The Physical Setting
of the wind-driven sand is limited to within a few feet
of the surface of the plain. Within this belt the rock
surfaces are cleaned while above it they preserve their
surface discolouration. The action of the wind is often
particularly well displayed where there is a pass between
two hills. The surfaces of hard rocks eroded by the wind
are usually remarkably polished. They may be smoothed
so that they appear as if covered by a film of varnish.
Rainstorms among desert hills certainly produce very
considerable effects. The falls are rare but when they
occur they may be torrential and the ground has no
covering of vegetation to form any protection. The
consequence is that these streams are very powerful and
sweep all before them. The results of water action are
often remarkably conspicuous in the desert. Care must
be taken not to attribute to desert conditions many of
the rock forms displayed there. The desert has not
always been desert and wider study suggests that some
of the hill forms have arisen tinder more humid conditions.
In fact, apart from the action of the wind and streams,
the desert is a region of great stability of form. The
sugar loaf type of hill as typified by Jebel Kassala is
the result of the slow processes of denudation acting on
rocks of different hardness. Jebel Kassala stands nearly
3,000 ft. above the surrounding plain and this means that
denudation has removed that amount more from the less
resistant rocks around it. A sugar-loaf form with &
fringe of boulders at the base seems to have arisen under
fairly humid conditions, as in the southern parts of the
Sudaa to-day where, at Loka, Gumbiri, a hill of similar
form can be seeai clothed in vegetation with a good deal of
soil among the boulders. The soil holds the moisture
against the base of the h21 and this decomposes and m
eaten into faster than the freely exposed upper parte.
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