Following on from yesterdays post on Arthurs Seat, here is a brief post on Castle Rock which Edinburgh Castle sits upon.
Edinburgh Castle atop the volcanic plug from wikipedia.
Castle Rock is the remains of an ancient roughly 350 million year old volcanic plug. A volcanic plug forms when magma within a conduit solidifies. Castle Rock has also shielded the sedimentary rock during glaciations which has allowed the formation of a sloped end. The rock itself is called Dolerite, which is a type of Basalt. Dolerite cools more slowly than other basalts and as a consequence can often have larger crystals.