40 EXPLORATIONS IN TURKESTAN. 
the Aralo-Caspian region is concerned, the expansion of the sea was the cause rather 
than the consequence of local climatic changes. Petrusevitch (1880) ascribes the 
shrinkage of the sea and the withering of the Murg-ab and the Tejen from an 
inferred connection with the Amu, to an assinned destruction of forests in the 
neighboring mountains. The probability of repeated Quate^lar^• expansions of the 
sea does not appear to have been considered ; but in this statement I may be doing 
injustice to Russian obser\'ers, whose more recent articles I have not been able to 
consult. 
THE PIEDMONT PLAINS. 
Since the withdrawal of the Pliocene sea, the eastern and southern borders of 
the plains of southern Turkestan appear to ha\'e been aggraded by the rivers that 
flow out upon them from the mountains. That a certain measure of such constnic- 
tive action has taken place is announced by the Russian geologists, but it is not 
apparent that tlie full measure of ri\er action has been recognized. Some of the 
strata of the plains are .said to be not fluviatile but lacustrine, because they are of 
fine texture and uniform structure, without the variable layers of gravel that are 
by implication supposed to be always indicati\'e of river work ; but this seems to be 
a simpler solution than the problem deserves. There are man}' rivers that do not 
carr\- gravel, and there are many river plains whose smooth surface nuist receive 
ver)" even and unifonn deposits of flood-laid silts o\'er large areas. Records of 
borings are quoted by Walther (1888, 210) which show river nuids on sand and 
loess to a depth of nearly 50 meters beneath the bed of the Amu River at Charjui, 
where the great railroad bridge was built. The record of a well boring at Askhabad, 
quoted by the same author (1900, 105) shows variable piedmont deposits over 2,000 
feet deep. It seems, indeed, as if we had in the plains of Turkestan and the Great 
Plains of our West one of the most striking of the many physiographic resemblances 
between Eurasia and North America ; and that there as well as here an increasing 
share may be given to the action of aggrading rivers in fonning the plains, as 
obseiA-ations are extended. It is well known that the tide of geological opinion in 
this countr\' has in recent years tunied more and more toward a fluviatile origin for 
the strata of the Great Plains that slope eastward from the Rocky Mountains, and 
the traditional lacustrine origin of the plains strata has been repeatedly questioned; 
so we may e.xpect, as closer attention is given to the details of river-laid fonnations, 
that a larger and larger share of the fresh-water strata that slope westward from the 
mountains of Central Asia may be interpreted as fluviatile rather than as lacustrine. 
In one respect, however, the comparison between the two continents reveals a 
contrast. In North .\merica the rivers that flow eastward from the Rocky Moun- 
tains are now dissecting the plains that they once built up, as has been so well 
shoxsTi by Johnson ; while in Turkestan the rivers that emerge from the mountains, 
heavily silt-laden, are still engaged in building up the plains. This is notably the 
case with the Murg-ab and the Tejen, as will be more full\- stated below, for these 
rivers wither away without reaching the sea, and ever>- particle of sand and silt that 
they bring from their headwater valleys in the moimtains must be laid down as 
the}- dwindle to dryness on the plains. Moreover, while the rivers at present bring 
abundant gravels out from the mountains and spread them on the nearer parts of 
