* * * * *
=GEOLOGY.—SEDIMENTARY ROCKS.=
* * * * *
Only a small proportion of South-Eastern Egypt is covered by
sedimentary rocks. As will be seen from a glance at the geological
map on Plate XX, sedimentary strata are to be found only in the
north-western and central portions of the area, where the Nubian
sandstone which covers so much of the country close east of the
Nile terminates in a long south-easterly projection, and along
certain portions of the coast, where small outliers of Nubian
sandstone and patches and strips of gypseous limestones and coral
reef occur. The oldest rocks of undoubted sedimentary origin, the
Nubian sandstones, are of Cretaceous age. Tertiary beds are absent,
except for the gypseous strata of Ras Benas, which may possibly be
Miocene. Quaternary deposits are represented by Pleistocene and
Recent coral reefs and beach deposits, and by accumulations of
alluvial downwash, blown sand, and calcareous tufa.
The sedimentary deposits may thus be classified into:—
(1) _Recent Deposits_ (coral reefs and raised beaches, blown sand,
alluvial deposits, and calcareous tufa).
(2) _Gypsum and Gypseous Limestones_ (of uncertain age, possibly
Miocene).
(3) _Nubian Sandstone_ (Cretaceous).
=Coral Reefs and Raised Beaches.=
Coral reefs fringe almost the entire coast-line of South-Eastern
Egypt, rendering the coast one of the foulest in the world for
shipping. Coral forms the islands of Wadi Gemal, Gulhan, Mukawar,
Mirear, Seyal, and Halaib, these rising to a few metres above the
sea; but in general it is not met with on the main land except close
to the shore. The reefs may extend, slightly raised, some distance
inland, but cannot be seen owing to the sand and downwash from the
mountains which covers the sloping coast-plain. At low water great
expanses of nearly level coral reef, stretching out for a kilometre
or more from the shore, are exposed in places, especially near Ras
Benas. Frequently a line of reefs can be seen running for many miles
parallel to the coast at a distance of a kilometre or more from it;
these outer reefs can be traced in windy weather by the breaking
of waves on them, and in calm weather by the lighter colour of the
water over them. There is frequently deep water between the outer
reefs and the shore. Openings in the outer reefs, through which boats
can pass into the sheltered water within, are called _mersas_ by the
Arab boatmen; they frequently occur opposite the mouths of the larger
wadis. Coral reef forms a tough, hard, and very porous limestone, with
an extremely rough surface on which the structure of the astraean and
other corals composing it can be clearly seen. It has been used for
building the jetty at Halaib, and rough blocks of it were anciently
employed for building at Berenice, as well as in the tombs at Suakin
el Qadim; but its porosity and intractability would never recommend
it as a building stone if any other could be found locally.
In close association with coral reefs are beach deposits of
Pleistocene age. These occur round the hills near the tip and on the
north side of the peninsula of Ras Benas, where they cover the slopes
of the gypseous limestone hills. Low hills and banks of very white
aspect occur near the sea further north; these are probably also
raised beaches, though they have not been visited and may possibly
be gypsum. Near the tip of Ras Benas these beach deposits are gritty
limestones with abundant casts of lamellibranchs and gasteropods,
mostly in bad preservation, as well as various echinoids, among
which M. Fourtau has identified the species _Brissus carinatus_
as one of the most abundant.
=Blown Sand.=
The dune-forming sand in South-Eastern Egypt is exactly similar to
that which covers such large areas in the Libyan Desert. It consists
of well-rounded quartz grains, averaging about a millimetre in
diameter; the grains are usually of a golden-yellow colour, owing
to their being coated with a thin superficial film of iron oxide.
The only area where sand dunes are of any considerable magnitude
or extent in South-Eastern Egypt is the tract called I Hubâl
(lit. “the sand dunes”) which extends from Wadi Meisah to
Wadi Aideib, fronting the hills of Ti Keferiai, Geror, Balatitda,
Sul Hamid, and El Sela. The north faces of all these hills and the
smaller hills north of them are swathed in thickly accumulated sand
to such an extent as to make travelling over them difficult. The
reason for the heavy sand accumulation at this particular place is
that the north-west wind, which is the prevalent wind over Egypt
proper, here meets with south-easterly winds coming up the Red Sea,
causing a local calm and a consequent dropping of the sand borne
from the Nubian sandstone areas by the north-west wind. Thus the
same climatic conditions which give rise to the rich vegetation on
the Elba mountains cause at the same time extreme desert conditions
in the tract immediately north-west of them.
