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=TECTONICS AND GENERAL GEOLOGY.=
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=Evidence of a former Pluvial Period.=
Given a sufficiently long period for their activity, the denuding
and transporting agencies at work at the present day are capable of
accounting for most of the superficial sculpturing of South-Eastern
Egypt. The country is not absolutely rainless, and within a decade
most of the dry valleys have been for a few hours the beds of
streams, the result of rain storms. There is practically no frost
in this part of the world, so that disintegration by the freezing
of water in crevices of the rock does not occur on any large scale;
the diurnal variations of temperature, are, however, so great that
this cause alone is very potent in breaking up rock material. The
disintegrated matter accumulates as heaps of debris and sand, ready
to be transported towards the Nile or the sea by the streams which
follow the next rainfall. Both in erosion and in the transport of
sand, wind is a very active agent, and accounts for the formation and
distribution of immense quantities of sand. Thus the mountains are
slowly being lowered, and the rocky valleys between them are being
widened and deepened, even at the present day, and the accumulations
of sand on the coast-plain and elsewhere are being slowly increased
in thickness.
But when we look at the great wadis, often hundreds of kilometres
in length, cut to a depth of fifty metres with a width of half a
kilometre through the sandstone plateaux which separate the mountain
ranges from the Nile, it is difficult to conceive that rainfall and
denudation have not in the past been greater than at present. In
our own day, it is but seldom that the great wadis convey streams
as far as the Nile or the sea, their waters being usually absorbed
by the sandy bed before the end is reached; erosion nowadays is
practically confined to the upper reaches of the wadis, and unless we
postulate greater rainfall in the past, inconceivable ages must have
been occupied in the erosion of these great channels. We are thus
driven to believe that what is now a very dry area was formerly one
of considerable rainfall. This belief is supported by the traces of
glaciation in Europe, for it is natural to infer that when temperate
Europe had an arctic climate, northern Africa had a temperate one;
the effect, whatever its cause, being practically equivalent to an
increase of latitude. This change of climate is equally evidenced by
geological observation in other parts of Egypt. It is even likely
that the climate of Egypt may be slowly changing at present; but
the change within the historical period has been so small as to be
practically negligible.
=Origin of the Red Sea.=
If the 200-metre contour of the bottom of the Red Sea, shown on Plate
I, be examined, it will be found to exhibit great indentations
towards the great mountain masses, while there is a curious
projection including the Island of Zeberged which mimics the present
Ras Benas. Some of the indentations of the contour line lie in the
direct prolongation of existing great wadis, such as those of Lahami,
Khoda, Hodein, Di-ib, and Serimtai. The obvious suggestion from this
coincidence is that the sea has encroached on the land since the
drainage-system had substantially its present form, and we infer a
sinking of the region at no very remote geological epoch. The central
parts of the Red Sea attain depths of over 2,000 metres; thus this sea
was a great and deep one even when the level of its waters, relative
to the land, was 200 metres lower than now. We have no information
which would give us a clue to the origin of this primitive sea, but
the inference from the contours is that the _present_ extent of the
Red Sea has been caused by a great general subsidence of the land,
and not by trough-faulting as has hitherto been usually stated.[138]
The subsidence just referred to was even greater than would be
gathered from a consideration of the present coast-line. At intervals
along the entire eastern coast of Egypt are hills of gypsum;
these are never found except close to the present sea-borders,
and the natural deduction is that the gypsum beds were deposited
when the sea was at a higher level than at present. At Ras Benas,
the gypseous strata reach altitudes of nearly 200 metres, so that at
the time when the gypsum was formed the Red Sea must have covered a
much greater area than now, extending in fact approximately to the
contour of 200 metres above present sea-level. As to the epoch when
this greater extent of the sea existed, we should have a clue if we
knew the age of the gypsum beds, which unfortunately is not the case;
but they are almost certainly younger Tertiary beds, possibly Miocene
or even Pliocene, so that in any case the Red Sea is a depression
of considerable antiquity.
