SUN AND WIND IN THE LANDS OF INFREQUENT RAINS
=The law of the desert.=—It is well to keep ever in mind that there is
no universal law which dominates Nature’s processes in all the sections
of her realm. Those changes which, because often observed, are most
familiar, may not be of general application, for the reason that the
areas habitually occupied by highly civilized races together comprise
but a small portion of the earth’s surface. In the dank tropical
jungle, upon the vast arid sand plains, and in the cold white spaces
near the poles, Nature has instituted peculiar and widely different
processes.
The fundamental condition of the desert is aridity, and this
necessitates an exclusion from it of all save the exceptional rain
cloud. Thus deserts are walled in by mountain ranges which serve
as barriers to intercept the moisture-bringing clouds. They are in
consequence saucer-shaped depressions, often with short mountain ranges
rising out of the bottoms, and such rain as falls within the inclosure
is largely upon the borders. Of this rainfall none flows out from the
desert, for the water is largely returned to the atmosphere through
evaporation.
The desert history is thus begun in isolation from the sea from which
the cloud moisture is derived, a balance being struck between inflow
and evaporation. Yet if deserts have no outlets, it is not true that
they have no rivers. These are occasionally permanent, often periodic,
but generally ephemeral and violent. The characteristic drainage of
deserts comes as the immediate result of sudden cloudburst. As a
consequence, the desert stream flows from the mountain wall choked
with sediment, and entering the depressed basin, is for the most part
either sucked down into the floor or evaporated and returned to the
atmosphere. The dissolved material which was carried in the water is
eventually left in saline deposits, and the great burden of sediment
accumulates in thick stratified masses which in magnitude outstrip the
largest deltas in the ocean.
=The self-registering gauge of past climates.=—From the initiation
of the desert in its isolation from the lands tributary to the sea,
its history becomes an individual and independent one. An increasing
quantity of rainfall will be marked by larger inflow to the basin, and
the lakes which form in its lowest depression will, as a consequence,
rise and expand over larger areas. A contrary climatic change will
bring about a lowering of the lakes and leave behind the marks of
former shorelines above the water level (Fig. 205). Deserts are thus
in a sense self-registering climatic gauges whose records go back far
beyond the historic past. From them it is learned that there have been
alternating periods of larger and smaller precipitation, which are
referred to as _pluvial_ and _interpluvial_ periods.
[Illustration: FIG. 205.—Former shore lines on the mountain wall
surrounding the desert of the Great Basin. View from the temple in Salt
Lake City (after Gilbert).]
From such records it is learned that the Great Basin of the western
United States was at one time occupied by two great desert lakes, the
one in the eastern portion being known as Lake Bonneville (Fig. 206).
With the desiccation which followed upon the series of pluvial periods,
which in other latitudes resulted in great continental glaciers and
has become known as the Glacial Period, this former desert lake dried
up to the limits of Great Salt Lake and a few smaller isolated basins.
Between 1850 and 1869 the waters of Great Salt Lake were rising, while
from 1876 to 1890 their level was falling, though subject to periodic
fluctuations, and in recent years the waters of the lake have risen so
high as to pass all records since the occupation of the country. As
a consequence the so-called Salt Lake “cut-off” of the Union Pacific
Railway, constructed at great expense across a shallow portion of the
lake, has been overflowed by its waters. The Sawa Lake in the Persian
Desert, which disappeared some five hundred years ago, again came into
existence in 1888 so as to cover the caravan route to Teheran.
[Illustration:
FIG. 206.—Map of the former Lake Bonneville (dotted shores), and the
boundaries of the Great Salt Lake of 1869 (smaller area) and that of
the present (after Berghaus).]
The record in the rocks of the distant past reveals the fact that in
some former deserts barriers were, in the course of time, broken down,
with the result that an invading sea entered through the breached
wall. The result was the sudden destruction of land life, the remains
of which are preserved in “bone beds”, now covered by true marine
deposits. A still later episode of the history was begun when the sea
had disappeared and land animals again roamed above the earlier desert.
Such an alternation of marine deposits with the remains of land plants
and animals in the deposits of the Paris Basin, led the great Cuvier
to his belief that geologic history was comprised of a succession of
cataclysms in which life was alternately destroyed and re-created in
new forms—a view which later, under the powerful influence of Lyell
and Darwin, gave way to that of more gradual changes and the evolution
of life forms.
=Some characteristics of the desert wastes.=—The great stretches of
the arid lands have been often compared to the ocean, and the Bedouin’s
camel is known as “the ship of the desert.” Though a deceptive
resemblance for the most part, the comparison is not without its value.
