The South East was covered by a shallow sea for nearly half of the Tertiary Period. Within the sea was an archipelago, or group of islands, consisting of ancient granitic rocks. These formed part of the original crust of the Gondwana supercontinent and are about 450 million years old. The region between Kingston and Keith is underlain by a shallow ridge of these crustal rocks which still protrude above the modern land surface as isolated rocky hills. The granite has been weathered into groups of large boulders known as 'tors'. These have been weathered and sculptured into varied shapes characteristic of granitic rocks in many parts of the world. The flat plains separating the tors are underlain in places by marine rocks and sediments indicating that at times the Tertiary sea covered much of the area.