The Rock of Gibraltar is an upside-down slab of Jurassic seabed
Ancient sailors treated this limestone mountain as one of the Pillars of Hercules, the edge of the known world. Geologists see something stranger: layers of seabed laid down some 175 to 200 million years ago, folded so hard that the oldest now sit on top of the youngest, riddled with caves and hollowed by soldiers' tunnels.
The Rock rises to 426 metres at O'Hara's Battery near its southern end, at the tip of a promontory about 9 kilometres long that is joined to Spain by a low sandy isthmus. About three-quarters of it is Gibraltar Limestone, holding worn fossils of corals, snails, brachiopods and stromatolites that date it to the Early Jurassic. It forms a deeply eroded, heavily faulted limb of an overturned fold. Its present shape owes more to events about 5 million years ago, when Africa jammed against Europe, the Mediterranean dried out, and the Atlantic then burst through the strait to refill it.
Rainwater slowly dissolving the limestone has carved more than 100 caves, with St. Michael's Cave halfway up the western slope the best known. In 1848 quarrymen at Forbes' Quarry found the skull of a Neanderthal woman, but no one grasped its importance until the Neander Valley discovery of 1856. Gorham's Cave, near sea level and now a UNESCO World Heritage Site, preserves traces of Neanderthals and their remarkably varied diet.
People have fortified the Rock for centuries. Moorish rule lasted 710 years, beginning when the Berber commander Tariq ibn-Ziyad landed in 711; the mountain still carries his name. During the Great Siege of 1779 to 1783, Sergeant-Major Henry Ince proposed boring a tunnel through the north face so guns could fire down on Spanish batteries, and the resulting galleries, finished in 1797, run nearly 304 metres. In the Second World War civilians were evacuated and more than 30,000 servicemen expanded the tunnels. A secret scheme, Operation Tracer, kept six men ready for years to be sealed inside as hidden observers had Germany seized it.
Roughly 40% of Gibraltar became a nature reserve in 1993. It shelters about 300 Barbary macaques, Barbary partridges and a local candytuft. The headland is also a bottleneck for migrating birds, with an estimated 250,000 raptors crossing the strait each year.
Source: Rock of Gibraltar
