Page 20 - A Hand book of Arabia Vol 1 (iii) Ch 1,2

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PHYSICAL FEAT UR HS 11

lower courses they spread out, and would create oases were it not
• that? the mas& of detritus, brought down by their floods, is so soon
left hi"h and dry, to be pulverized by the sun. Good examples are
the wadis of Western Yemen, which often fall some thousands of feet
in less than a hundred miles. The whole Red Sea coastal region,
fr/un Midian round..to the Hadhramaut, is a network of such wadis ;
whereas in the rest of Arabia, except in Oman, this familiar feature
of desert landscape does not often obtrude itself on the traveller s
attention.
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•• Deserts.
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The chief physical matter to note in the peninsula is desert—
* its different varieties and their local distribution.
There are four main varieties, distinguished by their surfaces :
’1. Dahanah : comparatively hard gravelly plain, covered at
intervals with parallel sand-belts of varying width. (This
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precise distinction is not everywhere recognized, desert of
nefud type being often called dahanah and vice versa.)
. ' 2. Nefud (or Nefudh) : a continuous area of deep -sand, forced
bjr wind-pressure into high sand-billows or dunes.
3. Ahqaf: very soft dune country with comparatively narrow
trough-intervals between continuous sand-billows, which are
of considerable height.
4. Harruh : a surface of corrugated and fissured lavas or scoriae,
overlying either plain or mountain. f

Of these varieties, Nos. 1, 2, and 4 are passable, if the tracts be
not more than a few days’ march in breadth, and sufficient water
and forage can be carried from oasis to oasis. No. 3, however,
must lie in very narrow belts, if it is to be crossed at all, the
physical labour involved in the march being excessive, even for
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•• :• camels.
But further qualifications must be noted :
1Dahanah is the kind of desert most likely to hold ground-water
at a depth which can be reached by well-sinkers. On the other
hand, it is very barren at all seasons, and its tracts are the most
extensive deserts in Arabia. Where provision of wells has not been
already made, a broad dahanah may be (as in the central southern
part of the peninsula) a wholly prohibitive barrier, especially if
bordered and interrupted by tracts of nefud or ahqaf.

— .-We/urf varies in passability according to (a) the frequency and
proximity of its sand-billows ; (6) the nature of its sand, whether it
be granite, sandstone, or limestone ; (c) its exposure to winter rain-

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