THE CONTEST BETWEEN MAN AND WIND-BORNE SAND
The Winds — Wind and Sand as Agents of Denudation — The Combat
of the Inhabitants with Sand-laden Winds — The Dune-Belts —
Origin of the Sand — Composition of Dune-Sand — The Forms of
Dunes — Dimensions and Rate of Movement — Irresistibility of
Blown Sand — Protection afforded by Topographical Features —
Effect of Artificial Obstructions on the Formation of Dunes —
Fixing of Dunes by Vegetation and Moisture — Storm-Walls and
Fences — The Dunes of Gennâh — Wind-borne Sand beneficial
in the South of the Oasis — Formation of Terraces of Wind-borne
Materials — Encroachment of Dunes in the South of the Oasis.
No one who has sojourned in Kharga long enough to compare it with
other parts of Egypt will feel inclined to dispute the statement that
the depression is one of the most windy places in the country. Only
one day in ten or eleven is calm, though if the wind has been
blowing fairly steadily during the day it generally drops at
night. Five times out of six its direction is from the north, and
when unaccompanied by sand the wind acts as a welcome moderator of
the temperature.
In a desert region, owing to the surface being unprotected
by vegetation, there is nearly always an abundance of weathered
material loosely exposed on the surface, the lighter portions of
which are quickly swept up and carried along as soon as the wind
attains even a moderate velocity. On the actual dunes even a light
breeze is sufficient to set the surface-layers in motion, while on
the open plain the wind becomes visibly charged with sand directly it
attains the velocity of a moderate breeze. The frequency with which
the wind blows has already been mentioned, and when we add that on
one day in three it attains a velocity sufficient to enable it to
easily carry sand in suspension, its power as a transporting agent
can readily be conceived. Not only does the wind carry the sand
along from one place to another, dumping it behind or in front of
every obstacle it may meet, but the combination of sand and wind
forms a denuding force of no mean power, capable of planing and
grooving the exposed surfaces of the hardest of rocks.
Now, although opinions may differ as to the part played in
the past by wind and sand in the shaping and formation of the
oases-depressions of the Libyan Desert, there can be no question
of the effects they are producing at the present day. Wind-borne
sand is indeed the curse of the oasis, and although its evil
effects may be mitigated or altogether staved off for a time,
the longer this force of Nature is opposed by man the greater and
more overwhelming is its final victory. If a full account of the
human occupation of the oases could be written, it would be very
largely the history of an incessant combat between man and Nature;
and although man may for a time gain the upper hand, and even make
the sand-laden winds to some extent serve his purposes, he is in
the end generally forced to abandon those places in which he has
been at such pains to establish himself.
In the Libyan Desert, both on the plateaux and in the depressions,
blown sand has a marked tendency to collect into dunes occupying
definite north and south belts of country, lying parallel to the
direction of the prevailing winds. This peculiar and fortunate
disposition depends on a number of circumstances, such as the
mean direction of the wind, the points of greatest supply of
the material, and on other causes which are not perhaps as yet
thoroughly understood.
In Kharga Oasis the best marked and most important belt lies somewhat
to the west of the central axis of the depression. At the north end,
in the neighbourhood of Um el Dabâdib, it consists of a number of
isolated but parallel lines of dunes, which southwards pass over
and round the hill-massif of Jebel Tarif, eventually uniting and
forming a more compact belt of sand, with an average width of 8 or
10 kilometres, which continues right through the depression into the
desert to the south. This line of sand sets a limit to the extension
of cultivation to the west. A second belt comes from the plateau to
the north of Qasr Lebekha, passes immediately to the east of Jebel
Têr and the village of Kharga, and continues southwards until it
spreads out and is arrested, more or less by artificial means, in
the vicinity of Gennâh. Other smaller and less defined belts exist
to the south of Ain el Tawîl, to the east of Headquarters, and near
the foot of, and parallel to, the eastern wall of the depression.
[Illustration: A BELT OF DUNES NEAR QASR LEBEKHA.]