=Possible Former Extent of the Eocene Rocks.=
Eocene rocks are entirely absent from the district, and the same is
the case with Cretaceous rocks younger than the Nubian sandstone. But
if we go westward along the parallel of Berenice, across the Nile
into longitude 23° 30′, we come to the plateau face of Gebel Garra,
where there are exposed thicknesses of about ninety metres of Eocene
limestone and 240 metres of Cretaceous marls. These beds cover great
expanses further west and north; they evidently once extended beyond
their present limits, and we may ask whether they ever reached over
the Red Sea mountains here, as is the case in North-Eastern Egypt. To
this question no answer can be given; from Berenice to Gebel Garra is
a distance of over 300 kilometres, and even a very gradual thinning
of the beds eastward would account for their absence from the main
mountains; at the same time the denudation which has removed every
trace of the hard Eocene limestone from the plain between Gebel
Garra and the Nile may well have done the same further east.
=Original Extent of the Nubian Sandstone.=
The Nubian sandstone is found on both sides of the watershed ranges,
and the question raised by its distribution is in regard to its
possible original continuity. Did the Nubian sandstone once extend
over the present igneous mountains, or was it laid down on either side
of a great island ridge? In other words, did the Red Sea mountains
exist as such in Cretaceous times, or have they been subsequently
elevated, and the Nubian sandstones which covered them denuded
away? To this question it may be stated at once that no certain
answer can be given; but a good deal of study has been devoted to
the facts bearing upon it, and these facts will be briefly enumerated
and discussed below.
_Evidence from Volcanic Intrusions._—That the Nubian sandstone
is younger than the igneous rocks in general is proved by the
almost complete absence of intrusions into the sandstone. Apart
from an interbedded diabase sheet in the north part of the region
and a small basic dyke near Gebel Awamtib, the igneous rocks appear
nowhere to penetrate the Nubian beds. The sandstone is _a fortiori_
younger than the metamorphic rocks into which the igneous masses
have been intruded.
_Evidence from Present Distribution._—Coming now to the distribution
of the sandstone, though we find it on both sides of the main ranges,
there is not a trace of sandstone among the higher mountains; this
of course proves nothing, for denudation would be most active among
the peaks and in districts of maximum up-thrust, and sandstone
is an easily erodible rock. The long tongue of sandstone plateau
which terminates in Gebel Anfeib extends indeed right across the
main watershed, but a reference to the orographical map (Plate I)
will show that this extension is along a north-west to south-east
general depression, where the Wadis Hodein and Garara form a cut
across the map separating the main mountain masses into north and
south groups. The presence of the sandstone tongue here would be
equally well accounted for whether the mountains were elevated before
or after the deposition of the Nubian beds.
Not only is the sandstone absent from the main mountains, but
it is never found in such close proximity to them as to render
a decisive answer to our question. Thus the eastern scarp of the
sandstone plateau in the north part of the area overlooks a broad
tract of low country between it and the mountains; and similarly
from Gebel Anfeib one overlooks low country to the north, east, and
south. On the eastern side of the mountains the sandstone deposits
are restricted to small patches considerably removed from the main
summits, and even in most cases from their foot-hills.
_Evidences from Structural Features._—With regard to the structural
evidences to be gathered from the sandstone itself, it was hoped
that the observations of the dip of the beds at different places
would throw light on the question as to whether their deposition
antedated the mountain formation or no.
To the north of latitude 24°, the eastern edge of the sandstone
scarp showed beds differing but little from the horizontal, though
very gentle folding in various directions is probable.
Further south, near Gebel Zergat Naam, much more decided evidences
of folding, and even of dislocation, were met with. The head of the
Wadi el Kreim, south-west of Zergat Naam (_see_ the geological map on
Plate XX) is probably a line of fault, for here on the south-west of
the wadi we have sandstones coming right down to the wadi floor, with
a dip of 30° to the south-west near the edge, becoming flatter the
further we go from the wadi, while on the other side are crushed and
brecciated schists. Going further west, in the sandstone hill-mass
which lies twelve kilometres west of Zergat Naam, the dip of the
beds is in the opposite direction, being 10° to 15° north-east,
and granite appears at the foot of the steep west-south-west slope;
this, with the preceding observation, seems to indicate a synclinal
fold terminated near Zergat Naam by a fault up-thrusting the schists
and syenite.
Where the Wadi Garara cuts through the sandstone hills to receive
the Wadi el Kreim, the beds dip markedly to the south, the observed
inclinations being 60° or more at the north edges, rapidly falling
to 20° or less further south; the north faces show granite and
schists at their base.
