PERSIA AS AN EXAMPLE OF AN ARID COUNTRY. 249 
order of superposition, namely, silts or other fine materials at the bottom, then gravel, 
and lastly wind-blown sand on top. It is probable that this order of superposition 
represents the ordinary- sequence of events in a countrj- where basin-making and 
desiccation are both in progress. 
The aqueous series of basin deposits. — The general appearance of the basin 
deposits is well described by Blanford («, pp. 495-496) : 
In the smaller plains, and in the larger deserts at a short distance from their margins, the sur- 
face usually consists of very fine, pale-colored rather sandy cart^h, which, although barren in gen- 
eral, is fertile wherever irrigation is practised, unless, as is not unfrequently the case, it is strongly 
impregnated with salts. . . . The margins of the desert plains . . . usually consist of a long 
slope covered with gravel and bowlders, and with a surface inclination of from one to three degrees. 
Such slopes often extend for a distance of from 5 to 10 miles from the base of the hills bounding 
the plains, the difference in level between the top and the bottom of the incline being frequently from 
1,000 to 2,000 feet or even more. What proportion of this depth consists of detritus it is impossible 
to say, but depth of the deposit must be great, because hills of solid rock but rarely emerge from it. 
The greater part of such slopes consists of sand and pebbles, the latter more or less angular and 
mixed with large blocks, all derived from the adjacent hills. . . . Fragments 2 or 3 feet in 
diameter are not uncommon, even at a distance of a mile or two from the base of the hills; but I 
only observed them near places where small streams issue from the higher ranges. At such spots 
the gravel deposits are naturally very often raised into a fan-shaped slope. Such a phenomenon is 
common enough in all countries, and so are strong slopes at the base of steep hills; but the peculiar- 
ity of these slopes in Persia consists in their great breadth, and in the enormous mass of detrital 
deposits which they contain. 
From many of the desert plains of Persia valleys of great width extend far into the more hilly 
regions. These valleys have, along their sides, precisely such long slopes of gravel as I have jost 
described. The presence of a s.tream in the midst of the valley is by no means constant ; but occa- 
sionally small rivulets coming from the sides run for miles along the slopes without descending to 
the bottom of the valley, and are finally absorbed by the soil, if not exhausted by being diverted for 
irrigation. 
Even at great ele\-ations (p. 497) up to 9,000 feet, similar immense accumula- 
tions of loose material occur in many places, and the higher peaks and ranges rise 
out of them. Among certain of the higher mountains, for example, near Shiraz 
(p. 498), where the rainfall is greater than usual and the streams flow perennially, 
the \-alleys present quite a different appearance, being flat-floored and havino- no 
gravel slopes along the sides. The reason for this peculiarit}-, as Blanford infers, 
is that the perennial streams are able to carr>' away the waste that is brought into 
the valley, whereas, if the streams come to an end at the base of the hills, it is 
inevitable that the detritus which they carr\- should be deposited at once and fans 
should be built up indefinitely. 
Blanford describes many fine examples of superficial deposits, especially of 
gravel slopes, and there are numerous others which might be mentioned. A small 
but typical example is the basin of Tabas, 60 miles east of Birjand, among the 
mountains, at an elevation of about 4,000 feet. The center of the plain, which is 
about 15 miles in diameter, is composed of the finest silt, and as water is relatively 
abundant, most of it is cultivated. Outside the cultivated area is a broad rim of 
fine gravel, difficult of cultivation, but very useful to the traveler, as we found 
during the melting of one of the occasional winter snows. The center of the plain 
was so mudd>- and slipper)- that it was utterly impassable for camels, which are 
the most helpless of beasts of burden when removed from their proper en\-iron- 
