folded sedimentary rocks in Great Britain, described by Greenly (1919) as an amazing revelation. The section is the type locality for the Monian South Stack Group, and includes the only known record of burrowing organisms Skolithos in the Mona Complex. The existence of these trace fossils has been cited in the support of a Cambrian, rather than a Precambrian, age for the exposures. The presence of clear sedimentary 'way-up' evidence in these cliffs provided critical evidence to Shackleton in his famous inversion of Greenly's stratigraphic sequence. More recently, the complex minor structures at South Stack have provided further evidence of great significance.

The coastal cliffs and the associated grassland and heaths are of major botanical interest. The South Stack fleawort Tephroseris integrifolia subsp. maritima isn't found anywhere else in the world and the nationally rare spotted rock-rose Tuberaria guttata occurs within the mosaic of heath and grassland communities above the cliffs, together with pale heath violet, Viola lactea. Other nationally scarce plant species on the cliffs include golden samphire, Inula crithmoides and the endemic rock sea-lavender Limonium britannicum subsp. celticum and L. procerum subsp. procerum. Juniper Juniperus communis, a locally uncommon plant, occurs on the cliffs and there are Atlantic bryophytes and ferns such as hay scented buckler fern Dryopteris aemula, Wilsons filmy fern Hymenophyllum wilsonii and Tunbridge filmy fern H. tunbrigense.

An extensive area of dry lowland heath of heather Calluna vulgaris western gorse Ulex gallii covers the flanks of Holyhead Mountain. Scree along the western edge of the mountain supports a more diverse bilberry Vaccinium myrtillus subcommunity of this heather and western gorse heath. Around the coastal margins heather western gorse heath of the spring squill Scilla verna subcommunity grades into heather Calluna vulgaris spring squill Scilla verna maritime heath. In wetter areas crossleaved heath Erica tetralix bogmoss Sphagnum compactum and deergrass Scirpus cespitosus crossleaved heath Erica tetralix wet heath communities are found.

On rocky ledges and at the top of the cliffs the vegetation comprises the thrift Armeria maritima common mouseear Cerastium diffusum maritime therophyte community. This generally forms rather sparse open turf with much bare ground; associated species include buckshorn plantain Plantago coronopus and kidney vetch Anthyllis vulneraria. On deeper soils above the cliffs is the cocksfoot Dactylis glomerata subcommunity of the red fescue Festuca rubra Yorkshire fog Holcus lanatus grassland. These areas are characterised by a very thick sward with associated Spring squill, wild carrot Daucus carota and sorrel Rumex acetosella.

The cliffs support important seabird colonies; guillemots, razorbills and puffins combine to create one of the largest colonies of breeding auks in North Wales. Fulmar and kittiwake also nest on these cliffs together with peregrine and chough, the latter using the heathland and adjacent areas extensively for feeding. Within the heathland stonechat, skylark, linnet and whitethroat all breed regularly.

The site supports a good range of invertebrates including the silver studded blue Plebejus argus and marsh fritillary Eurodryas aurinia has been recorded here in the past.


The site forms part of the Glannau Ynys Gybi: Holy Island Coast grade 1 NCR site (which also includes Port Diana SSSI and Pant yr Hyman SSSI) D A Ratcliffe (1977) A Nature Conservation Review. Cambridge University Press.

E Greenley (1919) The Geology of Anglesey. Memoir of the Geological Survey of Great britain, HMSO London 980pp (2 vols).

Part of the site has been selected asa result of the former Nature Conservancy Council's Geological Conservation Review, a national survey and evaluation of sites of geological and physiographical interest.

The site boundary was revised in 1986 and 2000

Part of the site is managed as a nature reserve by the Royal Society for the Protection of Birds (RSPB).

A voluntary ban agreed between CCW, RSPB and the British Mountaineering Council asks climbers to avoid specified cliffs where seabirds, chough and peregrine, nest from February to July inclusive.




The purpose of this note is to describe the nature and importance of a site, avoiding specialist terms, for the site owner and/or occupier. This note does not form part of the formal notification documents.


This great series of folded and metamorphosed rocks, laid bare in the lofty sea cliffs presents some of the most striking geological sections in Great Britain. The mainland cliffs here rise to a height of 131 m and these look down on the Stack itself. There can hardly be a finer section in the British Isles for the study of both large and small scale folding or of the relations of one to the other. Views from the Stack show great single folds sweeping up the whole height of the cliffs with an amplitude of more than 100m.

The rocks are Precambrian in age, about 570 million years old. They belong to what Edward Greenly, who wrote the geological memoir for Anglesey in 1919, termed the bedded series of the Mona complex. They are some of the oldest rocks to be found anywhere in southern Britain. Two types of bed exist - those which were originally sandy and those which were muddy. The intricate folding pattern seen today is directly related to the varied composition of these beds and to their different reactions when subjected to pressure.

Fossils are very rare in Precambrian rocks but Greenly reported the possible presence of worm burrows in some of the sandy South Stack beds. This has not been proved conclusively but it remains a stumbling block to those who submit that evidence for worms is found only in younger, Cambrian rocks.