Structurally, the whole of Arabia is a vast platform of ancient rocks, once continuous with north-east Africa. In relatively recent geological time a series of great fissures opened, as the result of which a large trough, or rift valley, was formed and later occupied by the sea, to produce the Red Sea and the Gulf of Aden.
The Arabian platform is tilted, with its highest part in the extreme west, along the Red Sea, sloping gradually down from the west to the east. The Red Sea coast, where the upward tilt is greatest, is often bold and mountainous, with peaks of 3,000 meters. Along the Red Sea coast, there is a narrow coastal strip (Tihama) which broadens out in the Jiddah area and provides access through the highlands to the interior. On the eastern side of the Kingdom, the Arabian Gulf coast is flat and low-lying. The shallow seas in this region deposited layers of younger sedimentary rock, allowing the creation of the vast oil reserves for which the area was to become famous. The coast is fringed with extensive coral reefs which make it difficult to approach the shore in many places.