QUATERNARY CLIMATIC CHANGES OF EASTERN PERSIA. 2/3 
THE NATURE AND THE METHOD OF ACTION OF THE QUATERNARY CLIMATIC 
CHANGES OF EASTERN PERSIA. 
If we admit that the terraces of Eastern Persia are due to climatic changes, we 
are at once confronted by the question of the nature of the changes and the method 
of their action. It seems reasonable to suppose that the nature of the changes was 
the same as that of the changes which took place in glaciated countries at the same 
time, although differing in degree. Till recently it has generally been a.ssumed 
that the glacial period was characterized by increased precipitation accompanied by 
greater cold. Penck and Bruckner have shown, however, that certain phenomena 
in the Alps can only be explained on the supposition that the precipitation remained 
nearly constant, while the degree of cold increased and evaporation therefor dimin- 
ished to such an extent that glaciers and inclosed lakes expanded greatly. Our 
knowledge of Persia is too slight to justify any conclusion as to whether the climate 
of the flu\ial and lacustral epochs was characterized chieflj- by greater cold or by 
greater precipitation. The question can not be wholly neglected in this report, 
however, for if, as seems probable, the last of the fluvial epochs occurred since the 
occupation of the countrv- by man, the character of the change must have had an 
important bearing on human development. 
A little light may be shed on the question by considering the conditions which 
must have prevailed during the formation of the terraces. The most important and 
universal condition for the production of climatic terraces seems to be that during 
an interfluvial epoch like the present the slopes of the mountains shall be ungraded, 
and during a fluvial epoch graded. A general view of Western Asia from Chinese 
Turkestan to Turkey shows that terraces are well developed among young mountains 
such as the Tian Shan range in Turkestan and the eastern part of the Taurus range 
in Turkey, where ungraded slopes are the rule. The)- are also numerous among 
mature mountains, provided the region is so arid that ungraded slopes are charac- 
teristic of maturity. Such a condition, as we have seen, is well illustrated in Eastern 
Persia. Among mountains which have reached the stage of maturity, and are not 
so arid as to remain ungraded, on the contrar\-, terraces are poorl)- developed, as is 
shown among many of the lower mountains of Turkey and to a less extent of 
Turkestan. The cause of the prevalence of terraces in regions where the slopes are 
to-da}- ungraded seems to be that in such regions a change of climate is able to 
produce marked effects upon the character of the slopes, either by causing more 
rapid weathering or by causing the slopes to assume a graded condition. 
.\nother condition of terrace formation is that terraces do not occur to any great 
extent in regions of deposition such as fans. When found upon fans they almost 
always soon die out downstream, showing that they owe their origin to impulses 
derived from farther upstream among the mountains. Accordingly, in considering 
the process of terrace-making we may confine our attention to the mountains and to 
those parts of the mountains where erosion is actively at work upon ungraded slopes. 
It is difficult to estimate the effect which an increased degree of cold with 
unchanged precipitation would produce upon ungraded slopes ; for among the moun- 
tains as tlie\' stand to-day the colder, more elevated portions are also subject to 
