National Institute of Informatics - Digital Silk Road Project
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Explorations in Turkestan : Expedition of 1904 : vol.2 |
Vertical Section of Interlapping Loess and Alluvium in 100-foot Cliff of Obu-sinb Canal at Crossing of Road from Samarkand to Kudu Sufi. |
284 | PHYSIOGRAPHY OP CENTRAL-ASIAN DESERTS AND OASES. |
exposing red layers of laminated sandy clay, and doubtless range across the direction of prevalent wind, as there is a constancy of leeward overhanging sides. Everywhere they are associated with heaps of sand derived from the silt, of which all finer material has been drifted away, doubtless to settle as loess in grassy mountain valleys. Anyhow, wherever the finer material is now, it has been totally removed by the wind that excavated the trenches and left their sand constituent behind. Another interesting feature is the frequency of large masses of sand piled on top of these ridges, to occupy spaces of calm in the eddies of windwork.
Proceeding still mountainwards, we soon find these trenches of deflation floored by hard gravel-beds, and in the course of a few miles the silt deposit thins out and dwindles into spits and iso-
lated areas on the gravel-plain , giving grass
Feet M
it a mottled aspect as seen from a 10° :..-...~..~ ,. :, :,..,.;i:.•:i;: Loess
distance—mottled only in shade and
texture, as both are red. This is _ Fine gravel
for in a short distance it is all one ~s. _ :.:..:. •:
vast expanse of gravel or cobbles 69
varying up to 4 or 5 inches in size.
. •~;~ F 1. ' •'
significant changes of conditions suc- I
' • :. Pure yellow loess
ceeding each other—first, a mown- with vertical
.. CleavagO:
tainward recession of alluviation
bringing its zone of fine deposits over its more ancient zone of coarse dc-
posits; second, a dissection of both
preceding zones by the channels now 15 x` <•:: <1.:
occupied, moving alluviation again 10
to a zone farther out than before the 0`~ • 1:~:: `. t :`• f '.: I :'':?::~ ''.':':
first change. It may be that the Canal water level
first resulted from a decrease of Fig. 462.--Vertical Section of Interlapping Loess and Alluvium in
100-foot Cliff of Obu-siob Canal at Crossing of Road from
precipitation corresponding to that Samarkand to Kudu Sufi.
extreme reaction which followed the
glacial period, as evidenced by moraine underlying the glaciers of Pamir. That the second resulted from an increase of grade caused by an uptilting of the margins of Tarim will be shown as we proceed.
Now we are perhaps 25 miles from the great sand, and our abandoned piedmont develops into a bad-land topography, an inclined table-land dissected into
a desert of red mountains rising ever higher above us as we ride slowly up the bottom of a canyon. At first the canyon walls are built entirely of piedmont conglomerates with here and there a layer I to 3 feet thick of silt, and all in slope conforming to that of the plain above. Then towards the bottom of the wall appears a surface beveling the tilted strata of a still more ancient piedmont series,
Loess Gravel
Loess
Fine cross-bedded quartz grit
Fine cross-bedded grit
Laminated silt Grit
Loess
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