The systematic examination of the geological structure of Egypt,
carried out by the Geological Survey during the last thirteen years,
has, in conjunction with previous studies, revealed the fact that
the surface of the country is composed of very varied materials:
limestones, sands, clays, sandstones, granites, schists, etc. The
co-ordination of view as to the distribution of these rocks[1]
presented by a geological map has further shown that they are not
arranged in a random manner, but that certain very definite relations
exist between them. If, for instance, the Moqattam hills behind
the Citadel be ascended, the lower part of the scarp is found to be
composed of white limestone, which is capped by a series of sandy
limestones and clays differing alike in colour and in the rapidity
with which they are worn away by the streams, due to the rare but
destructive winter rains. Finally, the summit is crowned by beds
in which boulders of flint and quartz play the most important part,
the matrix of sand which bound them together having long been removed
by the action of the wind.
Similarly, if we were to go southward towards Aswan, a change would
be noted in the character and composition of the rocks which form
the cliffs bordering the Nile Valley. From Cairo to Qena, limestones
predominate; but from thence southward to Esna, clays play a large
part at the base of the limestones, and the slipping of the latter
over these softer members has given rise to the “tumbled” country
which is so conspicuous a feature between Armant and Matana. South of
Esna the clays in their turn disappear, while the sandstone which is
seen underlying them near Mahamid becomes the dominant constituent
of the hills from near Edfu to the neighbourhood of Aswan; at this
locality the sandstone itself vanishes, except in so far as it forms
isolated caps on the granite, which is the principal rock in the
well-known district of the First Cataract.
The same lesson as to the order of succession of the rocks in Egypt
is forced home if we move from Qena eastward to the Red Sea hills,
or south-westward to Kharga Oasis. East of Qena the clays are a
conspicuous feature at the base of the outlying limestone hills (Abu
Had, etc.), and in their turn rest on sandstone, which forms striking
plateaus seamed by deep ravines giving entry to the heart of the Red
Sea hills. On traversing these gorges, a confused hill-country of
granite (worn into boulders on the surface), or dark-green schists is
entered, on whose summits the sandstone occurs as isolated outliers
near the main sandstone mass, but to the east disappears altogether.
Similarly, going westward and crossing the great limestone desert, on
reaching the edge of the scarp which bounds the oases, clays appear
from under the limestones, and in their turn overlie sandstones
forming the floors of the oasis depressions. Closer examination
reveals the fact that in the upper part of the sandstone series seams
of clay alternate with the sandy layers, and in that part of the cliff
where clay bands predominate, beds of limestone alternate with them.
In broad outline, it may be stated that in southern Egypt, limestone
rests on clays alternating with limestone, these on sandstones which
in their upper part alternate with clay, and the sandstone on granite
and metamorphic rocks (slates, schists, etc.). The inclination or dip
of the various beds is such that should a boring be made through the
limestone near Cairo, one might expect to reach first a succession
of beds where clay was predominant, followed by beds of sandstone,
and finally the series of which the granite of Aswan is a conspicuous
member.[2] In northern Egypt, the conditions are reversed, sandstones
and clays _overlying_ the limestones near Cairo, but never attaining
the extent and importance of those exposed to view in the south.
These facts require an explanation—the one furnished by the science
of Geology, being briefly as follows: Each of the rocks observed
has a definite origin or formation; the nature of their present
distribution is due to subsequent movement, or deformation; the
various meteorological agencies at work lead to the transformation of
the original structures, resulting in the formation of a new series
of rocks composed of materials derived from the wearing away of the
older deposits.