Editing Edinburgh Castle (section)

====Geology====

{{Main|Castle Rock, Edinburgh}}

[[File:Crag and tail.png|thumb|Diagram of a crag and tail feature, such as the Castle Rock: '''A''' is the crag formed from the volcanic plug, '''B''' is the tail of softer rock, and '''C''' shows the direction of ice movement. In the case of Edinburgh, the castle stands on the crag ('''A''') with the Royal Mile extending along the tail ('''B''')]]

The castle stands upon the [[Volcanic plug|plug]] of an [[extinct volcano]], which is estimated to have risen about 350 million years ago during the lower [[Carboniferous]] period. The Castle Rock is the remains of a [[volcanic pipe]], which cut through the surrounding [[sedimentary rock]] before cooling to form very hard [[dolerite]], a type of [[basalt]]. Subsequent [[glaciology|glacial erosion]] was resisted by the dolerite, which protected the softer rock to the east, leaving a [[crag and tail]] formation.<ref>McAdam, p. 16.</ref>

The summit of the Castle Rock is {{convert|130|m|ft}} above sea level, with rocky cliffs to the south, west, and north, rising to a height of {{convert|80|m|ft}} above the surrounding landscape.<ref>MacIvor (1993), p. 16.</ref> 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 self-evident, but the geology of the rock also presents difficulties, since basalt is extremely impermeable. Providing water to the Upper Ward of the castle was problematic, and despite the sinking of a {{convert|34|m|adj=on}} deep well, the water supply often ran out during drought or siege,<ref>Dunbar, p. 192.</ref> including during the Lang Siege in 1573.<ref name=Potter137/>