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More careful observation shows the plain, at first so simple in appearance, to 
be of complex form. Its conical convexity, indicated on the map by radiation of 
distributaries, may be demonstrated by watching a rider who crosses its lines dis- 
appear as does a ship at sea, whereas one passing straight down the slope will 
slowly fade into the heat waves of mirage. Our map shows it to be a character- 
istic subaerial delta with the general outlines of a fan, encroached upon by dunes 
from the north. Except for about 2 miles from its apex, it is everywhere bounded 
by dunes, without which its radius would be over 10 miles, as prolongations of bare 
clay still reach that far into the desert. At present outlying sandhills stand 4 
miles north of the apex, while its greatest width is but 5 miles. Careful study of 
the surface proves it by no means that of an even cone. It is everywhere broken 
with irregularities wrought by man; canals long since abandoned, mounds, and 
roadways, and most significant are its areas of many hundred acres several feet 
above the general surface, a difference caused by man’s control of alluviation, 
concentrating the stream with its depositions into limited areas of cultivation. 
These areas of concentrated deposition are bounded on the lower side with long 
bluffs varying up to 4 feet in height and of irregular plan, as shown on the map 
(fig. 486 and plate 65). 
For more complete explanation of these zones of concentrated deposition we 
may look to its present distribution. Except during exceptional flood, all the 
water of Anau Su is led into a system of canals irrigating fields with low dams 
on their down-slope sides. An irrigated area thus comprises a system of fields 
bounded round the lower side by an irregular, often more or less crescent-shaped, 
dam and merges above into the plain. This dam or dike may be only a foot or two 
in height, but it is always easier to rebuild or patch up the old one than to make 
it in a new place,so that a permanent barrier to deposition may exist for centuries 
on the lower border of irrigation; and since the whole stream is consumed in these 
areas, its depositions are concentrated therein and accumulate in the form of what 
we may term “‘irrigation terraces.’’ All phases of this process may be observed 
in the Turkoman grain-fields of to-day. We thus have an ancient delta surface 
surmounted by irrigation sediments concentrated into terraces near its apex. If 
man, their controlling factor, for any reason abandons them to carry on his agricul- 
ture elsewhere, they show their instability with the first flood; water-gates burst 
and dikes are rent by the stream thus set free to rush over terraces, falling down 
bluffs and gullying back. In the course of a score or so of years this channel will 
be carved to base-level and the excavated terrace material lie spread over the delta 
beyond. 
Pronounced irrigation terraces result only where a fixed area has been con- 
tinuously irrigated for a long time, and so the outlying, more erratically cultivated 
areas, comprising a large portion of the Anau delta, have aggraded in a less differ- 
entiated or more uniform way. The non-observant might cross such a terraced 
plain with never a doubt as to its uniformity. A sloping and often round-worn 
bluff, only 2 or 3 feet high and irregular in course, running perhaps a half mile 
and fading at either end into the plain, does not ordinarily arrest the eye nor 
does such a slight difference of level between two wide adjacent areas. To the 
