Portland stone has built half of British civic architecture, and Perryfield Whitbed is the variety most architects ask for by name. The bed runs through this quarry at heights useful enough to allow large blocks to come out intact - the bigger the block, the more flexibility the mason has for carving capitals, lintels, panels and steps. In 1993, Whitbed from Perryfield was used in the restoration of Westminster Abbey. In 2012, specially selected blocks were carved into classical panels and lintels for the Farringdon Street façade of the Sixty London development at Holborn Viaduct. The same beds supplied Waterloo Bridge when it was completed in 1945. Quarrying first began at Perryfield in the 1890s. Planning consent granted in 1951 covered 324 hectares of the island; the current permission runs to 2042, but the stone in the ground will outlast it.

Walk into the quarry pit and the cliff face reads like a textbook. The Whitbed is buff-white and shelly, a near-perfect freestone that masons love because it cuts in any direction without splitting along an awkward grain. Beneath it lies the Basal Shell Bed, the layer that yields the bivalves, gastropods, and ammonite fossils that quarrymen routinely turn up. The Purbeck Beds above are mostly buried now under rubble. The Titanites at the gate came from one of these beds. It is not an ornament - it is an advertisement, in the way of all good geological landmarks, for what this rock has been before it became a building. The quarry is open-cast, which is faster and gentler on the stone than tunnel-mining, and the working face moves westwards across the hill year by year.

Quarrying is a slow process and quarrying companies are landowners by default. Areas of Perryfield that were worked out decades ago have been left long enough for nature to climb in. In 1998 one of these abandoned, infilled areas was leased to Butterfly Conservation as a nature reserve, becoming part of the wider Bottomcoombe Site of Nature Conservation Interest. It joined Broadcroft Quarry, four years older as a reserve, in a small archipelago of butterfly habitat across Portland's quarrying landscape. The arrangement is unusual but logical: the quarry company keeps the lease cheap because the land has no industrial value, and Butterfly Conservation gets the rare habitat that limestone scrub and grassland provide. The conservation team's main task is keeping the encroaching scrub off the grassland - especially non-native plants like Cotoneaster horizontalis.

The silver-studded blue is Perryfield's signature species, with a thriving colony established on the warm limestone of the reserve. Look closely on a bright June day and the small males flutter in shimmering clouds over the low vegetation. Other blues join them through the summer: small blue in May and June, chalkhill blue in July, Adonis blue glowing electric in late summer. The grassland species cycle through too - dingy skipper early, small and large skippers, ringlet, meadow brown, marbled white. Migrant painted ladies and clouded yellows pass through. Moths take over at dusk: the cinnabar, the six-spot burnet, the Portland ribbon wave that takes its name from the very island it lives on. The sheltered tall grass on the slopes is one of the best sites on Portland for glow-worms after dark.

Above the butterflies, the copse of sycamores and the scrub-covered slopes down to the disused railway line form one of the best migrant-bird funnels on the southern Dorset coast. Portland sticks out into the Channel, and migrating birds that find themselves over open water at dawn often make for the first land they see. The reserve catches warblers in their hundreds during the spring and autumn passages. Green woodpeckers, kestrels, ravens, and peregrine falcons hunt here year-round. The quarry's other neighbours are residential now - Pennsylvania Heights, built on a former part of the workings, faces the cliff above Pennsylvania Castle - but the wildlife corridor along the disused railway and into the cliffs at Church Ope keeps the reserve connected to a much bigger landscape. Industry on one side of the fence, ammonites in the rocks, blues in the air. Portland does this kind of layered geography very well.