Although it has been stated by more than one writer that the great
sand accumulations of the Libyan Desert originate in the denudation
of the Nubian Sandstone which occupies large areas to the south of
the oases, I have never myself met with any evidence in support of
such a view. In my opinion the bulk of the sand must be regarded as
originating in the arenaceous deposits of post-Middle Eocene age,
which largely occupy and are exposed on the surface of the country
between the Mediterranean Sea and latitude 29° N. We know from
personal observation that some of the great sand-belts, which cross
the intervening limestone plateau to the southern oases, commence in
this region, and the supply of sand made available by the denudation
of the beds in question is inexhaustible. During the passage of
this sand across the great desert tableland the individual grains of
silica doubtless become much rounded and reduced by attrition, though
the total loss is probably more than counterbalanced by the very
considerable additions received in the form of calcareous grains,
derived from the limestones forming the surface of the high desert.
An examination of the dunes of Kharga Oasis at once shows that
the sand is not by any means composed solely of siliceous grains,
but that there is a considerable proportion of white granules of
limestone. A number of samples collected at random from the dunes
in the central part of the depression were all found to contain
a visible proportion of calcareous grains, and one of these,
collected from the big dune to the south-east of Headquarters,
was qualitatively examined for me by Mr. Garsed, and shown to
contain 7·7 per cent. of calcium carbonate. It would be difficult
to account for the presence of these calcareous grains on the
assumption that the sand is derived from the Nubian Sandstone,
though the denudation of the latter in and to the south of the
oases must give rise to a very large amount of siliceous sand,
which goes to swell the bulk of the dunes which have invaded this
country from beyond the limestone plateau to the north.
The dominant form of dune in the oasis is of crescentic or horse-shoe
shape, a form specially typical of desert regions where there
are prevailing winds in any one direction. These ‘barchans,’
as they are called, which are always disposed with the concave,
steeply-inclined sides facing southwards, are found of every size,
and exhibit many variations of the simple crescentic pattern. In
many parts of the oasis the sand-belts are made up of a number of
isolated and promiscuously disposed barchans; in others the dunes
have joined together into a compact mass, in which the typical
shape of the individual barchan is more or less obliterated.
Solitary barchans, being well-defined isolated masses, lend
themselves to observation better than the large and continuous
belts of sand. They occur of all sizes, from little baby crescents,
a metre or two across, to enormous masses 30 or 40 metres high and
200 to 300 metres in breadth. In all cases, except when their steady
march southwards has been temporarily interrupted by southerly
winds, the southern face, flanked by the horns of the crescent,
is a straight slope of from 30 to 33 degrees, its inclination is,
in fact, the angle of rest of loose dry sand, and it is formed
of the sand which is continually blown up the opposite slope and
dropped over the crest.
In a moderate breeze the whole barchan progresses steadily in a
southerly direction without loss or gain of sand; in stronger winds,
while still continuing its course, the dune may at the same time
be either losing or gaining in actual bulk. The rate at which the
barchans travel varies with their size. The larger the dune the
greater the amount of material to be moved, so that, as a general
rule, small barchans progress much more rapidly than large ones. The
actual rate may vary from 10 to 20 metres per annum, the average
being about 17. For instance, the big isolated dune on the road
between Headquarters and Kharga village moved 11 metres southwards
between December 8, 1907, and December 8, 1908, while during the same
period a small dune in the Bellaida district travelled 21 metres in
the same direction. The dimensions of these two dunes is as follows:
+------------------------------------+-------------+----------+
| | Kharga Road | Bellaida |
| | Dune. | Dune. |
+------------------------------------+-------------+----------+
| | Metres. | Metres. |
| Vertical height of centre of crest | 17 | 10·5 |
| Maximum breadth (east and west) | 259 | 160 |
| Maximum length (north and south) | 230 | 70 |
+------------------------------------+-------------+----------+
These are merely instances from dunes which I have had under
observation for a number of months, full details of which I hope
shortly to publish.
