The Rock of Gibraltar is a monolithic limestone promontory located in the British overseas territory of Gibraltar, off the southwestern tip of Europe on the Iberian Peninsula. It is 426 m (1,398 ft) high. The Rock is Crown property of the United Kingdom, and borders Spain. Most of the Rock's upper area is covered by a nature reserve, which is home to around 300 Barbary macaques. These macaques, as well as a labyrinthine network of tunnels, attract a large number of tourists each year. The Rock of Gibraltar was one of the Pillars of Hercules and was known to the Romans as Mons Calpe, the other pillar being Mons Abyla or Jebel Musa on the African side of the Strait. In ancient times the two points marked the limit to the known world, a myth originally fostered by the Greeks and the Phoenicians. The Rock of Gibraltar is a monolithic promontory. It is a deeply eroded and highly faulted limb of an overturned fold. The sedimentary strata comprising the Rock of Gibraltar are overturned (upside down) with the oldest strata overlying the youngest strata. These strata are the Catalan Bay Shale Formation (youngest), Gibraltar Limestone, Little Bay Shale Formation (oldest), and Dockyard Shale Formation (age unknown). These strata are noticeably faulted and deformed. Predominantly of shale, the Catalan Bay Shale Formation also contains thick units composed of either brown calcareous sandstone, soft shaly sandstone interbedded with bluish-black limestone, and interlayered greenish-gray marls and dark gray cherts. The Catalan Bay Shale Formation contains unidentifiable echinoid spines and belemnite fragments and infrequent Early Jurassic (Middle Lias) ammonites. The Gibraltar Limestone consists of greyish-white or pale-gray compact, and sometime finely crystalline, medium to thick bedded limestones and dolomites that locally contain chert seams. This formation comprises about three quarters of the Rock of Gibraltar. Geologists have found various poorly preserved and badly eroded and rolled marine fossils within it. The fossils found in the Gibraltar Limestone include various brachiopods, corals, echinoid fragments, gastropods, pelecypods, and stromatolites. These fossils indicate an Early Jurassic age (Lower Lias) for the deposition of the Gibraltar Limestone. The Little Bay and Dockyard shale formations form a very minor part of the Rock of Gibraltar. The Little Bay Shale Formation consists of dark bluish-gray, unfossiliferous shale, which is interbedded with thin layers of grit, mudstone, and limestone. It predates the Gibraltar Limestone. The Dockyard Shale Formation is an undescribed variegated shale of unknown age that lies buried beneath the Gibraltar's dockyard and coastal protection structures. Although these geological formations were deposited during the early part of the Jurassic Period some 175-200 million years ago, their current appearance is due to far more recent events of about 5 million years ago. When the African tectonic plate collided tightly with the Eurasian plate, the Mediterranean became a lake that, over the course of time, dried up during the Messinian salinity crisis. The Atlantic Ocean then broke through the Strait of Gibraltar, and the resultant flooding created the Mediterranean Sea. The Rock forms part of the Betic Cordillera, a mountain range that dominates south-eastern Iberia. Today, the Rock of Gibraltar forms a peninsula jutting out into the Strait of Gibraltar from the southern coast of Spain. The promontory is linked to the continent by means of a sandy tombolo with a maximum elevation of 3 m (9.8 ft). To the north, the Rock rises vertically from sea level up to 411.5 m (1,350 ft) at Rock Gun Battery. The Rock's highest point stands 426 m (1,398 ft) above the strait at O'Hara's Battery. The Rock's central peak, Signal Hill, stands at an elevation of 387 m (1,270 ft). The near-cliffs along the eastern side of the Rock drop down to a series of wind-blown sand slopes that date to the glaciations when sea levels were lower than today, and a sandy plain extended east from the base of the Rock. The western face, where the City of Gibraltar is located, is comparatively less steep. Calcite, the mineral that makes up limestone, dissolves slowly in rainwater. Over time, this process can form caves. For this reason the Rock of Gibraltar contains over 100 caves. St. Michael's Cave, located halfway up the western slope of the Rock, is the most prominent and is a popular tourist attraction. Gorham's Cave is located near sea level on the steep eastern face of the Rock. It is noteworthy because archaeological excavations in the cave have found evidence that Neanderthals used it as far back as 30,000 years ago. It is especially significant because plant and animal remains found in the cave (and others nearby) indicate that the Neanderthals had a highly varied diet. The flora and fauna of the Gibraltar Nature Reserve are of conservation interest and are protected by law.