GEOLOGICAL IILSTORY OF NORTHEASTERN PERSIA. 
239 
tains. The mountain tops are smoothly rounded ; the streams, which seem to be in 
process of becoming subsequent, sometimes follow the strike of the strata, but often 
break away, and there are a number of young gorges such as that of the Heri Rud. 
The histor}' of the mountains around the Heri Rud seems to have been similar 
to that of the more western parts of Kopet Dagh. Afteii the Tertiary era was well 
under way, the Cretaceous formation and the older Tertiary- strata were uplifted and 
the softer strata, at least, were highly folded. Erosion then reduced the countn,- to 
the rounded character which the mountain tops now show and to the flat condition 
which is seen in the flat-topped Tertiar>' ridge at Pul-i-Khatun. The waste from 
the mountains formed the vast alluvial or piedmont deposits through which the 
Heri Rud had now cut its way below the ridge. At length another uplift occurred 
corresponding to that of the Meshed fault, though on a much smaller scale, and 
possibly at a diflferent time. The land was raised to approximately its present 
height, and the work of cutting the Pul-i-Khatun gorge and other young valleys 
was inaugurated. Since the completion of that work, or during the latter part of 
its accomplishment, gravel terraces were formed. The topography of to-day consists 
of three portions — the mature topography of the mountains, remaining as an inherit- 
ance from a preceding c}cle, the >oung topography of the gorges, and the mature 
topography of the uplifted piedmont deposits, which is as young as the gorges in 
years, although n^ature in form. 
THE EXTINCT LAKE OF ZORABAD. 
Along the Heri Rud, within the area of Cretaceous limestone south of Pul-i- 
Khatun, lies the basin of Zorabad. On the Persian side of the river it extends 
northwest and southeast parallel to the mountains for a distance of about 20 miles, 
with a width of from 6 to 8 miles. On the Afghan side the limits are not known, 
but they are probably not extensive. This basin formerly contained a lake, as is 
shown b}- an extensive deposit of pure, bluish-white clay or marl, ver)- homogeneous 
and almost unhanded, into which the terraced streams have cut to a depth of at 
least 100 feet. Upon the clay, and sharph- contrasting with it, lies a hard cap of 
from 5 to 20 feet of coarse gravel. Near the Heri Rud, in the deepest portion of 
the basin, both foruiations increase in thickness. The clay, as would naturalh- be 
the case with a lacustrine deposit, preser\-es the same character. The conglomerate 
becomes finer in texture and at last passes into silt. Near the edge of the basin, on 
the south side at least, the clays and 
the overlying gravel are slightly 
warped and have been beveled by 
a grade plain fonned at the begin- 
ning of the cycle, during which the 
present valleys were incised in the 
lake deposits after the draining of 
the lake. Here it is seen that under 
the clays, and apparenth" conformable with them, lies a thick series of sediments, 
consisting of sandstone and conglomerate in the upper portion and the thick shale 
a=shale ; 6=sandstone and conglomerate ; c=lacustrine clay ; 
rf^recent gravel. 
Fig. 155. — Section near Khitayi, on the southern border of the 
Zorabad basin. 
