The course of events in Pleistocene times is at present obscure. As
far as can be judged it was during this period that a freshwater
lake, the precursor of the great Mœris, came into existence. It
might be thought that the early Pleistocene prehistoric lake was a
relic of the still older body of water of Pliocene times, in which
the gravel terraces and gypseous deposits were laid down. But such
a remnant would have been of a high degree of salinity and could not
have given rise to the fresh water Mœris. Most probably at the close
of the Pliocene period, after the formation of the gypseous deposits,
the area became elevated and cut off from the sea and from the Nile
Valley marine fiord; probably an extensive body of water remained as
an isolated lake, but this, cut off from external supply, would have
gradually evaporated, its salt being left as a superficial deposit on
the dried up bed. In early Pleistocene times we may presume the area
became dry and was gradually eroded to its present shape and depth. The
superficial deposits of salt and gypsum were for the most part removed
as the depression was deepened, while the continuous terrace of gravel
laid round the greater part of the rim was broken through, except
where protected in favourable localities, the constituents being
washed down and spread out over the lower ground. In course of time
the region was moulded to its present form and dimensions, or rather to
what it would be if the local alluvial deposits were stripped off and
the water of the lake baled out. The area was an inland depression,
probably sparsely vegetated like the rest of the higher country and
separated from the Nile Valley by a low rocky ridge surmounted by a
more of less continuous terrace of gravel of considerable height and
thickness. In our opinion desert conditions had already set in before
the early Nile broke down the ridge and formed a lake in the Fayûm;
the date when this important event first took place is a matter of
considerable doubt, as has already been mentioned (pp. 24, 25). We
know that at the close of the Pliocene period the Nile Valley was
a marine fiord (connected with the Fayûm and the Mediterranean)
which was replaced in Pleistocene times, probably in consequence of
slight elevation, by a series of fresh water lakes throughout the
valley. These lakes were probably two or three in number and drained
one into the other; the exact position of the barriers is as yet a
matter of conjecture. Within these lakes thick lacustrine deposits
were accumulated, so that the basins eventually became to a great
extent silted up. In later Pleistocene times drainage down the Nile
Valley appears to have become more pronounced, the barriers between
the lakes were broken down and the river cut for itself a channel
through the lacustrine beds, filling up the old lake basins. From
this time onwards to early prehistoric times the bed of the river
would appear to have steadily fallen, as it eroded its channel deeper
and deeper. That this early Nile was a river of considerable size is
evident from the amount of erosion it accomplished in the trough of the
valley, whence the older lacustrine beds have been almost completely
removed. Probably in the lower part of its course it swept against
the base of the dividing ridge between the Nile Valley and the Fayûm
depression. If, as we imagine, the river was at that time flowing
some 20 metres higher than at present its currents would have met
with little resistance from the loosely cemented terrace of gravel
which formed the upper part of the dividing ridge. Once this was
broken down the waters must have poured into the depression behind,
until a lake of considerable size was formed. The sediment spread out
over the floor in the shape of a fan, while at the same time deposits
of sands and fine clays, blown and washed into the waters from the
surrounding shores, were being slowly accumulated in the quieter and
more remote parts of the lake.
For some time subsequent to the first connection between the Fayûm
depression and the Nile, the latter continued to fall in level owing to
continued erosion along its course, possibly the Fayûm again became
completely isolated for a time. Subsequently in the earliest historic
times under changed conditions the river commenced to carry and lay
down the modern alluvial deposit of “Nile mud”, and from this time
to the present day its bed has gradually risen. This is shown by the
high Nile flood-readings on the early gauges of the Nile Valley; the
nilometer at Roda shows a difference of 1·22 metres in 1026 years, or
an annual rise of 0·12 centimetre, which is equivalent to 12 metres
in 10,000 years[90]. Whether there was ever complete disconnection
between the Nile and the originally formed lake in Pleistocene times
is uncertain, but even so it was probably only during a comparatively
short period: in early historic times the rise of the Nile bed must
have brought about a reconnection.
