The whole of Peninnis Head is coarse-grained Hercynian granite, part of the same Cornubian batholith that underlies all of Scilly. What makes this headland different is what has not happened to it. The Irish Sea Glacier stopped just north of the islands during the last ice age, which means the granite here was never scoured by ice. Three hundred million years of wind, rain and salt spray have done the work instead, etching and rotting the rock into the shapes that now stand against the sky. Thin podzolic soils have formed over the granite in pockets, supporting waved maritime heath, the wind-flattened vegetation that drapes the cliff edges. Spring squill and wild thyme grow in the species-rich grassland, both unusual plants for the Isles of Scilly.

The famous shapes on Peninnis Head have names because they have to. Pulpit Rock looks like exactly what it claims, a thick granite platform raised on a thinner column, as if some preacher had walked off mid-sermon. Other formations carry local names that come and go with generations of walkers. The cairns are not human-made. They are natural granite stacks, blocks fractured along the rock's own joint planes and then weathered until what is left looks deliberate. Geologists value the place as a Geological Conservation Review site precisely because of this absence of glaciation. By comparing Peninnis Head to glaciated granite on Tresco at the north of the islands, scientists can see what difference the ice did and did not make.

At the very tip of the headland, where the salt-laden wind never stops, a strange botanical garden survives. The dominant species is common sea ivory, Ramalina siliquosa, the grey-green tufted lichen that thrives at the edge of the British coast. But Peninnis Head also holds Roccella fuciformis and Roccella phycopsis, both nationally scarce. Teloschistes flavicans, the bright orange golden hair-lichen, clings to the granite in places. The endangered ciliate strap-lichen, Heterodermia leucomela, hangs on as well, a Biodiversity Action Plan priority species. The headland was first designated a Site of Special Scientific Interest in 1971, for both biological and geological reasons. The Duchy of Cornwall owns all the land within the SSSI.

Peninnis Lighthouse sits at the southern extremity of the headland. Built by Trinity House in 1911, it is a 45-foot circular tower of black steel open lattice with a white gallery and a black-domed top, an unusual design that owes more to industrial Victorian engineering than to the classic stone lighthouse. It replaced an older light at the centre of the island of St Agnes which had operated since 1680. The Peninnis light guides vessels into Hugh Town harbour through St Mary's Sound, the eastern entry. Originally the lamp burned oil gas from four pressurised tanks adjacent to the tower. In 1922 it converted to acetylene. In 1992 the structure went over to electricity. In 2011, on its centenary, an LED lantern replaced the old optic, and the visible range was reduced from 16 to 9 nautical miles.

The headland has another, quieter user community. Climbers have been bouldering and trad-climbing on Peninnis since 1977. Most of the routes on the southern coast of St Mary's are here. The hardest is a route called Rock Hard Peninnis, also known as When The Boat Comes In. It is graded E4 6a, which translates as: technically demanding, exposed, with serious consequences if you fall. It was first ascended by Andy Grieve, Sean Hawken and R. Plymouth in September 1996. The climbers come, set up their ropes, and work the same granite that the lichens and the geologists work, on a piece of coast where four entirely different specialisms of attention overlap. The visitor walking past usually does not notice.

Coordinates: 49.9059°N, 6.3038°W. Peninnis Head is the southern tip of St Mary's, jutting out about a kilometre south of Hugh Town and 800 metres south-west of Old Town Bay. From 2,000 feet the headland is unmistakable, a knot of pale granite ending in the black-and-white lattice of the lighthouse. St Mary's Airport (EGHE) sits 1.5 kilometres to the north-east. Approach from the west and the cairns silhouette dramatically against the Atlantic; from the east, the lighthouse marks the corner where St Mary's Sound bends into Hugh Town harbour.