Examination of the rock exposures at many localities in the
neighbourhood of Cairo bears witness to the fact that the strata of
limestone which were laid down horizontally upon the bed of the sea
are no longer in the position they once occupied, but are now inclined
to the horizontal plane at angles which are easily perceptible. This
is especially noticeable in the two shallow cuttings under the
Great Pyramid, where the strata are inclined 5 degrees to the
south, revealing the reason why the whole Pyramid slope descends so
rapidly in the southward direction. Again, if standing at the base
of the same mighty structure, the gaze be allowed to wander over
the broad expanse of the Nile Valley to the scarp of the Moqattam
hills behind the Citadel, it will be seen that the white limestone
of their lower slopes forms, not a long horizontal wall, but an arch,
sloping strongly both north and south of the Citadel. There has been
obviously change of form, but how has it taken place?
We learn with surprise that an earthquake shock in distant San
Francisco or in the inhospitable regions of Turkestan has recorded
itself through the delicately-poised recorder at Helwan, long before
the dire news of destruction has flashed along the telegraph wire
to the same destination. There is a realization of the instability
of the earth’s crust, in spite of the solidarity of its component
parts, but though the sudden shocks bring home this truth, it is
not so readily grasped that day by day and hour by hour parts of the
earth’s crust are slowly rising and others sinking, offering stern
problems to the dwellers on the shores where these changes are most
markedly taking place. On the eastern shores of England, towns and
forests are being submerged beneath the relentless advance of the sea;
on the coasts of the Scandinavian peninsula, etc., on the other hand,
beaches formerly beneath the sea now stand high above the influence of
its waves, and in Egypt coral-reefs which once grew beneath the waters
of the Red Sea rise in places to over two hundred metres above it in
bold hills or steep-sided terraces. Though possibly of far greater
importance and significance than the sudden convulsions which have
left so deep an impression on the mind of man, these movements are
nevertheless so imperceptible that they arouse little attention.
A second type of deformation is noted where heavy masses of one
rock rest on other and softer materials. The underlying beds are
then often compressed and contorted; clays, for instance, are
drawn out into thinner laminæ, giving rise to shales, while the
massive rocks above either slip on the surface of the lower ones,
if these be impermeable, and so permit of a water-layer forming
along the junction, or else descend by sheer weight, producing a
confused area of mixed materials in front of the still unshattered
cliffs. Examples of this nature abound in Egypt wherever the Eocene
limestones rest on the Cretaceous clays; on the railway from Armant
to Matana the resulting effects are well observed near Shagab. Here
in the main cliff the massive limestones are seen resting on the
soft bluish clays, while in front is a wilderness of low hills in
which limestones and clays are mixed, broken and contorted. But while
these deformations may be irregular and local, there remain greater
pressure-effects which have been regional in character, and by whose
agency the solid rocks have been folded in the most remarkable manner,
strata once horizontal being thrown into arches, or anticlines,
and basin-like curves, or synclines. The importance of these changes
cannot be over-estimated, and some of the marked features of Egyptian
scenery depend directly on these effects. Reference has already been
made to the slope of the Pyramid plateau, but the character of Egypt
in far broader outline depends on the results of these pressures and
the foldings so produced. A glance at the map suffices to show that
many of the salient features in this country present a remarkable
similarity and parallelism. The eastern cliff-wall of Kharga Oasis
preserves a notable parallelism to a portion of the Nile Valley,
though the latter be separated by many kilometres of wild desert
plateau from the oasis; another portion of the Nile Valley also
agrees with the Gulf of Suez in the broad outlines of its trend.
[Illustration: Fig. 1.—Two Anticlines and a Syncline.[3]]
A section taken from Baharia or Kharga Oasis to the shores of the
Red Sea reveals the fact that Egypt proper is bounded on the west
by a low flat arch which has brought the underlying sandstones
nearer to the surface, giving rise to the great oases.[4] These
are mainly present in the sandstone areas, and are in part bounded
by cliff-walls composed in many instances of clays at the base and
limestones at the summit. To the east, on the other hand, rise the
Red Sea hills, the central core of a steeply inclined arch in which
the resistant granites and schists now rise high above the low-lying
sandstone country which flanks them. Between these two arches is
the flat-bedded syncline in which the nummulitic limestone is the
conspicuous member, the strata in central Egypt having in consequence
a half cup-shaped form, of which one half, the northern, may have
disappeared by fracture beneath the waters of the Mediterranean
Sea. This cup-like structure may be due to Egypt having not only
been folded in a north-west and south-east direction but also almost
at right angles, this latter folding giving rise to such remarkable
features as the Wadi Araba, the Qena bend, and possibly affording the
fundamental explanation for the great S-shaped bends of the Nile. In
other and more mountainous regions, such as the Alps and Himalayas,
these solid earth-waves may be under such immense pressure that their
crests begin first to turn over like those of an advancing wave of
the sea, and then may be broken in such a way that mighty masses
of strata are rent asunder, those portions which are uppermost
being thrust for great distances over the underlying beds. These
extreme folds ending in fracture, or overthrusts, are as yet but
little known in Egypt, though Dr. Ball has reported an interesting
case from the neighbourhood of Abu Harba, and some of the phenomena
of dislocation observed near the borders of the Gulf of Suez may
possibly be explained as resulting from movements of this nature.
