The geological features of Egypt as presented to-day are the results
of the formation through varying conditions and the subsequent
deformation of the rocks composing its solid crust; we may next
consider the varied agencies through whose action these are now
undergoing transformation. A comparatively small portion of this
country is undergoing erosion by the sea, and if elevation be still
taking place, there is rather gain from the sea than destruction
by it.
Very different, however, are the meteorological agencies which are
at work fashioning the land as a whole, the effects of wind-blown
sand, rain, and river being of prime importance. Different in these
respects are the Eastern and Western Deserts of Egypt. If the western
border of the river be examined in Southern Egypt, and especially in
Nubia, the sight of huge masses of golden-tinted sand filling every
wind-sheltered hollow might well leave the impression that the vast
desert plain behind was covered by a pall of sand. Closer study
has shown that these wind-swept expanses afford no protection or
resting-place for the finer sands and that consequently their floor
must be formed of the more solid materials which wind cannot carry
before it. If this be realized, no astonishment will be felt that
the Libyan Desert surface is composed of limestone, or of coarse
gravels from between whose larger fragments all the finer sand has
been swept away.
[Illustration: Fig. 5.—Sand-erosion of Sandstone Cliff at Gebel
el Tunb. Wadi Qena, Eastern Desert.]
[Illustration: Fig. 6.—Amphitheatre in Side-valley of Um Leseifa,
north-east of Qena, due to erosion of Limestone by the action of
Temporary Torrents.]
[Facing p. 10.]
However, the wind-borne sand leaves its mark on the limestones,
which in some places are seamed by delicate grooves parallel to
the sand-blast, and in others, where they are softer, have been
sculptured into low hummocks often scattered over immense tracts
of country. The sand itself has a strangely local distribution,
advancing across the desert in lines of enormous length, and usually
trending in an almost meridional direction. The supply does not come
from the south or south-west, as might at first sight be expected,
but from the north, and most of the great dune-systems, which occur
around and beyond the great oases, have their termination in the
southern direction. Even the series of dunes, over 100 metres high,
which prevented the Rohlfs party advancing westward from Dakhla to
Kufra, come to an end further to the south, leaving the wide sandstone
plains bare.[6] In the oasis of Kharga the dominant longitudinal type
is replaced by huge crescentic dunes which, separated by broad spaces
clear of sand, follow one another along a north-south line, and are
not stopped in their onward march even by ridges of considerable
size. The reasons have still to be found for these lines of special
sand aggregation, though when studying the cataract district of Amara,
the initial formation of a dune-system was seen to be determined by
a local depression, which had given sufficient protection for the
formation of a sandy base on which the dune could then be built up.
[Illustration: Denudation Effects in a District of Sedimentary Rocks.
Fig. 7.—Um Leseifa camp (looking west). The harder beds of limestone
form precipitous ledges.]
[Illustration: Denudation Effects in a District of Igneous Rocks.
Fig. 8.—View of Diorite Hills near Gebel Sobeir, Eastern Desert
of Egypt.]
[Facing p. 11.]
Not only are limestones grooved and seamed, but certain areas of
the Western Desert are covered with curious melon-shaped masses,
harder concretionary portions remaining after the softer materials
of the beds have been carried away by wind-action, whereas in the
region to the east of the Nile these concretions are still enclosed
in the softer limestones.
Still more striking is the effect of the wind-blown sand in the
sandstone and granitic regions. Here the complex composition of
the granite has made it a ready victim. The softer felspars and mica
having been worn by the impact, leave the quartz grains loose upon the
surface, and give the rock a “frittered” appearance. Holes have
been formed in the windward side of the blocks, and the sand contained
in them clearly shows the agency to which they owe their origin.
Near the eastern edge of the Western Desert the effects of
water-action become more conspicuous, on the borders of the Fayûm
terrace-formation being a marked feature, and there is a transition
to the dominant characteristics which mark the limestone country
eastward of the Nile. Here, deep and intricate valley-systems have
been cut out from a plateau which a few kilometres from the Nile
is as flat as the great western plains. One of the best known
examples is the Wadi Hof, near Helwan, with its ramifications,
terminating in steep cliff-faces having all the appearances of “dry
waterfalls.” Any doubt as to the active agent in their production
is set at rest by an examination of the excellent photographs taken
by various observers during the great storms which almost annually
burst over this region. (The lecture was illustrated by a series of
slides showing Wadi Hof in flood, and the cascades descending the
“waterfalls” which terminate its side-channels, these being most
kindly lent to the writer by Herr Züst, of the Electrical Service,
Ministry of Public Works). The great annual and diurnal temperature
variations (over 50° C.) aid in the work of denudation by preparing
an immense amount of broken material through the contraction and
expansion which they produce. As every material expands and contracts
according as it is heated or cooled, so the different component
parts of the rocks composing the earth’s crust are in constant
movement with regard to one another, and the less homogeneous they
are the greater the effect in breaking them up into small masses or
particles. These loosened fragments which cover the surface of the
desert are thus ready to be swept away by the rain-waters, and as we
have already seen, it is owing to these effects, superadded to more
subtle changes next to be considered, that the old volcanic rocks
of Abyssinia yield the rich silt or mud of the Nile Valley. It would
be difficult to estimate the rapidity with which these wild ravines
are being deepened by any comparison with water-wearing effects in
Europe. Any beds of soft sands and clays are rapidly dissected by
the torrent waters, a feature which readily explains the absence
of conspicuous hills in the Eastern Desert east of Esna, where the
Cretaceous clays form the dominant constituent in the geological
structure of the country. Whatever the effects of sand-erosion in the
Western Desert or rain-erosion in the hills and on the plateaus of
the Eastern Desert of Egypt, they come relatively but little under
the notice of the dweller on the Nile, to whom the river-erosion
and the reformation of new materials become of primary importance.
