Arthur's Seat is the main peak of the group of hills which form most of Holyrood Park, described by Robert Louis Stevenson as'a hill for magnitude, a mountain in virtue of its bold design'. It is situated in the centre of the city of Edinburgh, about a mile to the east of Edinburgh Castle, near Holyrood Palace. The hill rises above the city to a height of 250.5 m (822 ft). Like the castle rock on which Edinburgh Castle is built, it was formed by an extinct volcano system of Carboniferous age (approximately 350 million years old), which was eroded by a glacier moving from west to east. The Salisbury Crags are a series of 46-metre (151 ft) cliffs at the top of a subsidiary spur of Arthurs Seat which rise in the middle of Holyrood Park (650-acre (260 ha) in area).