The Gulf of Akaba is directly in line with the Jordan Valley, a similar depression on a smaller scale, only partially occupied by water, the Dead Sea, while southwards we find another dry valley running through British East Africa and adjoining territories, a great trough bounded by plateaux, several thousand feet above its bottom. We can thus trace [Illustration: Diagram 7. Formation of a rift valley] this trough-like valley from Palestine to some degrees south of the Equator as a stupendous crack in the earth’s surface, well named “The Great Rift Valley[60].” The Red Sea is its greatest section, its total depth here being, say, 5000 feet from the summit of the mountains[61] to sea level and 6000 feet to the sea bottom, 11,000 feet in all. The formation of such a valley, by the dropping down of a series of strips of country below the level of the remainder, is illustrated by Diagram 7. To study the simplest possible case we draw a section through the ground and imagine it formed of three kinds of rock, of which two form horizontal sheets, AA and BB, over the third CC. These were originally unbroken, and in the positions shewn by the lines of dashes, but were broken by the dropping down of the central part to form the valley shewn here in section. The floor of the valley has the same structure as the original surface of the ground, the same three beds, A, B and C, occurring in the same positions, but at a lower level. They are found again in each of the steps on the valley’s sides, their regular reappearance in this way being conclusive proof of the earth movements postulated. The vertical lines FFFF, between each step and the next drop, along which the continuity of the beds is broken, are termed “faults,” a geological term which should be remembered. Rift valleys are found elsewhere in the world, but are exceptional, ordinary valleys, with their winding courses and rounded outlines, having been formed by the action of streams, which slowly wash away the ground and hollow out their courses to the sea. The actual structure of the middle portion of the Red Sea Valley is shewn diagrammatically by the section on page 145. Five steps are shewn, Nos. 2 and 3 being further separated by a minor fault valley. The details are described later. The southern part of the sea, below Masawa, has recently been subjected to volcanic[62] action; many of the islands there are quite well-preserved volcanic cones, but as regards the rest of the sea, though earth movements have been frequent and considerable, there are now no traces of volcanic action, and the movements that have occurred have not necessarily involved cataclysms greater than severe earthquakes.