Both are closed basins, and it is in this respect that the desert and
the ocean may be said to most resemble each other, for none of the
water and none of the sediment is lost to either except as boundaries
are, with the progress of time, transposed or destroyed. Flatness of
surface and monotony of scenery both have in common, and the waters and
the sand are in each case salt; yet the ocean, from the tropics to the
poles, has the same salts in essentially the same proportions, while in
the desert the widest variations are found both in the salts which are
present and in their relative quantities.
Upon the borders of the ocean are found ridges of yellow sand heaped up
by the wind, but these ramparts are small in comparison to those which
in deserts are found upon the borders (plate 7 A).
The desert is a land of geographic paradoxes. As Walther has pointed
out, we have rain in the desert which does not wet, springs which yield
no brooks, rivers without mouths, forests preserved in stone, lakes
without outlets, valleys without streams, lake basins without lakes,
depressions below the level of the sea yet barren of water, intense
weathering with no mantle of disintegrated rock, a decomposition of the
rocks from within instead of from without, and valleys which branch
sometimes upstream and sometimes down.
Within the deserts curious mushroom-like remnants of erosion afford a
local relief from the searching rays of the desert sun. Pocket-like
openings large enough for a hermit’s habitation are hollowed out by the
wind from the disintegrated rock masses. Amphitheaters open out from
little erosion valleys or wadi, and isolated outliers of the mountains
stand like sentinels before their massive fronts.
Because of the general absence of clouds above a desert, no shield
such as is common in humid regions is provided against the blinding
intensity of the sun’s rays. Sun temperatures as high as 180°
Fahrenheit have been registered over the deserts of western Africa.
Every one is familiar with the fact that a blanket of thick clouds is a
prevention of frosts at night, for, with the setting of the sun and the
consequent radiation of heat from the earth, these rays are intercepted
by the clouds, returned and re-returned in many successive exchanges.
Over desert regions the absence of any such blanket of moisture is
responsible for the remarkable falls of temperature at sunset. Though
shortly before temperatures of 100° Fahrenheit or greater may have been
measured, it is not uncommon for water to freeze during the following
night. Much the same conditions of sudden temperature change with
nightfall are experienced in high mountains when one has ascended above
the blanketing clouds.
[Illustration:
FIG. 207.—Borax deposits upon the floor of Death valley, California
(after a photograph by Fairbanks).]
=Dry weathering—the red and brown desert varnish.=—In desert lands
the fierce rays of the sun suck up all the available moisture, and
the water table may be hundreds of feet below the surface. Roots of
trees a hundred feet or more in length have been found to testify to
the fierce struggle of the desert plant with the arid conditions. In
humid regions the meteoric water dissolves the more soluble sodium
salts near the surface of the rock and carries them out to the ocean,
where they add to the saltness of the sea. In the desert the rare
precipitations prevent an outflow, but the sun’s strong rays suck out
with the moisture the salts from within the rock, and evaporating upon
the surface, the salts are left as a coat of “alkali”, which is in part
carried away on the wind and in part washed off in one of the rare
cloudbursts. In either case these constituents find their way to the
lowest depressions of the basin, where they contribute to the saline
deposits of the desert lakes (Fig. 207).
[Illustration: FIG. 208.—Hollowed forms of weathered granite in a
desert of central Asia (after Walther).]
Certain of the saline constituents of the rocks, as they are thus
drawn out by the sun’s rays, fuse with the rock at the surface to form
a dense brown substance with smooth surface coat, known as _desert
varnish_. Within the interior a portion of the salts crystallize within
the capillary fissures, and like water freezing within a pipe, they
rend the walls apart. As a direct consequence of this disintegrating
process the interior of rock masses may crumble into sand; and if the
hard shell of varnish be broken at any point, the wind makes its
entrance and removes the interior portion so as to leave a hollow
shell—the characteristic “pocket rock” (Fig. 208) of the desert. The
nummulitic limestone of Mokkatan and many of the great hewn blocks of
Egyptian limestone sound hollow under the tap of the hammer, and when
broken, they reveal a shell a few inches only in thickness (Fig. 209).
[Illustration: FIG. 209.—Hollow hewn blocks in a wall in the Wadi
Guerraui (after Walther).]
The brown desert varnish is one of the most characteristic marks
of an arid country. It is found in all deserts under much the same
conditions, and is especially apt to be present in sandstone. When
scratched, the surface of the rock becomes either cherry-red,
indicating anhydrous ferric oxide, or it is yellowish, due to the
hydrated iron oxide which we know as iron rust. Thus it is seen that
the sands of deserts, in contrast to those yielded by other processes
within humid regions, have a characteristic red color, and this may
vary from brownish red upon the one hand to a rich carmine upon the
other.
=The mechanical breakdown of the desert rocks.=—The chemical changes
of decomposition within desert rocks are, as we have seen, largely due
to the action of concentrated solutions of salts at high temperatures.