Sand accumulations of less magnitude occur in and about the lower
reaches of the Wadi Hodein, in the great valley to the north of
Berenice, on the plain west of Gebel Um Harba, and at some few
other places, but they are seldom so extensive as to cause a serious
hindrance to travel.
Besides transporting the ordinary dune-forming sand, the action of
the wind is responsible for a considerable amount of redistribution
of the coarser granitic and other sand produced by the disintegration
of igneous rocks. In high winds this coarse sand, in angular grains
as large as peas, is blown about on the coast-plain to considerable
distances from the places where it was originally deposited by
water transport.
=Alluvial Deposits.=
The degradation of the mountain and hill masses which is constantly
going on under sub-aerial agencies (disintegration by variations of
temperature, and transport by water during occasional rain storms)
has in the course of centuries resulted in immense accumulations of
alluvial material (sand and gravel) along all the drainage lines and
on the plains. Disintegration is in many places greatly facilitated
by the crushed nature of the rocks, due to the tectonic movements
which they have suffered during the process of folding and mountain
formation. This fact is specially evident in the case of the more
basic rocks. Serpentines, for instance, are frequently so crushed
and cracked that one may search in vain over a whole mountain for a
place where the rock is sufficiently free from cracks to allow of a
hand-specimen of the ordinary size being extracted without breaking
up into fragments under the hammer. In the more acid rocks, like the
granites, crushing has been a less powerful aid to disintegration,
and here the diurnal temperature-range has been the principal factor;
the rock, being an aggregate of crystals of different materials with
different coefficients of expansion, has been easily broken up at its
surface by the constant differential motion of its component grains,
resulting in the formation of coarse granitic sand. Chemical action in
denudation is of quite subordinate importance. Though some chemical
action has taken place, as for instance in the kaolinisation of the
felspars, we find as a rule that the alluvial accumulations consist
principally of unaltered rock fragments.
In material, the alluvial accumulations are of the same varied
nature as the mountains from whose degradation they have originated,
ranging from very basic mineral substances, such as serpentine,
to the most acid, such as quartz. The relative proportions of
the different materials in the alluvial deposits do not, however,
correspond as a rule with the proportions in which they enter into the
mountain masses. Not only are heterogeneous rocks, such as granites
and diorites, far more rapidly disintegrated by temperature-changes
than are the more homogeneous rocks such as felsites and serpentines,
but the fragments produced by the disintegration of granitic rocks are
rounder and of much smaller size than the fragments resulting from
the breaking up of felsites, schists, and serpentines. The result
is that granitic detritus is transported to greater distances and
distributed over larger areas than the detritus of the closer-grained
and more homogeneous rocks. In approaching a mountain composed of
about equal parts of, say, granite and serpentine, we accordingly
find the lower portions of the wadi consist chiefly of granitic sand,
while the coarser and more angular detritus of serpentine is only seen
in abundance in the higher parts of the drainage channel. The nature
of the alluvium of the wadi floor occasionally gives its name to the
wadi; thus, of the two wadis which by their junction form Wadi Khoda,
one is called Wadi Salib el Azrak, because its alluvium consists of
dark rock fragments, while the other is called Wadi Salib el Abiad,
because its floor is formed of white granitic sand. (_Azrak_ = dark;
_Abiad_ = white).
In size, the fragments composing alluvial detritus vary from huge
blocks weighing tons down to the finest sand whose grains are only
a fraction of a millimetre in diameter. As a rule, of course, the
larger fragments are found near the heads of wadis, and the finer
sands in their lower reaches and on the plains. But often we find
great boulders mixed with the sand at immense distances from the
parent mountain mass; the heavier rain storms which occasionally
break over the mountains produce for short periods such immense
rushes of water down the wadis, that great boulders are swept along
and incorporated in the finer material which is deposited in times
of more normal rainfall. Thus it is no uncommon thing in sinking
a well in a wadi fifty kilometres or more from the mountains to
encounter great boulders in the sandy alluvium.
