254 THE nASIN OF EASTERN PERSIA AND SISTAN. 
THEORIES OF TERRACE FORMATION. 
Tliere are three generally accepted theories of terrace fonnation. ( i ) The best- 
known theory explains terraces as the resnlt of tectonic movements of the earth's 
crnst. The nplift of a given region accelerates the streams and causes them to 
deepen their channels. A period of rest allows the streams to cnt down to grade 
and to broaden their flood-plains. A repetition of this process produces terraces, 
provided the broadening of the flood-plains during each successive period of rest is 
less tlian during the preceding period. For brevity I shall refer to this as the tectonic 
theory- of terrace formation. (2) A second theony- explains a large number of ter- 
races as due to the growth of stream-laid deposits in the valleys downstream from the 
foot of glaciers during successive glacial epochs and the dissection of the deposits 
by the streams during iuterglacial epochs. (3) Other terraces have been explained 
by Davis (d) as the result of the normal swinging of rivers in alluvial deposits 
during a single phase of downcutting. Under this supposition a river cuts laterally 
until it reaches the rock wall of the valley, where it is stopped by the rock. At its 
next swing in the same direction the stream is at a lower level, and, because of the 
narrowing of the valley downward in vertical section, reaches the valley wall before 
swinging so far as before. Thus it is not able to cut so far lateral!)-, and a terrace is 
fonned. (4) In addition to these three theories there is a fourth, which has been but 
little discussed. It has been outlined in the preceding report on Txirkestan, where 
the conclusion is reached that the terraces of that country are due to the alternations 
of climate which occurred during the glacial period in regions where no glaciation 
took place. I shall refer to this as the climatic theorj' of terrace fonnation. 
Of the four theories mentioned, the second and third can not possibly appl\- to 
Eastern Persia, for no trace of glaciation has been detected there, and the terraces 
frequently continue for many miles without approaching the rock walls of their 
valleys. The explanation of the Persian terraces seems to lie either in the first or 
the fourth theory — the tectonic or the climatic. In the following pages the terraces 
of Eastern Persia and the neighboring portion of Transcaspia are described and an 
attempt is made to ascertain which tlieor}- best fits the facts. To avoid confusion I 
shall use the terms "fluvial" and "interfluvial,"' or "lacustral" and " interlacustral," 
when speaking of the climatic equivalent of the glacial period in non-glacial regions. 
These terms must not be understood as bearing any implication as to the cause of 
the glacial period. Either an increase in cold or an increase in precipitation would 
cause the lengthening of the rivers and the expansion of the lakes. Hence the 
climatic equivalent of a glacial epoch is appropriately termed a fluvial epoch when 
we are considering river action, and a lacustral epoch when we are considering 
lake action. 
EXAMPLES OF TERRACES. 
THB NORTHERN SLOPE OF KOPET DAGH. 
In his report on Transcaspia Professor Davis has described the terraces of Kizil 
Arvat, at the western end of the Kopet range, 140 miles northwest of Askhabad, 
and also those of the portion of Kopet Dagh immediately to the west and south 
