<P> Like the rock on which Edinburgh Castle is built , it was formed by an extinct volcano system of Carboniferous age ( lava samples have been dated at 341 to 335 million years old ) , which was eroded by a glacier moving from west to east during the Quaternary ( approximately the last two million years ) , exposing rocky crags to the west and leaving a tail of material swept to the east . This is how the Salisbury Crags formed and became basalt cliffs between Arthur 's Seat and the city centre . From some angles , Arthur 's Seat resembles a lion couchant . Two of the several extinct vents make up the ' Lion 's Head ' and the ' Lion 's Haunch ' . </P>
when was the last time arthur 's seat erupted
lava samples have been dated at 341 to 335 million years old