The limestone arch that defines Durdle Door began forming 140 million years ago when tectonic forces folded horizontal sediment layers into vertical ridges. Wave action over millennia carved the opening, leaving a 25-metre span that frames the Channel in a geometry textbooks now cite as a teaching example of coastal erosion. Lulworth Cove, two kilometres east, owes its horseshoe perfection to a single breach in the Portland Stone rampart that guards the coast; once seawater reached the softer Purbeck and Wealden clays behind, it excavated a near-circular basin whose symmetry survives in the modern shoreline. Together, the two landmarks anchor the central corridor of the Jurassic Coast, the 155-kilometre World Heritage Site that exposes 185 million years of Earth's narrative in a legible west-to-east sequence. Bournemouth sits 27 kilometres east of the cove, close enough that a Lulworth Cove & Durdle Door mini coach tour from Bournemouth delivers both sites in a half-day window. The route follows the A35 and B3070 through Wool and West Lulworth, depositing visitors at the clifftop car park 143 steps above Durdle Door beach. The arch itself is best photographed from the western headland at mid-morning, when the sun strikes the seaward face; the eastern vantage offers a profile view that includes Man o' War Bay in the middle distance. Lulworth Cove's horseshoe curve measures 180 metres across the mouth and 230 metres at its deepest penetration, dimensions that make it both a natural harbour and a textbook illustration of differential erosion. The Heritage Centre on the village approach holds fossil displays and stratigraphic cross-sections, though most Jurassic Coast tours allocate more time to the viewpoints than the exhibition halls. Geologists value the site for its clarity. The Lulworth Crumple, a series of vertical limestone folds visible on the cove's western arm, compresses what would ordinarily take hundreds of kilometres of lateral distance into a 30-metre outcrop. The rock sequence reads like a vertical library: Portland Stone at sea level, Purbeck Beds above, then Wealden Clay and finally Chalk on the inland ridges. Each layer represents a distinct marine or terrestrial environment, and the folding that tilted them upright happened during the Alpine orogeny 20 million years ago. The arch at Durdle Door exposes the same Portland Stone, its honey colour a result of iron oxide staining. Erosion continues at a measured pace; the arch's western abutment shows fresh fracture lines, and geomorphologists estimate the structure has perhaps 10,000 years before collapse. For now, it remains the most photographed natural feature on the southern English coast, a claim supported by visitor numbers that exceed 500,000 annually.