10'
N. lat., the western shore is higher and the water deepens considerably,
being over one-half of the area 50 fathoms, while the maximum depth
(between 41 deg. and 42 deg. N. lat.) reaches 437 fathoms. This, the
middle section of the Caspian, which extends as far as the Apsheron
peninsula, receives the Terek and several smaller streams (e.g. Sulak,
Samur), that drain the northern slopes of the Caucasus. At Derbent, just
north of 42 deg. lat., a spur of the Caucasus approaches so close to the
sea as to leave room for only a narrow passage, the _Caspiae Pylae_ or
_Albanae Portae_, which has been fortified for centuries. The eastern
shore of this section of the sea is also formed by the Ust-Urt plateau,
which rises 550 ft. to 750 ft. above the level of the Caspian; but in 42
deg. N. lat. the Ust-Urt recedes from the Caspian and circles round the
Gulf of Kara-boghaz or Kara-bugaz (also called Aji-darya and
Kuli-darya). This subsidiary basin is separated from the Caspian by a
narrow sandbar, pierced by a strait 1-1/4 m. long and only 115 to 170
yds. wide, through which a current flows continuously into the gulf at
the rate of 1-1/2 to 5 m. an hour, the mean velocity at the surface
being 3 m. an hour. To this there exists no compensating outflow current
at a greater depth, as is usually the case in similar situations. The
area of this lateral basin being about 7100 sq. m., and its depth but
comparatively slight (3-1/2 to 36 ft.), the evaporation is very
appreciable (amounting to 3.2 ft. per annum), and sufficient, according
to von Baer, to account for the perpetual inflow from the Caspian. South
of the Kara-Boghaz Bay the coast rises again in another peninsula,
formed by an extension of the Balkhan Mountains. This marks (40 deg. N.
lat.) the southern boundary of the middle section of the Caspian. This
basin may be, on the whole, considered as a continuation of the
synclinal depression of the Manych, which stretches along the northern
foot of the Caucasus from the Sea of Azov. It is separated from (3), the
southern and deepest section of the Caspian, by a submarine ridge (30 to
150 fathoms of water), which links the main range of the Caucasus on the
west with the Kopet-dagh in the Transcaspian region on the east. This
section of the sea washes on the south the base of the Elburz range in
Persia, sweeping round from the mouth of the Kura, a little north of the
Bay of Kizil-agach, to Astarabad at an average distance of 40 m. fr