In shape, the alluvial fragments vary very much, according to
their mineral nature and the amount of rolling they have received
in transport. Foliated rocks, like schists and slates, produce
typically flaky fragments which seldom attain any high degree of
rounding; the same is true of serpentines which have been shattered
by crushing. Granitic rocks produce generally more symmetrical
fragments, though granitic sand is typically angular unless it has
been transported for considerable distances. The larger blocks are
generally rounded in the case of granitic rocks, even when they
have not travelled for any great distance; for if a cubical block
of granite be exposed to weathering, disintegration takes place
most rapidly at the corners, and the block eventually becomes
more or less spheroidal without having moved at all. This can be
well seen in such masses as Gebel Selaia, where the granite boss
is thoroughly well rounded _in situ_. The same phenomenon can be
noticed in certain dolerites, where portions of the rock detached
by joint planes have weathered _in situ_ into forms like cannon
balls. The more homogeneous rocks like felsite and serpentine,
as well as the foliated rocks like gneisses, schists, and slates,
do not show much of this rounding _in situ_, as they yield to the
stresses of expansion and contraction by breaking along planes of
crushing or foliation, producing separate masses which preserve most
of their angular shape. In the lower reaches of the wadis and on the
plains, of course, all the materials are more or less rounded, owing
to the attrition to which they have been subjected during transport.
Of the thickness of the alluvial sands and gravels very little is
known. But that it exceeds eight metres in many of the wadis is
proved by wells sunk to that depth. On the coast-plain it may be
vastly thicker, for Grabham[122] mentions that borings made in the
maritime plain near Port Sudan have been carried down to 1,000 feet
(305 metres) in the deposits. Only a very small proportion of the
alluvial detritus brought down the wadis enters either the sea or the
Nile; for the storm-formed streams, though violent in the mountainous
tracts, commonly cease by being absorbed in the thirsty wadi floors
and plains before reaching either the sea or the river. Bearing this
in mind, and the ages through which the actions of denudation and
transport have gone on, we can well imagine how great must be the
total quantity of detritus accumulated.
In some wadis there are high banks of old detritus through which the
streams have cut their way. In the Wadi Meneiga, for instance, there
are terraces of alluvium, some eight or ten metres high, on either
side of the present water channel; these terraces, which are about
two kilometres lower down the wadi than the wells, are covered with
rude rubble ruins, the remains of dwellings erected out of reach of
the occasional streams which rush down the wadi after rain.
At the east foot of Gebel Hamra Dom are some well stratified friable
sand rocks and sandy clays, about ten metres thick, ending abruptly
against the granites and schists of the hill-mass. These beds appear
to be ancient alluvial deposits. They contain ferruginous scales and
show sun-cracks in places, while the materials are finer and more
distinctly stratified than one would expect to be brought down from
the mountain under present conditions.
It is to the alluvial material in the wadis that the Eastern Desert
owes most of its perennial water supplies and the vegetation
it possesses. The alluvial deposits are the great conservers of
water. Pools form, it is true, in the bare rocky beds of the higher
drainage lines, and may last for many months where they are screened
by the walls of a gorge from wind and sun, and are in consequence not
subject to rapid evaporation. But such pools are difficult of access,
and afford but a precarious source of water supply because they are
liable to dry up if a long period passes without rain. The alluvium
of the wadis absorbs the rainfall and protects it from evaporation,
so that even in very dry years water may be found by excavating in it
at suitable places to depths of a few metres. Almost all the wells
which are so important to travellers crossing the desert from the
Nile are of this character. The abundance of trees which flourish
in so many of the wadis likewise derive their nourishment from the
water conserved in the alluvium of the wadi floor.
=Calcareous Tufa.=
Small deposits of calcareous tufa of recent origin have been noted in
the Wadi Um Tundeba and in a gorge on the east side of Gebel Ghuel,
as well as round the little trickling spring called Megwel Hamida
in the south part of the region.
At Um Tundeba the deposit occurs in a little gully close to a well
known _galt_ or pool of rain water. The deposit is not extensive,
and has doubtless been formed by the evaporation of trickling
drainage-water which had absorbed lime from the rocks. The tufa
(10,374) is a pale brown rock of rather porous nature; it envelops
fragments of schist-debris from the surrounding rocks.
The calcareous tufa of Gebel Ghuel is a more impressive deposit,
though its total mass is not very great. It occurs at a point of
sudden fall in a narrow rocky gorge leading to the Wadi Ghadir in
about latitude 22° 53′. Proceeding up the gully one is confronted
with a great curtain-like mass of tufa covering the face of a high
ledge like a solidified cascade. Here also the origin is clearly due
to trickling of lime-laden drainage-waters over the ledge of rock,
which forms a step in the wadi floor.
