268 PHYSIOGRAPHY OF CENTRAL-ASIAN DESERTS AND OASES.
and there with low monadnocks and bordered by the Alai and Peter-the-Great Range. But to attain even that stage of maturity in the heart of a great mountain region; to erode what in Pliocene time must have been a region of colossal ranges to its metamorphic cores, must have taken vastly longer than all three succeeding cycles of erosion taken together. The sum total of these later cycles has resulted in no more than an immature dissection of the ancient topography, and, though for our purpose they should be termed erosion cycles, they are by no means comparable to that which closed the Pliocene and should be regarded as mere phases of a Quaternary striving towards base-level. We shall find corresponding phases of Quaternary erosion over other regions and term them cycles for the sake of a tentative correlation.
Fig. 445.—A Bridge over the Third-cycle Terrace in Karategin.
Terraces of the second stage lie about half-way up the valley sides, but are found only at rare intervals, usually where tributaries join, and badly preserved because the third-stage valley-floor has widened to nearly obliterate the transition. But those of the third stage are in remarkable preservation and form the great feature of Karategin, the spacious plains and gentle slopes of its oases. Broadly speaking, it gives a concave sweep to the valley bottom, for the most part 4 miles across and traversed by the present river channel, about half a mile wide, of rectangular section, varying up to nearly 30o feet in depth.
At Damburachi (junction with the Muk Su), this stage widens into a triangular junction-terrace* of over 3o square miles area, traversed by abandoned distributary channels of the Muk Su between 200 and 30o feet above stream. Two
IIIFカラー高解像度
