IIIF Color HighRes
National Institute of Informatics - Digital Silk Road Project
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| IIIF Color HighRes |
| Southern Tibet : vol.7 |
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Citation Information
588 RECAPITULATION OF THE OROGRAPHY AND MORPHOLOGY OF TIBET.
went on, debris and detritus were washed down by temporary and occasional rains and melting snows, as well as by winds, into the lowest parts of the depressions.
The relative altitudes of the mountains decreased, the wild accentuated forms were
smoothed and rounded at the same time as the valleys, and the basins became broader, shallower and more even. The steepness of the gradients all over the country
diminished gradually and coincidentally with the decreasing differences in altitude
between the mountains and the depressions. The enormous beds of subaerial deposits in the region of the Upper Satlej prove that an epoch of great aridity prevailed
before a period of moist climate with heavy precipitation again set in. The latter
was followed by a new period of aridity which still goes on , though the present area of self-contained basins is less extensive than that of the earlier arid epoch.
On map LXXXIX the present extension of the self-contained area of Tibet is marked with a thick, blue line, and the areas of the basins mentioned in chapter LV are indicated with thin blue lines.
On the accompanying figures (p. 5 8 9) I have sketched four successive stages in the course of development which in earlier as well as in the present period of aridity
is going on in Tibet. The depressions become gradually filled with blocks, gravel,
sand, dust and material in the finest state of division. By the increasing deposits the mountain-sides become more and more hidden, and only occasionally solid rock
crops up from the beds of debris. Simultaneously, weathering and denudation con-
tinue on the still projecting crests. By this levelling activity the folding land is transformed into a plateau-land, the even plains of which are interrupted by rounded
ranges. In some cases no doubt comparatively low ranges have already become completely embedded in the deposits. In other cases low and rather short ranges still raise their rounded crests above the surrounding plateau-land. But of course there are here and there in the interior, and more particularly near the edges of the plateau-land, magnificent mountains which still bid defiance to the destructive activity of weathering and denudation.
On Pl. XC I have drawn a profile of my route from Camp LXXXV on the Bogtsang-tsangpo to the Tsangpo below Camp CXXV. The vertical scale is nearly
I o times greater than the horizontal scale. The single object of this profile is to
show the great morphological change which takes place in the relief and sculpture of the highlands as soon as we pass from the interior, self-contained parts of Tibet
to the peripheric region with an outflow to the sea. On our profile this boundary is situated in the Sela-la. To the north of this water-parting Transhimalaya pass we meet softer forms with slowly rising gradients, and basins filled with deposits. To the south of Sela-la, on the other hand, we find an accentuated relief and deep-cut valleys where the still comparatively abundantly running water washes away all fine material, and where thus no mighty deposits may be formed.
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