270 THE BASIN OF EASTERN PERSIA AND SISTAN. 
There are two hypotheses in explanation of the terraces — either the terraces 
are due to warping of the earth's cnist or they are due to the changes of climate 
which in colder regions caused the successive epochs of the glacial period. The 
terraces of each valley, taken by themselves, can be explained on the first of 
these theories as due to warping of the earth's crust. Such warping is essentially 
a local manifestation. The force which produces it may act simidtaucously over 
large areas, but the manner of manifestation is almost sure to varj' in details from 
place to place. Moreover, the force is an internal agency, and its manifestations 
can not be expected to coincide universally with such puny surface features as indi- 
vidual \-alle\s. When we examine the scores of valleys in which terraces have been 
noticed, it appears that the cause of the terraces has acted in just the way that 
tectonic forces can not act. The same phenomena occur everywhere with the same 
details as to the number of terraces, the method of filling and then re-e.\cavating 
the valleys, and the grouping of the successive changes. The incidence of the cause, 
moreover, must be taken to be that of an exterior, not an interior agency, because 
it has so acted as to produce the same effect upon all similar external features, 
whether they be remote from one another or whether they be closely and intricately 
interlocked. The more broadly the terraces are viewed the more unlikely does it 
become that they are the product of warping. 
The theory of climatic changes is of directly the opposite character in these 
respects, and seems to fit all the facts. It is not local, but almost universal in its 
application, since a change of climate in one place implies a corresponding change 
in other places. In a region such as we are discussing the details of climatic change, 
and hence the manifestation of those changes, would be almost identical everywhere. 
In the ne.xt place, climate is external in its origin, and so may be expected to adapt 
itself to the minute details of mountain and valley, and to produce the same effect 
upon all similar parts, whether they be remote or whether they be closely interlocked. 
In addition to these more general reasons for adopting the climatic rather than 
the tectonic theory of the origin of the terraces, there are others of a more specific 
character. At the heads of some of the valleys are old moraines, whose relation to 
the terraces proves that the two forms were in process of constniction at the same 
time. At the lower ends of certain valleys are inclosed lakes whose old shorelines 
show that while the terraces in the suiTounding valleys were being formed the lakes 
were subject to pronounced changes of level. One such lake is so closely connected 
with the terraces of the Heri Rud as to make it almost certain that the changes in 
the lake took place simultaneously with the terracing of the river. Both moraines 
and ancient shorelines are well known to indicate changes of climate. It is highly 
improbable that at the very time when climatic changes were taking place and were 
producing certain sets of fluviatile terraces any other agency should be at work which 
would produce the same type of terraces in almost the same region. Still another 
reason for accepting the climatic theory is that it alone seems competent to explain 
the habitual superposition of coarser deposits upon finer deposits in the filling of the 
valley bottoms. Lastly, the phenomena of Eastern Persia agree exactly with what 
we should theoretically expect to find if the climatic changes of the glacial period 
