 About 180 million years ago, a mantle plume under southern Gondwana caused bulging of the continental crust in the area that would later become southern Africa.Within 10 – 20 million years rift valleys formed on either side of the central bulge, which became flooded to become the proto-Atlantic Ocean and proto-Indian Ocean.The stepped, steep walls of these rift valleys formed escarpments that surrounded the newly formed Southern African subcontinent.