THE AKHAL-TEKIN OASES. 41 
the plains, the ri\ers of late Tertiar>' time must have had a much smaller quantity' 
of coarse detritus ; for, at that epoch, the mountains had been reduced to relatively 
low relief, as will be shown particularly in Mr. Huntington's report, and the 
waste that they then shed must have been for the most part of fine te.xture. It 
appears, therefore, that a ver}' careful examination of the fresh-water Tertiary- and 
Quaternary .strata in the plains of Turkestan should be made with a view of deter- 
mining not only the date, but also the physical conditions of their deposition. It is 
evident that the opportunity for organic life, and especially for human life, would 
have been very different, according as the plains are of lacustrine or flu\iatile origin. 
Inasmuch as man}- mounds and ruins occur within the area of debatable action, the 
solution of this problem has a close relation to the objects of our expedition. 
THE .\KHAL-TEKIX OASES. 
The gently sloping plain that lies piedmont to the Kopet Dagh and the asso- 
ciated ranges — the mountains that divide Persia and Turkestan — is a case in point 
The plain here receives from point to point sufficient water from the mountains to 
support a series of \-illages, known as the Akhal-tekin oases. The Central Asiatic 
railroad, from Krasno\-odsk to Tashkent, naturally was constructed through this settled 
belt on the way into the interior. A section of the mountains at Kizil Ar\-at is given 
by Bogdanovitch (1887), in which the north-dipping ^Miocene limestones and clays 
on the crests and flanks are followed b)- the horizontal layers of the plains, which 
are labeled Aralo-Caspian. Again, Konshin's sketch map (1896) of the Quaternary- 
sea brings its border close to the mountain base at Kizil-Ar\-at. He had earlier 
(1883, 383) reported the occurrence of variegated clays in the gorges back of Kizil- 
Ar\-at, which he referred to the Pliocene Aralo-Caspian. 
During our brief stop at Kizil- Ars-at we rode out to the mountains and had a 
good view of their structure. The mountain-making rocks are hea\-y- limestones, 
underlaid and overlaid with clays or shales, all compressed into great folds, and much 
denuded. We looked from one of the anticlinal limestone ridges into an inner syn- 
clinal valley, where the weak clays that overlie the limestones were terraced. Tlie 
stream from this inner \alley cuts a narrow gorge near the end of the west-pitching 
limestone anticline, but the road follows a valley around the west end of the anti- 
cline. Where the stream issues from the mountains it has terraced the reddish 
and yellowish clays on the northern slope at several levels, and has strewn gravels 
on the terrace floors. The terraces decrease in height northward, as if they would 
merge in the plain, but the front of the higher terrace has been much consumed 
and eroded into a sort of bad-land topography-, shown in fig. 22, during the produc- 
tion of the lower terraces. This suggests a recent uplift, with its greatest measure 
in the range and rapidly decreasing toward the plains. The spurs of the mountain 
ridges hereabout seem to ha^•e been graded to moderate slopes with reference 
to the uppennost terrace, while narrow ravines and gulches are cut in the 
mountain flanks with respect to the present valley floors. Evidently- a much 
longer time must have been de\-oted to the erosion of the highest terrace floor, 
which once extended continuously along the mountain base, than in opening the 
