THE ARALO-CASPIAN SEA IN THE KARA-KUM. 37
The plains of southern Turkestan are described by Mushketof and others as occupying an extensive area of depression which has received the waste washed into it from the surrounding higher lands. These higher lands are as follows : The Ust-urt plateau on the west is an uplift of late Tertiary strata, being covered in its western part at least by the Akchlagyl formation of Andrussof (1902). Krasnovodsk lies southwest of this upland, and the sections already given in fig. 2o, as well as the escarpment of the Ust-urt farther east, suggests that the upland is bordered by a fault along its southern and southeastern margin. The Kopet Dagh, on the south, has been described by Bogdanovich (1887). The range is largely composed of Mesozoic and Tertiary limestones, folded in a somewhat orderly fashion, with axes trending west-northwest. The northwestern part of this line of disturbance is known as the Kurian Dagh, the Small and Great Balkhans, and the Kuba Dagh (the last rising back of Krasnovodsk). The farther extension of the saine line leads to the Caucasus range. The plains are bounded on the east by the out-reaching members of the great mountain systems of Central Asia, well known to involve late Tertiary and post-Tertiary uplifts, as will further appear in Mr. Huntington's report. To the north the plains continue far beyond the region here considered.
The depressed area between these higher lands, the southern Turkestan depression, is called a graben by Mushketof, at least that part between the Ust-urt and the Kopet Dagh. It seems to have been kept below the surrounding highlands by repeated differential movements, and it has therefore long been receiving their waste. It slopes away from the higher borders after the fashion of piedmont fluviatile plains, of which it seems to be in large part an excellent example. Its surface materials are coarse near the margin, but become finer farther forward. Many of the streams that descend from the mountains wither away on the plains ; only the largest rivers, the Amu and the Syr, succeed in reaching the Aral. The Tejen and the Murg-ab disappear on the southern plains ; the Zerafshan, greatly reduced by use in irrigation in Bokhara, approaches but fails to reach the Amu ; and the Chu wastes away on the plains farther north.
It is the district south of the Ainu with which we are at present concerned. This part of the plains is chiefly a barren waste, the Desert of the Black Sands, the Kara-Kum.
THE QUATERNARY ARALO-CASPIAN IN THE KARA-KUM.
The deposits of the Pliocene Aralo-Caspian are described by some writers as underlying all the Kara-Kum, but there does not seem to be entire agreement on this point. The Quaternary Aralo-Caspian is believed to have been of less extent, but it has not been well defined in Turkestan (cf. Mushketof, 1886, I, 696), probably because of the difficulty of exploration in the desert. Jakalof (1882) speaks of the general belief that the Kara-Kum is the bed of the expanded Aralo-Caspian Sea, but notes that sea shells are not found in the desert. Sjögren (1888) briefly states that the last rise of the Caspian covered the Kara-Kum. Konshin (1896) gives a sketch-map of the sea, showing its area at the beginning and at the middle of the Quaternary period and at the opening of the present or historic period, and thus