Approached on a bright sunny day from the south side, the angle of
slope within the crescent of a barchan is most deceptive. Nine out
of ten persons will confidently state that the face of the dune
cannot be less than 60 or 70 degrees from the horizontal, though,
as a matter of fact, it never exceeds by more than two or three
degrees a third of a right angle. Unless held together by vegetation
or compacted by moisture, it cannot exceed the angle of rest of dry
blown sand, which is about 32 degrees. In normal weather this face
of a barchan is a straight, unrippled, even slope of loose sand from
top to bottom; when a light or gentle breeze is blowing from the
north, the dune can be seen ‘smoking’ at the top of the crest,
the sand being blown up the much more gentle and markedly rippled
northerly slope, and dropped over the crest on to the steep, smooth,
southerly face. The bulk of this sand in a light breeze falls on the
upper part of the slope, equilibrium being adjusted from time to time
by a downward sliding of the superficial layers. A cross-section of
the dune, from north to south, would indeed show it to be composed
of well-stratified layers of sand, dipping steadily southwards at
a little over 30 degrees.
In this district I have never observed southerly winds hold for a
sufficient length of time to do more than slightly modify the general
form of a barchan, though during their continuance the progress of
the dune is arrested, the outline of the crest becomes considerably
altered, ripple-marks appear on the steep southerly face, and an
apron of sand of greater or lesser dimensions forms at its foot.
The irresistibility of drift sand is well seen where isolated
hills lie in the path of the dunes. When such hills are narrow,
and elongated in a north and south direction, they generally act
as a wedge, divide the wind, and force any sand it may be carrying
to pass on either side. When, however, a hill has northerly flanks
of any considerable width, it will only form a temporary check to
the southward passage of the sand. The progress of the latter may
be arrested for a while, until hollows have been filled in and the
general slopes reduced; but eventually the sand will pass up along
the lines of least resistance to the summit. This is beautifully
illustrated at Jebel Tarif, the great isolated, flat-topped,
hill-massif lying between Um el Dabâdib and Kharga village,
standing over 300 metres, or 1,000 feet, above the surrounding
plain. Instead of being diverted by this obstacle, the sand, coming
from the plateau to the north and north-west of Um el Dabâdib,
surmounts the slopes, and continues in its normal direction across
the flat-topped summit. The latter has been cut up into numberless
parallel north- and south-disposed ridges, separated by deep troughs,
which are now to a considerable extent filled with sand. Active
erosion must, therefore, at the present day be confined to the upper
portions of the slopes of the valleys and to the intervening ridges.
Naturally in such a region the inhabitants have been quick to take
advantage of any protection afforded by topographical features, and
it is not surprising to find that the most important of the ancient
monuments, as well as the greatest area of modernly cultivated land,
lie in a comparatively sheltered position to the south of Jebel
Têr. Even there, however, trouble is constantly experienced on the
east side, owing to the tendency of the belt of sand-dunes, which
comes from the vicinity of Qasr Lebekha, to spread laterally. Along
the east side of the village the dunes are close up against the
outlying houses, and the sand continually surmounts and pours over
the walls which from time to time are erected by the natives to
act as barricades.
Sheltered places of the above description are rare, the majority of
the smaller settlements being situated on the open plain, where the
only precaution which can be taken is to select the sites for wells
as far away as possible from the well-defined sand-belts. But in
windy weather the sand-blast is not confined to the neighbourhood of
the dunes; the wind rages across the open plains, carrying dense
clouds of sand in suspension, and devastating the works of man
wherever the latter offer the slightest obstacle to its progress.
[Illustration: SAND EROSION ON SUMMIT OF JEBEL TARIF.]
In order to protect his crops from the fierce onslaught of wind-borne
sand the native is compelled to erect fences and barricades along the
northern boundaries of his fields. These serve his purpose for the
time being, the sand which would otherwise have cut and beaten down
the growing crops being deposited on either side of the fence, where
the velocity of the wind is checked. The arrested sand accumulates
with increasing rapidity as the size of the obstruction becomes
greater, a veritable dune, constituting a really efficient protection
to the cultivated lands, being formed over the site of the original
small and insignificant hedge. Unfortunately, this sand continues
to collect, and although the cultivator may have satisfactorily
insured himself against the frequent damage caused to his crops by
their exposure to wind-driven sand, he is now confronted with the
still more serious prospect of having his lands overwhelmed by the
ever-increasing dunes, which have grown to considerable proportions
as the result of his efforts to protect himself.
