Castle Rock
is a
volcanic plug
in the middle of Edinburgh upon which
Edinburgh Castle
sits. The rock is estimated to have risen some 350 million years ago during the lower
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 type of basalt. Subsequent
glacial erosion
was resisted by the dolerite, which protected the softer rock to the east, leaving a
crag and tail
formation.