Further west, on the way to Gebel Um Harba, the sandstone of the
hills showed dips to the east of 15°. At Gebel Um Harba itself there
are thick beds of sandstone dipping 13° east-north-east, while all
around the mountain one looks out over beds having approximately
the same inclination.
At Gebel Um Khafur, the dip is 13° to 14° to the north-north-east,
and is very constant over a large area. From the north side,
where the plain is 380 metres above sea-level, the hills rise
with a succession of dip slopes and basset-edges over a horizontal
distance of 2·9 kilometres (measured perpendicular to the strike)
to the triangulation beacon at 560 metres above sea, the beds all
along dipping at 13° or 14°. Unless there is step-faulting here
along the strike-wadis (_see_ Fig. 60) the total thickness of Nubian
sandstone here is over 450 metres. This is a much greater thickness
than has been noted anywhere else in Egypt, and I am inclined to
think that there is step-faulting along at least two of the strike
wadis which separate the hill-mass into ridges.
[Illustration: Fig. 60.—Section of Sandstone at Gebel Um Khafur.]
In approaching Gebel Awamtib from the north-west, I crossed over a
small patch of diorite in the sandstone at the pass from Wadi Um
Terbi into Wadi Awamtib, and a basic dyke was found cutting the
sandstone of a spur of Gebel Awamtib. The beds of Awamtib itself
dip pretty uniformly a little north of west.
[Illustration: Fig. 61.—Sketch section of junction of sandstone
with granite, west of Gebel Um Keit.]
About nine kilometres to the south of Gebel Awamtib, a station was
taken on a sandstone headland with schists and quartz veins at its
floor, and afterwards I skirted the limit between sandstone and
granitic rocks on the way to Gebel Um Reit. All along this route
there was no suggestion of sharp folding or faulting; the beds were
nearly horizontal right up to the limit, where the granite hills
rise suddenly (_see_ Fig. 61).
From the top of Gebel Um Reit, which itself is granite, the sandstone
limit could be seen to the north and east; the beds dip 30° in
places; in those on the east the direction of dip is about south. The
region round Um Reit is evidently one of considerable disturbance,
but the sandstone is too far off for one to get any precise idea of
its nature from the summit.
[Illustration: Fig. 62.—Sketch of faulting near Wadi Saalek.]
Going northwards from Um Reit across the Wadi Saalek and up one
of its branches into the head of Wadi Muegil, there is a good
exposure of faulted sandstone overlying schists just before reaching
the pass. Here (_see_ Fig. 62) one of the faults is a distinct
overthrust, with a north-easterly strike, and there are other faults
in a parallel direction.
From the Wadi Muegil northwards past the Galt el Aguz to Gebel Um
Harba there are gentle dips and curvings of the sandstone beds,
with a predominant dip about north-east. At the Galt el Aguz the
sandstone rocks are much tumbled about, but this appears to be due
to fall of over-hanging beds.
At the pass from Wadi Um Arta into the head of Wadi Silsila,
the dip of the sandstone is north-north-east. In its lower part,
Wadi Silsila passes between sandstone ridges the dip of which is
constantly east-north-east.
Round Bir Abraq and the triangulation station on Gebel Abraq
(_see_ large scale map on Plate XV) the main direction of dip is
east-south-east; the pool of Bir Abraq itself is under projecting
slabs of sandstone dipping in this direction. Bir el Sunta, a little
further north, appears to lie in a syncline, the beds of the hills to
the north dipping south-east, while those to the south dip north-west.
Approaching the sandstone from the east by Wadi Hodein, one traverses
granite country with felsite dykes and enters suddenly into high
sandstone plateaux. The beds at the Abu Saafa Springs dip about
5° northwards.
The beds of Gebel Dif and Gebel Anfeib have a prevalent dip on the
west side to the east and north-east, while on the east face the dip
appears to be in the reverse direction. This great mass of sandstone
plateau may thus be a syncline with a north-westerly strike (_i.e._,
a strike roughly parallel to the main direction of the watershed
mountain ranges), but the eastern face was not examined in detail,
and it may be that the whole mass dips to the north-east; some
outlying small sandstone masses near the head of Wadi Edunqul show
no reversal, the dip being still east-north-east. At Bir Dif the
sandstones are variously tilted, and probably faulted.
