Castle Rock ( gd, Creag a' Chaisteil, IPA: �kʰʲɾʲekˈaˈxaʃtʰʲɪl is a

volcanic plug A volcanic plug, also called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if rising volatile-charged mag ...

in the middle of

Edinburgh Edinburgh ( ; gd, Dùn Èideann ) is the capital city of Scotland and one of its 32 Council areas of Scotland, council areas. Historically part of the county of Midlothian (interchangeably Edinburghshire before 1921), it is located in Lothian ...

upon which

Edinburgh Castle Edinburgh Castle is a historic castle in Edinburgh, Edinburgh, Scotland. It stands on Castle Rock (Edinburgh), Castle Rock, which has been occupied by humans since at least the Iron Age, although the nature of the early settlement is unclear. ...

sits. The rock is estimated to have formed some 350 million years ago during the early

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

period. It is the remains of a

volcanic pipe Volcanic pipes or volcanic conduits are subterranean geological structures formed by the violent, supersonic eruption of deep-origin volcanoes. They are considered to be a type of ''diatreme''. Volcanic pipes are composed of a deep, narrow cone of ...

which cut through the surrounding

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

, before cooling to form very hard

dolerite Diabase (), also called dolerite () or microgabbro, is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grained ...

, a coarser-grained equivalent of

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

. Subsequent

glacial erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

was resisted more by the dolerite, which protected the softer rock to the east, leaving a crag and tail formation. The summit of the castle rock is above sea level, with rocky cliffs to the south, west and north, rearing up to from the surrounding landscape. 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 An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...

. Providing water to the Upper Ward of the castle was problematic, and despite the sinking of a deep well, the water supply often ran out during drought or siege, for example during the Lang Siege of 1573.

References

External links

{{Hills of Edinburgh Carboniferous volcanoes Hills of Edinburgh Volcanic plugs of Scotland