Even the powerful agency of frost cannot be entirely dismissed from
consideration in Egypt. Owing to the expansion of water when converted
into ice, the rocks in whose cracks the water has collected are split
asunder, and as we have recently noted, temperatures lower than 2°
C. have been recorded in Cairo during the present winter (1910). On
the great desert plateau which extends from Kharga Oasis to the Nile
Valley, temperatures of 24° F. and 30° F. were also observed, and
in the Red Sea hills and Sinai frost must be of common occurrence,
as one mountain in the latter peninsula was ascended in snow, and
the higher peaks are frequently covered in a white pall.
The nature of a river system need not be dealt with in much detail
here, as I have already discussed this subject in “Survey Notes”
for April, 1907, under the title of “River Characteristics as
illustrated by the Nile.” It may be well, however, to recall that a
normal river passes through three distinct phases of activity. In its
mountain tract (for most large rivers arise in the higher altitudes)
there is maximum erosion and backward growth of the river system. In
its central portion, or valley tract, the stream is acting as a
transporter of eroded material, and such erosion as there is, is
downward rather than sideward. Finally, the plain tract is the region
of deposition of the materials so carried, erosion being lateral,
and the growth of the stream bed forward in the form of a fan-shaped
delta where the transported sands and clays enter the sea.
But this general succession may be further complicated by
circumstances depending upon the geological conditions. In Egypt and
the Sudan the Nile passes from areas where it flows peacefully and
quietly, usually of considerable breadth and bounded by fertile lands,
to others in which it is restricted, dashing down steep slopes in
rapids and cataracts. A geological examination has shown that in
the first case the river is flowing over and between sedimentary
homogeneous rocks, such as the limestones and sandstones, while in
the second instance it has entered regions composed of heterogeneous
igneous and metamorphic rocks, such as the granites, gneisses and
schists. The production of these rapids is due to the combination of
steep slope and the difference between harder and softer materials,
the rapidly-moving waters wearing away those more easily denuded,
while the compact members remain as obstacles to their advance,
and are only slowly worn away along joint-planes and other lines of
weakness. In the Third Cataract, hard bars of granite rising through
softer gneiss at right angles to the river course have produced the
main rapids; elsewhere, as at the Bab el Kebir, near Wadi Halfa, the
river has taken advantage of a thin dyke of soft rock traversing an
extremely hard diorite, so that the stream has worn a narrow gully
between steep rock-walls, where the intensity of the rush of water
is greatly exaggerated owing to its being restrained and fettered
by the narrowness of the passage. In some cases the same result has
been produced owing to the existence of a line of fracture, or fault,
across the stream, the waters taking advantage of this line of least
resistance. The general erosion in these rapids is accompanied
by great local effects where eddies and whirlpools are produced,
and the sand and rocky fragments act as abrading agents. Pot-holes
are formed in the solid rock, and rapidly deepened by the intense
effects of this nature produced during times of flood, the result
being splendidly illustrated in some of the smaller islands of the
First Cataract at Aswan.
A river is, in fact, the main agent combining the effects of
transformation and reformation, new strata being produced in its
plain tract as the result of the eroding activities in its upper
reaches. Much of the detrital material is also carried seaward to
form deposits of marine sands and muds along the shore-lines of the
continents, these themselves becoming, should subsequent differential
movement of land and sea take place, the sandstones and clays of
future continental areas.
But there are other agencies at work as transformers on and within
the earth’s crust. There are in most rocks a series of divisional
planes, which may be either vertical or inclined, and to which
the name of joints has been given. These may arise from various
causes. Both in sedimentary and igneous rocks they are in part
due to contraction during consolidation—in the former when they
lose their contained water, in the latter when they solidify from a
molten condition. Joints may also be called into being by the effects
of internal pressures and movements within the earth’s crust,
such structures having been experimentally reproduced by Daubrée
in materials under stress by torsion and by simple pressure. The
granite of Aswan displays such jointing to a marked degree, giving
rise to remarkable hills composed of huge boulders of granite piled
on one another.