That there is a certain mechanical rending of these rocks, due to the
“freezing” of salts within the capillary fissures, has been already
mentioned. A further strain effect arises in rocks like granite, which
are a mixture of different minerals. Heated to a high temperature
during the day and cooled through a considerable range at night, the
different minerals alternately expand and contract at different rates
and by different relative amounts, so that strains are set up, tending
to tear them apart. The effect of these strains is thus a surface
crumbling of rocks.
But rock is, as already pointed out, a relatively poor conductor of
heat, and hence it is a relatively thin skin only which passes through
the daily round of wide temperature range. This outer shell when heated
is expanded, and so tends to peel off, or exfoliate, like the outer
skin of an onion. The process is therefore described as _exfoliation_.
In all rocks of homogeneous texture the continued action of this
process results in convexly spherical surfaces, the material scaled
off in the process remaining as a slope or talus which surrounds the
projecting knob (Fig. 210). Naked, these projecting domes rise above
the rim of débris at their bases. Not a particle of dust adheres to the
fresh rock surface—no dirt interferes with its glaring whiteness. Yet
close at hand lie masses of débris into which wells may be carried to
depths of more than six hundred feet without encountering either solid
rock or ground water. The bare walls of granite sometimes mount upwards
for thousands of feet into the air, as steep and as inaccessible as the
squared towers of the Tyrolean Dolomites.
[Illustration: FIG. 210.—Smooth granite domes shaped by exfoliation
and surrounded by a rim of talus. Gebel Karsala, Nubian Desert (after
Walther).]
Rock is such a poor conductor of heat that special strains are set
up at the margin of sunlight and shade. This localization of the
disintegration on the margin of the shaded portions of rock masses is
known as _shadow weathering_ (see Fig. 215, p. 206).
There is, however, still another mechanical disintegrating process
characteristic of the desert regions, which is likewise dependent
upon the sudden changes of temperature. Rains, though they may not
occur for a year or more, come as sudden downpours of great volume and
violence. Rock masses, which are highly heated beneath the desert sun,
if suddenly dashed with water, may be rent apart by the differential
strains set up near the surface. That rocks may be easily rent as a
result of sudden chilling is well known to our Northern farmers, who
are accustomed to rid themselves of objectionable bowlders by first
building a fire about them and then dashing water upon their surface.
Thus split into fragments, even the larger bowlders may be handled and
so removed from the farming land. The natural process of rock rending
by the occasional cloudburst may be described as _diffission_. Blocks
as much as twenty-five feet in diameter have been observed in the
desert of western Texas, soon after being broken into several fragments
at the time of a downpour of rain (Fig. 211).
[Illustration:
FIG. 211.—Granite blocks in the Sierra de los Dolores of Texas, rent
into several fragments by the dash of rain (after Walther).]
=The natural sand blast.=—Because of the saucer-like shape, the vast
expanse, and the absence of wind breaks, the potency of wind as a
geological agent is in desert areas not easily overestimated. While
most of its work is accomplished with the aid of tools, it has been
proven that even without this help, considerable work is done through
the friction of the wind alone, particularly when moving as powerful
eddies in cracks and crannies. This wear of the wind, unaided by
cutting tools, is known as _deflation_.
The greater work of the wind is, however, accomplished with the aid
of larger or smaller rock particles, the sand and dust, with which it
is so generally charged above the deserts. Unprotected by any mat of
vegetation the materials of the desert surface are easily lifted and
are constantly migrating with the wind. The finest dust is raised high
into the air, and is carried beyond the marginal barriers, but none of
the sand or coarser materials ever passes beyond the borders.
[Illustration:
FIG. 212.—“Mushroom rock” from a desert in Wyoming (after Fairbanks).]
The efficiency of this sand as a cutting tool when carried by the wind
is directly proportioned to the size of the grain, since with larger
fragments a heavier blow is struck when carried at any given velocity.
These more effective grains are, however, not lifted far above the
ground, but advance with a squirming or hopping motion, much as do
the larger pebbles upon the bottom of a river at the time of a spring
freshet. To quote Professor Walther: “Whoever has had the opportunity
to travel over a surface of dune sand when a strong wind is blowing has
found it easy to convince himself of the grinding action of the wind.
At such times the ground becomes alive, everywhere the sand is creeping
over the surface with snake-like squirmings, and the eye quickly tires
of these writhing movements of the currents of sand and cannot long
endure the scene.”
[Illustration:
FIG. 213.—Windkanten shaped by the desert sand blast (after Chamberlin
and Salisbury).]
A direct consequence of this restriction of the more effective cutting
tools to the layer of air just above the ground, is the strong tendency
to cut away all projecting masses near their bases. The “mushroom
rocks”, which are so characteristic of desert landscapes, have been
shaped in this manner (Fig. 212). Another product of the desert
sand blast is the so-called _Windkante_ (wind-edge) or _Dreikante_
(three-edge), a pebble which is usually shaped in the form of a pyramid
(Fig. 213).