Fragments of tufa were also seen round about Gebel Allawi, though
the rock was not traced to its source. As only a relatively small
number of rocky gullies have been explored, it is likely that similar
deposits to those above described occur in many other places.
=Gypsum and Gypseous Limestones.=
The occurrences of gypsum and gypseous limestone beds in South-Eastern
Egypt are restricted to the coastal regions. The most prominent of the
deposits are those on Ras Benas (_see_ map on Plate XXI), where the
beds form white hills rising to 188 metres above sea-level. Further
north, gypseous deposits have been found by Dr. Hume to exist near
the sea at Bir el Ranga, and by Mr. Ferrar the same beds have been
recorded as occurring near the coast in the neighbourhood of Wadi
Igli. It appears probable that the gypseous strata form a continuous
or nearly continuous strip extending along the coast down to latitude
24° 22′, as shown on the geological map on Plate XX, but more
complete observations may show that the distribution of the beds is
somewhat different from that indicated. On the coast-plain south
of Ras Benas, gypseous beds are not exposed except in the extreme
south-east corner of Egypt, where they form small patches at Halaib
and round the wells of Ti Kureitra.
At Ras Benas, where the beds have been studied in most detail,
they consist of gypsum and anhydrite [11,513][123] alternating with
sandy marls and marly sands, forming hills much cut-up by steep sided
narrow ravines. The weathered faces of the rocks are very soft, and
the disintegrated material forms a stretch of soft gypseous sand,
into which one’s feet sink four or five centimetres at every step,
between the hills and the shore. At Halaib, gypseous limestones crop
out from under the gravel of the coast-plain and form low banks;
the gypsum is here associated with calcareous grits [12,152, 12,114]
and conglomerates, the latter having boulders of igneous rock set
in a calcareous matrix. Near Ti Kureitra wells, the gypsum exposures
contain much crystalline selenite, blocks of this material being used
in the masonry lining of the wells. Both at Bir el Ranga and on Ras
Benas the gypsum is found to contain small pockets of native sulphur.
No fossils have been found in the gypseous strata, and their
geological age is uncertain. The beds are younger than the Nubian
sandstone, since they overlie that formation at Bir el Ranga. Their
occurrence only near the coast leads one to regard them as having been
formed after the Red Sea occupied its present position; but whether
they originated as direct gypseous deposits, or were produced by
the alteration of pre-existent Cretaceous or Tertiary limestones is
not yet quite certain. It is noteworthy that at Ras Benas, where the
gypseous strata rest on diorite and hornblende granite, the igneous
rocks are considerably altered as if by weathering, with a strong
brick-red colour due to the oxidation of ferruginous matter.
PLATE XXI.
[Illustration: GEOLOGICAL MAP OF RAS BENAS
Ball. Geography & Geology of South-Eastern Egypt.
Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190)]
=The Nubian Sandstone.=
The Nubian sandstone covers a comparatively small portion of
South-Eastern Egypt, occupying only about one-tenth of the total area
under consideration. The greater portion of it forms the eastern
termination of the vast sandstone plateaux which stretch eastward
from the Nile towards the igneous and metamorphic back-bone of the
Red Sea mountains, but there are in addition some small patches on
the eastern side of the watershed which are interesting as showing
that the deposition of the sandstone was not confined to the western
side of the mountain ranges.
The most northerly exposure of Nubian sandstone in the district
here described extends with some breaks southward from near Gebel
Sufra to near Gebel Homr Akarim, forming low hills. Further south,
it comes in again on both sides of the Wadi Garara, forming the hills
of Felieiti and Abu Hashim. In the sandy plain around the Wadi Timsah
are outlying sandstone hills of considerable height and extent,
the principal being Gebels Nuggur, Mulgata, and Ziraga. Further
south-west, the Nubian sandstone forms an extensive high broken
plateau stretching southward from near Gebel Zergat Naam to near
Gebel Um Reit, sending out a long tongue south-eastwards across Wadi
Dif to near Gebel Nigrub el Tahtani. This tract of broken sandstone
plateaux bears various names in different parts, Gebels Um Harba, Um
Khafur, Dagalai, Shebakhit, Awamtib, Um Sididad, Abraq, Hodein, Dif,
Anfeib, and Kala, all forming parts of the same great sandstone mass,
though separated by wadis from each other. Small outliers of Nubian
sandstone cap the hills of Gebels Reietit, just north of latitude
23°, near the meridian of 34°, and Gebel Seiga, in latitude 22°
44′, longitude 34° 16′.