Fortunately for the occupiers of these outlying settlements, the
progress of such dunes, formed more or less artificially on the
northerly sides of the cultivated tracts, is not at the same rate as
that of isolated dunes moving across the open desert plain. Owing
to the presence of a certain amount of moisture in the superficial
layers of the ground in the neighbourhood of the wells and the
cultivated lands, plants of various kinds are enabled to thrive,
with the result that the dunes become to some extent ‘fixed’ by
wild vegetation. The most valuable plant in this connection is the
tamarisk, which by means of immensely long roots continues to thrive
and keep its head above growing dunes with remarkable facility. To
a certain extent, moreover, the moisture from the soil itself
rises by capillarity into the base of the sand, and assists the
vegetation in binding it into a compact mass, which is not liable
to movement in the same degree as dry sand. The rate of progress
of such dunes may therefore be reduced to a considerable extent,
but the movement can never be altogether stopped. As we have seen
in the case of the hamlet of Meheriq, the sand eventually gets the
upper hand, blotting out the arable lands, burying the palm-groves,
and forcing man to abandon his habitations.
In the neighbourhood of the Corporation’s Headquarters a
considerable area of the floor of the depression has been brought
under cultivation during the last three years. This tract is exposed
on every side, there being no protection whatever in the shape of
natural features. Although occasional sandstorms are experienced
from the south, and even from the east and west sides, practically
the whole of the trouble encountered comes from northerly sandstorms
and sand-laden winds. It was found quite impossible to raise crops
unless the fields were protected by storm-walls or fences, the young
plants being cut down as if by a scythe. So long as the reclamation
of land proceeded from south to north at a fairly rapid rate, the
accumulation of sand against these fences was of little moment,
being, indeed, rather welcome than otherwise, as the sand was found
to be of considerable value when spread out and ploughed into the
heavy clay soil with which this area is superficially covered. When,
however, a storm-wall remains for several consecutive months
exposed to the open desert to the north, the accumulation of sand
becomes a more serious matter, and care has to be taken to avoid
the formation of dunes, which it might be afterwards difficult to
get rid of. A fairly satisfactory mode of procedure is the planting
of narrow belts of ‘sesban,’ or other quickly-growing shrub,
along the northern limits of the newly reclaimed lands; a hedge of
this description forms an efficient shelter to the fields, while at
the same time it allows a good deal of the sand to filter through
and become absorbed by the soil.
[Illustration: THE CORPORATION’S HOMESTEAD (HEADQUARTERS).]
Most of the dunes in the Gennâh district are more or less covered
with tamarisk and other wild vegetation, but that their progression
southwards has only been retarded, and not prevented, is evident
from an examination of the country. Large areas of land have been
abandoned owing to the invasion of the dunes, and the magnificent
wells of Gennâh are threatened with destruction within a very short
term of years, unless special measures are adopted to cope with
the advance of the sand. The available land in the vicinity of the
village has long been insufficient to utilize the water discharged
by these wells, which has in consequence to be led by a channel of
several kilometres in length to lower ground south-east of the Gorn
el Gennâh. This channel is being constantly pushed to the south,
and as the ground rises in that direction, it may eventually become
impossible to maintain the conduit at a sufficiently low level to
carry the water of the wells to this outlying district.
The southern portion of the oasis is the only area where sand and
wind can be said to be of any actual benefit to the inhabitants,
but in this area, owing to the broad plains of alluvial clay,
the suspended material consists to a very great extent of fine
argillaceous particles. At many of the wells in the Dakhakhin and
Beris districts the cultivators encourage the gradual deposition
on their fields of the sand and clay-dust borne by the northerly
winds. The mixture forms an excellent loam, and an annual dressing
of the material is regarded as desirable, and even necessary, on
account of its value as a fertilizer. The result of this gradual
deposition of wind-borne material has been that the cultivated
terraces have in many cases attained to such elevations that the
discharges of the wells have suffered very considerable diminution,
owing to the increased height of the outlets.
To the west of the villages of Maks and Beris the great belt of sand
has already enveloped a number of wells, and appears to be still
encroaching—as the individual dunes follow their normal course in
a direction slightly east of south—on the area occupied by the
existing cultivated lands. Fortunately there are still extensive
tracts of cultivable land comparatively free from accumulations
of drift sand, and doubtless the inhabitants, when driven by the
irresistible forces of Nature from the localities they now occupy,
will again, as they have so frequently been compelled to do in the
past, migrate to pastures new.