Castle Rock (Edinburgh)

Castle Rock
Castle Rock as seen from Princes Street
Highest point
Elevation	140 m (460 ft)[1]
Prominence	c. 80 m (260 ft)
Listing	Tump
Coordinates	55°56′55″N 03°12′03″W / 55.94861°N 3.20083°W / 55.94861; -3.20083
Geography
Castle Rock Edinburgh, Scotland Show map of Edinburgh Castle Rock Castle Rock (the City of Edinburgh council area) Show map of the City of Edinburgh council area Castle Rock Castle Rock (Scotland) Show map of Scotland
Topo map	OS Landranger 66
Geology
Rock age	~350 million years
Mountain type(s)	Crag and tail, volcanic plug
Climbing
First ascent	Unknown

Castle Rock (Scottish Gaelic: Creag a' Chaisteil, Scottish Gaelic pronunciation: [ˈkʰʲɾʲekˈaˈxaʃtʰʲɪl]) is a volcanic plug in the middle of Edinburgh upon which Edinburgh Castle sits. The rock is estimated to have formed some 350 million years ago during the early Carboniferous period. It is the remains of a volcanic pipe which cut through the surrounding sedimentary rock, before cooling to form very hard dolerite, a coarser-grained equivalent of basalt. Subsequent glacial erosion was resisted more by the dolerite, which protected the softer rock to the east, leaving a crag and tail formation.^[2]

The summit of the castle rock is 130 m (430 ft) above sea level, with rocky cliffs to the south, west and north, rearing up to 80 m (260 ft) from the surrounding landscape.^[1] This means that the only readily accessible route to the castle lies to the east, where the ridge slopes more gently. The defensive advantage of such a site is clear, but the geology of the rock also presents difficulties, since basalt is an extremely poor aquifer. Providing water to the Upper Ward of the castle was problematic, and despite the sinking of a 28 m (92 ft) deep well, the water supply often ran out during drought or siege,^[3] for example during the Lang Siege of 1573.^[4]