The sandstones on the east side of the watershed ranges, near Gebel
Ranga, were observed by Dr. Hume to dip towards the sea. In the south
part of the region there are some low hills forming two series of
ridges between the sea and Gebel Kolaiqo, separated by hills of red
aplitic granite; these hills were not actually visited, but were
mapped from some little distance, and the dip of the beds was not
measured, but the nature of the rocks was confirmed by specimens of
sandstone brought back by guides sent to the place.
Summing up the dips and disturbances of the sandstone beds at the
different places, it is clear that the strata are the more disturbed,
the more closely they approach the main mountain ranges; this and
the seaward dip of the beds near El Ranga are so far in favour of
the view that the sandstone may once have extended right over the
present mountains. But it will be seen that the disturbances of
bedding are of an irregular character, especially between Gebels
Zergat Naam and Um Reit, where the folding and faulting are often
in directions quite distinct from that of the main mountain axes,
and it may be that these disturbances are due to later movements
rather than to the elevation of the main mountain ranges. So far as
observations on the sandstone itself go, therefore, the question as
to whether the Nubian sandstones of the Red Sea border ever directly
joined those on the west of the mountains remains open; the observed
disturbances of the beds give us clear proof that considerable earth
movements have taken place since the sandstone was deposited, but
they are inconclusive as to whether these same movements caused the
elevation of the great mountain ranges.
=The Igneous and Metamorphic Rocks.=
Turning now to the main mountain-forming rocks, we find them composed
of the two great classes, igneous and metamorphic. Though in places
we have transition members, such as granites passing into gneiss, yet
on the whole the two groups are quite distinct, and we find typical
igneous masses rising in the midst of equally typical schists. Under
these circumstances there can be no hesitation in considering the
two groups to be of different ages, and that the igneous rocks must
be the younger.
The igneous masses are divisible geologically into the three classes
of (_a_) lavas, (_b_) dykes, and (_c_) plutonic rocks.
_Lavas._—Amongst the igneous rocks, the lavas are those most
poorly represented. Apart from the andesite of Gebel Sufra, and the
diabase sheets in or under the Nubian sandstone, there are only a
few occurrences of volcanic rocks in all the area, a circumstance
which is easily explained by the enormous denudation which has
gone on and the fact that lavas, being superficial out-pourings,
will have been most exposed to denuding forces.
_Dykes._—Dykes seam the schists and igneous rocks, frequently in
such numbers and with such parallelism as to give to the land the
form of a succession of ridges separated by long narrow depressions,
forming “dyke country.” In nature, the dykes vary from extremely
acid rocks like aplites and quartz-felsites, to very basic forms
such as diabase and basalt. There is on the whole a preponderance
of basic over acid types. As already remarked, only one instance
of a dyke cutting the Nubian sandstone has been observed in the
district. Another significant feature in the distribution of dykes
is that while dykes of all kinds are found cutting schists and acid
plutonic igneous rocks, the basic plutonic masses are as a rule free
from dykes; this suggests that the basic plutonic rocks may be on
the whole younger than the acid forms.
_Plutonic Rocks._—The plutonic rocks include granites, syenites,
diorites, gabbros, pyroxenites, amphibolites, and peridotites. But
these are not all of equal importance, and a natural division from the
field observations is a two-fold one into acid and basic groups, which
are tolerably well marked off from each other and are most likely of
different ages. The acid class on this view consists of the granites
and syenites, while the basic class contains all the other rocks
above-mentioned. The granites are on the whole of a very acid type;
syenite is very scarce and nearly always occurs in close association
with granite. Of the basic group, probably the most abundant and
most typical rock is gabbro; diorites and the various ultra-basic
rocks are almost always closely associated with gabbro. Where acid
and basic rocks occur in proximity (as for instance near Bir Abraq,
where an acid granite occurs side by side with a serpentine) there
is a sharp change, without transitional forms.
The much greater abundance of dykes in the plutonic rocks of the acid
group as compared with those of basic composition, as mentioned above,
inclines us to regard the basic group as the younger—a view which
is further supported by the greater frequency with which a tendency
to gneissose structure is noticeable in the acid rocks. The fact that
the basic members are frequently in a higher state of decomposition
than the acid ones is not contrary to this view, being due to the
greater ease with which their constituents undergo weathering; and
as explained on p. 315, the fissured state of the altered peridotes
is probably not due to the same earth forces which have sheared the
granites, but to internal stresses set up by the expansion of the
rocks on serpentinisation.
