THE GRAVELS OF SISTAN. 29I 
strata would be acquired through oxidization. A succession of ten fluvial and inter- 
fluvial epochs would account for all the observed facts of the clays of Sistan. There 
is therefore reasonable ground for the working hypothesis that Sistan, and pre- 
sumably the Iran basin, has passed through at least ten fluvial epochs during the 
Quaternary era. The number of these epochs is surprising and adds interest to the 
question of their relation to the glacial epochs of other countries. 
THE GRAVELS OF SISTAN. 
Before we can consider the question of the relation of the fluvial period of Persia 
and the glacial period of other lands, it will be necessary- to ascertain the history- of 
Sistan since the volcanic outbreaks which elevated the clays. The first part of this 
history is recorded in gravel deposits, and the later part in terraces. A reference to 
the diagrams of the sections (plate 5) exposed in the lake bluffs shows that at 
the top of each a stratum of gravel lies uncomformably on the cla)s. In sections C 
to F the gravel is greatly developed and reaches a thickness of from 100 to 150 feet. 
These sections are located a few miles south of Kuh-i-Chaku, in a region which 
received the full force of the uplift due to the volcano, but was not covered with la\-a 
like A. Their situation is most advantageous for the preservation of a record of all 
the non-volcanic events since the eruption of Kuh-i-Chaku. 
The eruption took place during a time of gravel deposition, as appears from 
the gravel under the la\-a in section A and the much greater thickness of gravel 
which accumulated nearby outside the lava-covered area. The change from the 
deposition of silt to that of gravel was probably due to an uplift of the borders of 
the basin, whereby the slope of the streams was steepened and opportunity gi\-en 
them to carry away the material which had accumulated upon the graded slopes of 
the mountains a few miles to the west, or in the small basins scattered among them. 
Evidence of such an uplift is found in the stage of dissection of the mountain range 
on the western border of Sistan, and in certain lava sheets. West of Bendan, on 
the road to Neh, a large sheet of dark lava, from one to three hundred feet thick, 
fonns a high mesa, like Kuh-i-Khoja and Kuh-i-Chaku. The lava does not rest upon 
clays, however, but upon a broad, smooth expanse of relatively soft limestone and 
shale, both of which are evenly tnmcated by a surface of erosion. This surface 
could only have been reduced to such smoothness by long erosion at a lower level, 
for the strata vary much in hardness and are highly folded. Most of the mountains 
round abotit are young in appearance, although few of them stand higher than 
the mesa. The peaks are shai^p and well defined, even though some of them con- 
sist of the softer strata. The slopes are naked and steep, and the valleys, which to 
a large extent follow subsequent courses along the softer strata, are narrow, with 
ungraded sides. Rising above the mesa and the lower peaks are a number of large, 
flat-topped mountains, most, if not all, of which are composed of more resistant 
limestone. Apparently at the time of the eruption which fonned the lava-sheet 
capping the mesa, the country- stood lower than now and consisted of hills of 
gentle relief, from among which rose residual mountains of limestone. Such a 
mature country is exactly what might be expected at the end of the long, quiet 
period during which the alternating pink and green clays of Sistan were deposited. 
