262 THE BASIN OF EASTERN PERSIA AND SISTAN. 
the gravel bluff; second, the level of the old outlet; third, the level of the upper 
terrace, although this may be the same as the old outlet ; and fourth, the level of 
the lower terrace and the beach which may be reached to-da}- in time of flood. 
At a little distance from the lake we find evidence of still other changes in the 
behavior of the water-courses. If the le\-el top of the gravel bluff south of the 
lake be followed northwestward, it will be found that it merges smoothly into the 
plain of Kalagak. In the plain the .streams have incised themselves to a depth of 
from 4 to 10 feet, in response apparently to the changes in lake level indicated by 
the little terraces. On the edges of the plain two terraces of quite a different kind 
present strong escarpments heavily capped with gravel. These correspond to the 
uppermost of the terraces of the Heri Rud, as appears by following them to the 
south, where tliey are well displayed. Along the Jam River seven terraces can be 
seen in certain places, but as two of them, near the bottom of the series, do not 
seem to be permanent, we shall consider them as adventitiotis and leave them out 
of account. The third terrace, counting the upper and oldest as the first, corresponds 
to the Kalagak plain and to the top of the bluff south of the lake. The fourth ter- 
race, that is, the one next to the bottom, if it be traced toward the lake, is found to 
coincide with the bottom of the old outlet. A little tributar}- of the Jam is now 
gnawing back into the soft gravel in which the fonner outlet is trenched, and will 
in time cut through the bluff and drain the lake. The fourth and fifth terraces along 
the Jam seem to correspond to the low terraces northwest of the lake and to the 
slight channeling of the Kalagak plain. The terraces along the Jam are strong 
while the others are weak, because the main ri\er was able to continually deepen its 
channel, wliile the lake furnished abut slightly changeable base-level and prevented 
its tributaries from cutting deeply. 
Turning now from a mere statement of facts to a consideration of causes, we 
must first sum up the historj' of the lake of Kogneh and the neighboring rivers. 
Originally an uninteiTupted stream must have flowed from Kalagak to Daniduo, 
where, after passing what is now the site of Kogneh Lake, it joined the Jam River, 
and the combined streams emptied into the Heri Rud. For some reason this whole 
river system was subjected to certain s}'stematic changes by wliich the streams were 
at first induced to dejxjsit abundant gravel and to wander widely from side to side. 
Then other conditions ensued under which the streams acted in exactly the opposite 
fashion and cut deeph- into their beds, carrying away much of the gravel, cutting 
even into the underlying rock and forming high terraces. Just how many such 
alternations took place we are unable to say, but there were at least two before the 
formation of Kogneh Lake. During the third time of gravel deposition and river 
wandering, the large Jam River deposited its load so rapidl}' across the mouth of the 
Kalagak stream that the latter could not keep an outlet clear. Thus a bar was 
formed across the mouth of the Kalagak \'alle)-, and behind this the brook spread 
out into the lake of Kogneh, finding an outlet to the main stream as best it could 
among the gravels of its bigger neighbors. Up to this point the history of the 
region is explicable either on the tectonic or the climatic theory ; from this time 
onward only the climatic theory seems competent to account for all the facts. 