Though we may be fairly sure that the basic plutonic rocks are
on the whole younger than the acid ones, we have no certain guide
from which to estimate the geological age of either. We know from
their relations to the sandstone that they are both older than the
Upper Cretaceous, but we cannot say how much older; they may be
anything from Archæan to Jurassic; the circumstance that similar
rocks underlie Carboniferous strata in Sinai inclines us to place
them at least as far back as Palæozoic times.
_Gneisses and Schists._—The gneisses and schists which cover so much
of the country are obviously older than the plutonic masses which
are intruded in them, and for these metamorphic masses we need have
small hesitation in speculating on an Archæan age. They exhibit a
wonderful variety of composition. Most of the gneisses are doubtless
sheared ancient igneous rocks, such as granite and diorite, and a
similar origin may be assigned to many of the schists. Other rocks,
as for instance the clay-schists, graphite-schists, and marble,
have almost certainly originated from the metamorphism of ancient
sedimentary rocks. But in a large number of cases we have at present
no clue to the parent rock from which the schists have been formed.
=Summary of Geological History of South-Eastern Egypt.=
Having in the foregoing pages discussed the evidences for the relative
ages of the different classes of rocks and their mutual relations,
we may now endeavour to reconstruct the past geological history of
this part of Egypt from the information gathered. In this process
we shall reverse the order of consideration taken above, and begin
with the oldest rocks.
The schists and gneisses probably represent, not the original crust
of consolidation of the earth, but a complex of ancient sedimentary
and igneous rocks, laid down in pre-Palæozoic times and subsequently
crushed, folded, and faulted into mountains which were subsequently
denuded and worn down. In parts of the main mountain-masses we may
possibly still have the cores of some of these ancient elevations,
but most of the present mountain peaks are formed of a later series
of igneous rocks.
The first igneous intrusions into the schists and gneisses were
the granites which form such peaks as Gebels Nugrus, Faraid, and
Elba. Then followed crushing and folding of these rocks, giving
them often a gneissose structure and opening fissures, up which came
later intrusions, mostly of basic rocks, in the form of dykes. With
the dykes or later came great basic intrusions of gabbroid rocks,
forming such masses as Gebels Dahanib, Gerf, and Meisah.
A long interval now followed of which the rocks contain no record. We
do not know whether the area was submerged or not in Palæozoic and
early Mesozoic times; but if it was, all trace of the deposits of
these ages has vanished in the great denudation which surely took
place before the Upper Cretaceous sea swept over the country and
deposited the Nubian sandstone. We do not know whether the Red Sea
mountains then stood up as islands, or whether they were subsequently
elevated. Nor do we know whether the area remained wholly or partly
submerged during the Eocene period. But we are sure that at some time
between the Upper Cretaceous and Oligocene epochs there was a great
elevation of the land, with folding and faulting, especially in the
areas now occupied by the great mountain masses, and possibly the
mountains had their origin in this movement of elevation. The Red
Sea depression may well have originated in a complementary sinking
at this same period. The depth of the sea is of the same order
(2,000 metres) as the height of the mountains on the adjacent land.
In the succeeding Oligocene period the land was being sculptured
into something like its present form, and probably the main drainage
lines of to-day were then formed.
About the Miocene epoch there was a sinking of the crust in this
region. The Red Sea increased in area, and then, probably as the
results of evaporation in a closed sea, deposits of gypsum were
laid down. A subsequent elevation in Pliocene or post-Pliocene times
raised these deposits along the present shores.
In geologically recent times a further gentle elevation has gone on,
giving rise to slightly raised coral-reefs and sea-beaches. During
the glacial period of Europe, the rainfall in Egypt was probably
greater than at present, and during this period the great wadis
received almost their final sculpturing. After the change from this
rainy climate to the dry one of to-day, erosion still went on, though
more slowly, in the hills; but on the plains and along the coast
accumulation of sands took place, partly owing to wind transport,
and partly owing to the insufficiency of the drainage waters to carry
their load as far as the Nile or sea. The abundance of coral-reefs in
the Red Sea is largely conditioned by the lack of in-flowing streams
of silt-laden water; for the coral animal flourishes only where the
water is clear.
* * * * *
[Footnote 138: SUESS, in _The Face of the Earth_ (English edition,
Vol. I, p. 374), states that “the Red Sea itself is, however,
a trough-subsidence, probably the greatest in the world.”]