Whenever a rock face, open to direct attack by the drifting sand, is
constituted of parts which have different hardness, the blast of sand
pecks away at the softer places and leaves the harder ones in relief.
Thus is produced the well-known “stone lattice” of the desert (Fig.
214). Particularly upon the neck of the great Sphinx have the flying
sand grains, by removing the softer layers, brought the sedimentary
structures of the sandstone into strong relief.
[Illustration:
FIG. 214.—The “stone lattice” of the desert, the work of the natural
sand blast (after Walther).]
When guided both by planes of sedimentation and planes of jointing,
forms of a very high degree of ornamentation are developed. Some of
the most remarkable forms are due to the protection afforded to the
sun-exposed surfaces by the shell of desert varnish. In the shaded
portions of projecting masses there is no such protection, and here the
sand blast insinuates itself into every crack and cranny. In this it is
aided by shadow weathering due to the differential strains set up at
the border of the expanded sun-heated surface. As a result, projecting
rock masses are sometimes etched away beneath and give the effect of a
squatting animal. These forms, due to shadow erosion, have also been
likened to projecting faucets. (Fig. 215).
[Illustration:
FIG. 215.—Projecting rock carved by the drifting sand into the form of
a couchant animal as a result of shadow weathering and erosion. Cut in
granite on the north Indian Desert (after Walther).]
Worn by its impact upon neighboring sand grains while in transport, but
much more as it is thrown against the ground or hard rock surfaces,
the wind-driven or _eolian_ sand is at last worn into smoothly rounded
granules which approach the form of a sphere. Compared to the surface
which sea sand acquires by attrition, this shaping process is much
the more efficient, since in the water the beach sand is buoyed up
and is more effectively cushioned against its neighboring grains. The
grains of beach sand when examined under a microscope are found to be
much more irregular in form and usually display the original fracture
surfaces only in part abraded.
[Illustration:
FIG. 216.—Cliffs in loess 200 feet in height which exhibit the
characteristic vertical jointing (after von Richtofen).]
=The dust carried out of the desert.=—When, standing upon the mountain
wall that surrounds a desert, the traveler gazes out to windward over
the great depression, his field of view is generally obscured by the
yellow haze of the dust clouds moving across the margins. Upon the
mountain flanks and extending far outside the borders, this cloud of
dust settles as a shrouding mantle of impalpable yellow powder, which
is known as _loess_. These deposits are continually deepening, and
have sometimes accumulated until they are hundreds or even thousands
of feet in thickness. Before reaching its final resting place the dust
of this deposit may have settled many times, and has certainly been in
part redistributed by the streams near the desert margin. In it are
the ingredients which are necessary for the nourishment of plants, and
it constitutes the most important of natural soils. Continually fed by
new deposits from the desert, and refertilized from below by a natural
process so soon as the upper layers become impoverished, it requires no
artificial fertilization. Without artificial aids the loess of northern
China has been tilled for thousands of years without any signs of
exhaustion.
[Illustration:
FIG. 217.—A cañon in loess worn by traffic and wind. A highway in
northern China (after von Richtofen).]
Though easily pulverized between the fingers, loess is none the less
characterized by a perfect vertical jointing and stands on vertical
faces as does the solid rock (Fig. 216), but it is absolutely devoid
of layers or bedding. Its capacity of standing in vertical cliffs the
loess owes to a never failing content of lime carbonate which acts as a
cement, and to a peculiar porous structure caused by capillary canals
that run vertically through the mass, branching like rootlets and lined
with carbonate of lime. This texture once destroyed, loess resolves
itself into a common sticky clay.
By the feet of passing animals or by wheels of vehicles, the loess is
crushed, and a portion is lifted and carried away by the wind. Thus in
the course of time roadways sink deep into the mass as steep-walled
cañons (Fig. 217). A portion of the now structureless clay remaining
upon the roadway is at the time of the rains transformed into a thick
mud which makes traveling all but impossible, though before its
structure has been destroyed the loess is perfectly drained to the
bottom of its deposits.
The particles which compose the loess are sharply angular quartz
fragments, so fine that all but a few grains can be rubbed into the
pores of the skin. Fine scales of mica, such as are easily lifted by
the wind, are disseminated uniformly throughout the mass. The only
inclosures which are arranged in layers consist of irregularly shaped
concretions of clay. These show a striking resemblance to ginger roots
and are called by the Chinese “stone ginger”, though they are elsewhere
more generally known by their German name of _Loessmännchen_, or loess
dolls. These concretions are so disposed in the loess that their longer
axes are vertical, and they were evidently separated from the mass and
not deposited with it.