Castle Rock (, ) is a

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

in the middle of

Edinburgh Edinburgh is the capital city of Scotland and one of its 32 Council areas of Scotland, council areas. The city is located in southeast Scotland and is bounded to the north by the Firth of Forth and to the south by the Pentland Hills. Edinburgh ...

upon which

Edinburgh Castle Edinburgh Castle is a historic castle in Edinburgh, Scotland. It stands on Castle Rock (Edinburgh), Castle Rock, which has been occupied by humans since at least the Iron Age. There has been a royal castle on the rock since the reign of Malcol ...

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

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

period. It is the remains of a volcanic pipe which cut through the surrounding

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

, before cooling to form very hard dolerite, 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 Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

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 material, consisting of permeability (Earth sciences), permeable or fractured rock, or of unconsolidated materials (gravel, sand, or silt). Aquifers vary greatly in their characteristics. The s ...

. 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