The geological evidence for the existence of a great freshwater lake
in Pleistocene and prehistoric times is afforded by the well-marked
lacustrine clays and sands which are found over such a large area of
the northern and western deserts of the Fayûm; the great extent of
this lake will be seen by an examination of the accompanying maps. Its
area must have been about 2250 square kilometres or about ten times
the size of the modern Birket el Qurûn. The western limit may even
have been further west than shown on fig. 10.,[91] as some of the
desert in the neighbourhood of Gar el Gehannem is very low-lying;
or there may have existed subsidiary lakes in that direction. The
upper limit of these ancient lacustrine clays is between 22 and 23
metres above sea-level, which exactly agrees with the figure adduced
by Sir Hanbury Brown as the height of the more modern Lake Moeris
from the evidence of levels. That Lake Moeris was simply the older
prehistoric lake placed under artificial control admits of no doubt;
the difficult question being as to when the lake first came into
existence in prehistoric times.
The clays abound in freshwater shells and semi-fossil fish-bones of
exactly the same species of fish (siluroid, etc) as still inhabit the
Birket el Qurûn; probably some are even hardly different specifically
from the Middle Eocene forms of the district. In addition remains
of large animals are common, and include _Hippopotamus_, _Elephas_,
_Bubalis_, sheep or goat, and _Canis_, with crocodiles and turtles,
etc.[92]
[Illustration: PLATE XV.
ISOLATED SAND-DUNE NEAR GAR EL GEHANNEM.]
Martens[93] has described the following species of mollusca from
Schweinfurth’s collections:—
_Unio abyssinicus_, Mart.
_U. Schweinfurthi_, Mart.
_Corbicula fluminalis_, var. _consobrina_ Caill.
_Neritina nilotica_, Reev.
_Valvata nilotica_, Jick.
_Cleopatra pirothi_, Jick.
_C. pirothi_, var., _unicarinata_, Mart.
_Bithynia_ aff. _Boissieri_, Charp.
_Melania tuberculata_, Müll.
_Limnaea natalensis_, Krauss.
_L. mœris_, Mart.
_L. palustris_, Müll.
_Planorbis subangulata_, Phil.
Blanckenhorn has pointed out[94] that this fauna is of special
interest and differs from all fossil and living faunas in Egypt. It
might be compared with the _Melanopsis_-fauna of the Nile Valley if the
exceptional _Limnaea_ were replaced by _Melanopsis_ or _Paludina_. Its
_Unio Schweinfurthi_ recalls the youngest alluvial deposits of the
Nile Valley, 2nd Cataract, Kom Ombo and Silsila; at these places,
however, the beds containing the species in question are at least 20
metres above mean water level of the present day.
The sub-fossil fauna of the Fayûm alluvium, in addition to those forms
everywhere met with in the Nile Valley, includes _Neritina nilotica_
and _Melania tuberculata_, which are common forms of the _Melanopsis_
stage, as well as _Unio abyssinicus_ and _Valvata nilotica_. In
common with the present fauna of the Birket el Qurûn it has the five
forms belonging to the genera _Corbicula_, _Neritina_, _Valvata_,
_Melania_, and _Planorbis_. The sub-fossil fauna, which passes into
the modern fauna of the Birket el Qurûn, shows connection with the
Mediterranean and Blue Nile, but has a total absence of White Nile
forms such as _Ampullaria_, _Lanistes_, _Cleopatra bulimoides_,
_Spatha_ and _Aetheria_. Moreover _Limnæa palustris_, although
identical with the form found on other Mediterranean coasts, is as
yet entirely unknown from the Nile Valley. Blanckenhorn concludes that
the diluvial subfossil deposits of the Fayûm were produced when the
climate of Egypt was damper and more Europæan, the Nile carrying
more arenaceous sediment in place of the mud of to-day and running
at higher level, as it did when the shells of _Unio Schweinfurthi_
were enclosed in the deposits of Jebel Silsila. Blanckenhorn thinks
the Nile obtained access to the depression during the last European
ice period. This last supposition, coupled with the above comparison
of the Fayûm fresh-water fauna with the Melanopsis stage of the Nile
Valley Pleistocene series, shows that in regarding the early Fayûm
lake as dating from prehistoric times Blanckenhorn and the writer
are in agreement.