国立情報学研究所 - ディジタル・シルクロード・プロジェクト
『東洋文庫所蔵』貴重書デジタルアーカイブ

clay sediment to form the takyrs (adobe flats). The area occupied by the delta-
oasis, or living delta, is proportional to the amount of water available for keeping
the dunes in check. It is evident that during a continued decrease in the annual
supply of water the delta will die at its borders and the dunes advance, diminishing
the area of the oasis. After that, only a long-continued increase in water-supply
could recover for the oasis its lost area by burying the bordering dunes in takyr
and thus extending the foot of the living delta.
The continued process of aggrading on the three zones of the delta is, therefore,
of a very complex nature and dependent on varying factors; at the apex, there
remains the greater part of the coarsest material—boulders, cobble, gravel, and
coarse sand; the middle zone receives in overflow much of the finer silts; while the
rest of the finest silts accumulate on the lowest slopes as far as the dune-barrier;
and here, too, as well as beyond in the dune-locked depressions, are deposited the
coarse and fine sediments rolled along its bottom or carried in suspension by the
stream.
These are the contribution from the mountains. Parallel with this is the
contribution from the desert on the north. As we have seen, a part of the
sand from the desert is distributed and assimilated by the living delta. Besides
this the desert whirlwinds come laden with fine dust, and where this falls on the
delta it remains caught in the vegetation, and it, too, enters as a loess-constituent
into the delta structure. It is, therefore, clear that the zone of deposition and
depression along the base of the mountains is being continually loaded, not only
with the products of current destruction of the neighboring mountain masses,
but also with materials brought from the desert. And while much of this came
originally from the mountains near at hand, much must also have originated
in the distant highlands of Afghanistan and the Pamirs.* Thus it is probable
that the load on part of the zone of depression may be greater than that removed
from the neighboring mountain masses.
We may extend this analysis still further, to a picture of the internal consti-
tution and structure of the zone of depression as a whole. Ever since Central
Asia became a "central region," without efflux of its waters to the ocean, it has
necessarily been an area of desert wastes with a climate the aridity of which was
tempered probably only during periods of glacial expansion. Of equally long
duration has been the activity of the processes I have outlined. Deltas have
continually grown up at the mouths of streams emerging from the mountains, and
where the streams were separated by short intervals their deltas merged together;
where the intervals were greater the sands, as they were stopped in their onward
course by the mountain barrier, piled up in hills which rose high above the interven-
ing living deltas.