The 639-hectare (1,580-acre) site is located on the north Cornish coast, by the Celtic Sea of the Atlantic Ocean. The 12 miles (19 km) length of coastline stretches from Boscastle in the south to Widemouth Bay to the north.[2][3] The coast on either side of Crackington Haven is characterised by the cliffs collapsing and rather than having steep vertical slopes such as in west Cornwall, there is a series of 'undercliffs' which are thickly vegetated. Some are grazed by cattle and horses and the resulting mosaic of semi–natural habitats is of national importance for the coastal heath and grassland. High Cliff (grid referenceSX125943) at 223 metres (732 ft) is the highest cliff in Cornwall.[4]
The coast between Boscastle and Widemouth is characterised by high slumped cliffs leading to areas of thickly vegetated, sloping lower undercliffs. The underlying rock, a type of shale which is easily fractured, is known to geologists as the Crackington formation and the layered strata contorted by earth movements can best be seen at Crackington Haven and Millook.[4] The rocks date to the Namurian stage roughly 326 to 313 Ma (million years ago) and were folded during the Variscan orogeny a period of mountain building caused by the collision of two continents.[5]
During the Quaternary Period the area was covered in periglacial deposits called head. At the time the area was tundra and was just to the south of an ice sheet which covered most of Britain.[1]
The valleys at Crackington Haven, Cleave, Dizzard and Millook show good examples of the zonation of habitats from splash zone lichen communities, to cliff vegetation with maritime grass, heath and scrub into woodland communities.[1][5]
The discovery of the large blue butterfly (Phengaris arion) at Millook by E A Waterhouse in 1891 was described as both unexpected and remarkable. It had been extinct in Northamptonshire for thirty years and was declining in areas such as the Cotswolds and along the south coast of Devon. It had probably been overlooked in isolated Cornwall until just before the coming of the railways to Camelford in 1893 and to Bude in 1898. The butterfly was found to inhabit valleys as far west as Tintagel and in some places found in great abundance, which led to some collectors visiting year after year and taking hundreds for collections. Not surprisingly by 1925 the species was in decline, although not solely due to collecting. In 1902 many acres of the original habitat at Millook had been enclosed and ploughed, and P M Bright noted how since 1911 "the gorse has been allowed to grow up over the whole area ... especially in the neighbourhood of Millook. This has choked out its (food plant) in many places where it was abundant, and has also driven away the ants". The temporary demise of rabbits (Oryctolagus cuniculus) caused by the rapid spread of myxomatosis after its introduction in 1953 would also have led to the spread of gorse, and in that year Millook valley was described as "by then totally overgrown and without a vestige of wild thyme, but several other localities, including parts of Crackington Haven, appeared to be, and in fact still are much less obviously changed". In 1963 a search of twenty-three sites in Cornwall found only eight small colonies left, all to the north of Bude and thus outside the area of this SSSI; the large blue was last seen in Cornwall in 1973 (although it was introduced to a north Cornwall site in 2000).[7][8]