Red Sea

RED SEA, a narrow strip of water extending south-south-east from Suez to the Strait of Bab el Mandeb in a nearly straight line, and separating the coasts of Arabia from those of Egypt, Nubia and Abyssinia. Its total length is about 1,200 m., and its varies from about 250 M. in the southern half to 13o m. in 27° 45' N., where it divides into two parts, the Gulf of Suez and the Gulf of Aqaba, separated from each other by the penin sula of Sinai. Structurally, the Red sea is part of a great rift-valley system forming one of the most marked features of the earth's crust. The rift valley of the Jordan and the Dead sea is con tinued southward by the Wadi el Araba, and its submerged southern section gives the Gulf of Aqaba. This well marked north-north-east to south-south-west line meets the north-north west to south-south-east line, already marked in the Gulf of Suez, and together they form, to the south, the main basin of the Red sea. The rift may be traced still farther southward among the great African lakes.

This great structural depression is probably of Tertiary age, being let down between two ancient Archaean blocks—Arabia and North Africa. Secondary and Tertiary deposits appear on both flanks in Egypt and Arabia, while Archaean material appears at the surface in many places on the Red sea coast. These exten sive earth movements were accompanied by much volcanic activity, traces of which are still evident. A group of volcanic islands occurs in 14° N., and on Jebel Teir, farther north, a vol cano has only recently become extinct. The margin of the Red sea itself consists, on the Arabian side, of a strip of low plain backed by ranges of barren hills of coral and sand formation, and here and there by mountains of considerable height. The greater eleva tions are for the most part formed of limestones, except in the south, where they are largely volcanic. The coasts of the Gulf of Aqaba are steep, with numerous coral reefs on both sides. On the African side there are, in the north, wide stretches of desert plain, which towards the south rise to elevated tablelands, and ultimately to the mountains of Abyssinia. The shores of the Red sea are little indented; good harbours are almost wanting in the desert regions of the north, while in the south the chief inlets are at Massawa, and at Kamaran, almost directly opposite. Coral for

mations are abundant ; immense reefs, both barrier and fringing, skirt both coasts, often enclosing wide channels between the reef and the land.

Depths.

The mean depth of the Red sea is 267 fathoms. The Gulf of Suez is shallow, and slopes regularly down to the northern extremity of the Red sea basin, which has a maximum depth of 64o fathoms, and then over a shoal of 6o fathoms goes down to 1,200 fathoms in 22° 7' N. The Gulf of Aqaba is separated from the Red sea by a submarine bank only 7o fathoms from the surface, and in 28° 39' N. and 43' E. it attains the depth of 700 fathoms. South of the 1,200-fathom depression a ridge rises to soo fathoms in the latitude of Jeddah, and, south of this again, a similar depression goes down to 1,190 fathoms. Throughout this northern part, i.e., to the banks of Suakin and Farsan in 2o° N., the ioo-fathom line keeps to a belt of coral reef close inshore, but in lower latitudes the shallow coral region, 30o m. long and 7o to 8o m. across, extends farther and farther seaward until, in the latitude of Hodeida, the deep channel (marked by the ioo-fathom line) is only 20 m. broad, all the rest of the area being dangerous to navigation, even for small vessels. In the middle of the gradu ally narrowing channel three depressions are known to exist; soundings in two of these are : 1,209 fathoms in 20° N. and 890 fathoms in 16° N., a little to the north of Massawa. To the north-west of the volcanic island of Zebayir the depth is less than 500 fathoms; the bottom of the channel rises to the ioo-fathom line at Hanish island (also volcanic), then shoals to 45 fathoms, and sinks again, in about the latitude of Mokha, in a narrow channel which curves westward round the island of Perim (depth 170 fathoms), to lose itself in the Indian ocean. This western channel is 16 m. wide in the Strait of Bab el Mandeb; the eastern channel of the strait is 2 m. broad and 16 fathoms deep. Estimates of the total area of the Red sea vary considerably, but it may be taken at 177p3o sq.st.m. approximately, and the volume at 53,700 cb.st.m. approximately.