On the eastern side of the watershed, Nubian sandstone extends
for some distance near the coast opposite the Gulhan islands, and
also forms small thin outliers south of the Wadi Hodein. The most
southerly point where the sandstone has been met with in the area
is a small outlier in latitude 22° 36′, a few kilometres east of
Gebel Hamra Dom.
In petrographical characters the Nubian sandstone is remarkably
uniform, consisting of medium-sized silica grains set in a more
or less ferruginous cement; the colour varies from nearly white,
through various shades of brown, to nearly black, according to the
amount of iron present. It is generally well bedded, and frequently
much jointed. There is usually a marked absence of the clay beds
which are associated with the Nubian sandstone in other parts of
Egypt. The basal beds are generally pebbly, forming conglomerates in
some places. Concretions are found in the rock at some points, notably
to the north of Gebel Um Harba. Some of the concretions are hollow,
the shell being of hard dark ferruginous sandstone while the interior
is filled with a powder of snow-white chalky matter containing sand
grains. The only fossils observed in the sandstone within the area
described were collected by Mr. Charteris Stewart in the plain of
Um Harba; they consist of crocodile scales and various shells. From
these and the fossil shells found further west in sinking a well in
Wadi Abu Rahal,[124] it may be concluded that the Nubian sandstone
here, as in the Nile Valley, is of Cretaceous age.
Intrusions of igneous rock into the sandstone have only been observed
at two points in the area, _viz._, near the western foot of Gebel
Awamtib, where a basic dyke runs through the sandstone, and on the
Red Sea coast, near Bir el Ranga, where Dr. Hume has found andesite
interbedded in the sandstone. A bed of diabase, now much altered,
underlies the sandstone near Wadi Muelih, and is possibly of
contemporaneous origin.
In thickness, the Nubian sandstone attains a maximum of about
350 metres in several places round about Bir Abraq and the Wadi
Hodein. Its thickness must at one time have exceeded this, for the
upper surfaces have suffered much denudation, and are not capped by
younger rocks.
Some interest attaches to the maximum altitude reached by the Nubian
sandstone beds. The highest point at which it has been observed is at
Gebel Seiga, whose sandstone cap is 905 metres above sea-level. The
highest point reached by the rock on the great plateau round about
Wadi Hodein is Gebel Kala, 846 metres above sea.
In contradistinction to the same beds which form the plateau between
longitude 34° and the Nile, the Nubian sandstones within the area
here described show considerable disturbances from their original
horizontal bedding, the dips being as a rule greater and more variable
the nearer one approaches to the watershed mountain-ranges. These
tectonic disturbances, which are important in connexion with the
geological history of the region, will be considered in Chapter XI.
* * * * *
[Footnote 122: _Geol. Mag._, Decade V, Vol. VI (1909), p. 271.]
[Footnote 123: The numbers in square brackets are the specimen-numbers
in the Cairo Geological Museum.]
[Footnote 124: Wadi Abu Rahal is a small feeder of Wadi Abad,
joining the latter from the south in latitude 25° 0′, longitude
33° 30′. At the point of junction of the two wadis, which lies
on the usual camel road from Edfu to the Baramia mine, a well was
sunk by the Mines Department in 1906, in the hope of obtaining a
water supply. I visited the well in May 1906, when it had attained
a depth of fifty-four metres. The strata passed through were ten
metres alluvium, then thirty-seven metres of sandstones and clays,
followed by a thin band of bituminous shale, and seven metres of
dark grey clays. From near the bottom of the well I collected
specimens of _Lingula_ and a mytiloid shell which Mr. Bullen
Newton compared to _Septifer linearis_; the latter shell differs
but little from specimens obtained from the English Gault, and thus
tends to show that the Abu Rahal beds are of Cretaceous age. (_See_
HUME, _Preliminary Report on the Geology of the Eastern Desert
of Egypt_. Cairo, 1907. p. 29). Since my visit, the well has been
deepened to seventy-three metres in sandstones with a bituminous seam,
but water was not reached, and the well has been abandoned.]