Gulf of Suez. In
the western desert, however, those large sand accumulations which are
usually associated with a desert are met with. They occur as lines of
dunes formed of rounded grains of quartz, and lie in the direction of
the prevalent wind, usually being of small breadth as compared with
their length; but in certain areas, such as that lying S.W. and W. of
the oases of Farafra and Dakhla, these lines of dunes, lying parallel
to each other and about half a mile apart, cover immense areas,
rendering them absolutely impassable except in a direction parallel
to the lines themselves. East of the oases of Baharia and Farafra is a
very striking line of these sand dunes; rarely more than 3 miles wide,
it extends almost continuously from Moghara in the north, passing
along the west side of Kharga Oasis to a point near the Nile in the
neighbourhood of Abu Simbel--having thus a length of nearly 550 m. In
the northern part of this desert the dunes lie about N.W.-S.E., but
farther south incline more towards the meridian, becoming at last very
nearly north and south.

_Oases._--In the western desert lie the five large oases of Egypt,
namely, Siwa, Baharia, Farafra, Dakhla and Kharga or Great Oasis,
occupying depressions in the plateau or, in the case of the last
three, large indentations in the face of limestone escarpments which
form the western versant of the Nile valley hills. Their fertility is
due to a plentiful supply of water furnished by a sandstone bed 300 to
500 ft. below the surface, whence the water rises through natural
fissures or artificial boreholes to the surface, and sometimes to
several feet above it. These oases were known and occupied by the
Egyptians as early as 1600 B.C., and Kharga (q.v.) rose to special
importance at the time of the Persian occupation. Here, near the town
of Kharga, the ancient Hebi, is a temple of Ammon built by Darius I.,
and in the same oasis are other ruins of the period of the Ptolemies
and Caesars. The oasis of Siwa (Jupiter Ammon) is about 150 m. S. of
the Mediterranean at the Gulf of Sollum and about 300 m. W. of the
Nile (see SIWA). The other four oases lie parallel to and distant 100
to 150 m. from the Nile, between 25 deg. and 29 deg. N., Baharia being
the most northerly and Kharga the most southerly.

Besides the oases the desert is remarkable for two other valleys. The
first is that of the natron lake