[Illustration: Fig. 2.—Overfolding of Strata.]
These intense movements become masked under the influence of the
denuding hand of time, the contorted strata may again sink beneath the
sea, new beds are laid down horizontally on the upturned edges of the
older series and the result is the production of an unconformity
between the two members, which differ in age, in inclination,
and in fossil character. Sometimes rolled fragments of the older
stratum are interposed between it and the new overlying beds, further
revealing the activity of denudation before the newer member began
to be deposited.
If fracture by overthrust be unknown in Egypt, another type of
fracture has produced effects of a far-reaching character. In certain
regions of the earth, folding is no longer the conspicuous method
by which the rock-components of its crust are displaced with regard
to one another. In many instances small fissures have been observed
in which the beds on one side have been thrown down to a lower level
than they are on the other. These faults are frequently the result of
great earthquakes, many striking examples of such occurrences having
been noted during the major earth-movements within the last hundred
years. It is probable, indeed, that many of the most important of
these, such as the destructive earthquake of Messina, are due to
further settling of the strata in relation to fracture-lines already
determined. Naturally, this implies that there are certain spaces
formed deep beneath the earth’s surface, which permit of these
efforts to restrengthen the weak spots by filling up the gaps. In the
extreme outskirts of Egypt these fault-lines have produced marvellous
and striking effects, the most conspicuous being the remarkable
depression which, commencing at the Gulf of Aqaba, penetrates far
into the continent of Asia, giving rise to the Dead Sea (many hundred
metres below the level of the Mediterranean), to the Jordan Valley,
to the Lake of Tiberias, and the valley which continues this line
northward. I have had the opportunity of personally studying some
of these fault-lines in the Sinai peninsula, tracing them from an
arched fold in the north, whose sides were being let down by faults
in a series of steps, to a trough-fault, in which the younger strata
are displaced bodily between the older ones. Where undisturbed, the
succession in Sinai shows sandstone lying on granite, and limestone
(containing definite groups of fossils) on sandstone. In the valleys
due to these trough-faults, the granite hills tower 500 metres on each
side, capped by small outlying fragments of the overlying sandstone,
whereas in the valley itself the only rocks visible are limestones
and sandstones often tilted at high angles, and thus revealing the
tremendous displacement to which they have been subjected.
[Illustration: Fig. 3.—Unconformity exhibited in the Fayûm.]
[Illustration: Fig 4.—Fault exhibited by a Coal-seam.]
Considering the earth-movements of Egypt as a whole, the evidence
shows folding to have been the more important type of displacement on
its western side, and intense faulting to have been most conspicuous
in the east, while between these two extremes, the relative importance
of folding and faulting will remain a contested point. The discussion
will only close when the borders of the Nile Valley and the Gulf
of Suez have been mapped with the accuracy and precision of a
Geological Survey in Europe—a pious aspiration, whose realization
can scarcely be hoped for while the broad geological picture is
still being filled in.
Egypt, then, has passed through a long history, of which the
following seems to be the record put in dogmatic form. The ancient
schists and granites which form the central core of its eastern
arch, or anticline, the Red Sea hills, are witnesses to a period
of sedimentation, of volcanic action, and of the influence of
deep-seated molten igneous masses. Step by step these were revealed
by denudation, becoming part of an ancient continent. In addition to
a brief marine invasion during the Carboniferous period, there was a
second advance of the sea in Cretaceous times, which is represented
first by the deposit of such coarse detrital materials as the sands
composing the Nubian sandstones. Then, as the depression increased,
only the finer clayey materials reached the Egyptian region, and
finally the sea covered, if not the whole, at least the greater
part of its area for a lengthened period. With the close of the
Eocene epoch a reverse movement set in, by which the pure white
marine limestones of the Moqattam hills were capped by the sandy
limestones and clays forming the brown-tinted beds of their upper
portion, the whole being, in its turn, covered by the coarse flint
and quartz gravel which containing silicified trees[5] is the chief
desert-former in the immediate neighbourhood of Cairo, as also of
a large area in north-west Egypt. Any gain by the sea since this
period has been comparatively local and limited, and the present
conditions suggest that the land is gaining on the sea rather than
the reverse. The great fold-movements which we have seen to determine
so many important features are relatively of very late date in this
series of events, on their nature and position depending the present
character and visible extent of the different formations.