THE FEATURES IN DESERT LANDSCAPES
[Illustration:
FIG. 218.—Diagrams to illustrate the effects of obstructions of
different types in arresting wind-driven sand. _a_, An unyielding
obstruction which permits the wind to pass through it; _b_, a flexible
and perforated obstruction; _c_, an unyielding closed barrier (after
Schulze).]
=The wandering dunes.=—Over the broad expanse of the desert, sand
and dust, and occasionally gypsum from the saline deposits, are ever
migrating with the wind; on quiet days in the eddying “sand devils”,
but especially during the terrifying sand storms such as in the windy
season darken the air of northern China and southern Manchuria. This
drift of the sand is halted only when an obstruction is encountered—a
projecting rock, a bush, or a bunch of grass, or again the buildings of
a city or a town. The manner in which the sand is arrested by obstacles
of different kinds is of great interest and importance, and is utilized
in raising defenses against its encroachments. If the obstacle is
unyielding but allows some of the wind to pass through it, no eddies
are produced and the sand is deposited both to windward and to leeward
of the obstruction to form a fairly symmetrical mound (Fig. 218 _a_).
An obstruction which yields to the wind causes the sand to deposit
in a mound which is largely to leeward of the obstruction (Fig. 218
_b_). A solid wall, on the other hand, by inducing eddies, is at first
protected from the sand and mounds deposit both to windward and to
leeward (Fig. 218 _c_ and Fig. 219).
Except when held up by an obstruction, the drifting sand travels
to leeward in slowly migrating mounds or ridges which are known as
_dunes_. Their motion is due to the wind lifting the sand from the
windward side and carrying it over the crest, from where it slides down
the leeward slope and assumes a surface which is the angle of repose
of the material. In contrast with this the windward slope is notably
gradual, being shaped in conformity to the wind currents.
[Illustration:
FIG. 219.—Sand accumulating both to windward and to leeward of a firm
and impenetrable obstruction. The wind comes from the left (after a
photograph by Bastin).]
The dunes which are raised upon seashores, like those of the desert,
are constantly migrating, those upon the shores of the North Sea at the
average rate of about twenty feet per year. Relentlessly they advance,
and despite all attempts to halt them, have many times overwhelmed the
villages along the coast. Upon the great barrier beach known as the
_Kurische Nehrung_, on the southeastern shore of the Baltic Sea, such
a burial of villages has more than once occurred, but as in the course
of time further migration of the dune has proceeded, the ruins of the
buried villages have been exhumed by this natural excavating process
(Fig. 220).
[Illustration:
FIG. 220.—Successive diagrams to show how the town of Kunzen was
buried, and subsequently exhumed in the continued migration of a great
dune upon the Kurische Nehrung (after Behrendt).]
┌─────────────────────────────────────────────────────────────────────┐
│ PLATE 7. │
│ │
│ [Illustration: _A._ Ranges of dunes upon the margin of the Colorado │
│ Desert (after Mendenhall).] │
│ │
│ [Illustration: _B._ Sand dunes encroaching upon the oasis of Wed │
│ Souf. Algeria (after T. H. Kearney).] │
└─────────────────────────────────────────────────────────────────────┘
=The forms of dunes.=—The forms assumed by dunes are dependent to
a very large extent upon the strength of the wind and the available
supply of sand. With small quantities of sand and with moderate winds,
sickle-shaped dunes known as _barchans_ (Fig. 221) are formed, whose
convex and flatter slopes are toward the wind and whose steep concave
leeward slopes are maintained at the angle of repose. The barchan is
shaped by the wind going both over and around the dune, constantly
removing sand from the windward side and depositing it to leeward.
With larger supplies of sand and winds which are not too violent a
series of barchans is built up, and these are arranged transversely to
the wind direction (Fig. 222 _b_). If the winds are more violent, the
minor depressions in the crests of the dunes become wind channels, and
the sand is then trailed out along them until the arrangement of the
ridges is parallel to the wind (Fig. 222 _c_). The surfaces of dunes
are generally marked by beautiful ripples in the sand, which, seen from
a little distance, may give the appearance of watered silk (plate 7 A).
[Illustration: FIG. 221.—View of desert barchans (after Haug).]
[Illustration:
FIG. 222.—Diagrams to show the relationships in form and in
orientation of dunes to the supply of sand and to the strength of the
wind. _a_, barchans formed by small supplies of sand and moderate
winds; _b_, transverse dune ridges, formed when supply of sand is large
and winds are moderate; _c_, dune ridges formed with large sand supply
and violent winds (after Walther and Cornish).]
Under normal conditions dunes are not stationary but continue to wander
with the prevailing winds until they have reached the outer edge of
the zone of vegetation near the base of the foothills at the margin of
the desert. Here the grasses and other desert plants arrest the first
sand grains that reach them, and they continue to grow higher as the
sands accumulate. Some of the desert plants, like the yuccas, have so
adapted themselves to desert conditions that they may grow upward with
the sand for many feet and so keep their crowns above its surface.
=The cloudburst in the desert.=—Such clouds as enter the desert
through its mountain ramparts, and those derived from evaporation
from the hot desert soil, usually precipitate their moisture before
passing out of the basin. Above the highly heated floor the heavy
rain clouds are unable to drop their burden. The rain can sometimes
be seen descending, but long before it has reached the ground it has
again passed into vapor, and through repetition of this process the
clouds become so charged with moisture that when they encounter a
mountain wall and are thus forced to rise, there is a sudden downpour
not equaled in the humid regions. Desert rains are rare, but violent
beyond comparison. Often for a year or more there is no rainfall upon
the loose sand or porous clay, and the few plants which survive must
push their roots deep down until they have reached the zone of ground
water. When the clouds burst, each small cañon or _wed_ (pl. _wadi_)
within the mountain wall is quickly occupied by a swollen current
which carries a thick paste of sediment and drowns everything before
it. Ere it has flowed a mile, it may be that the water has disappeared
entirely, leaving a layer of mud and sand which rapidly dries out with
the reappearance of the sun.
[Illustration: FIG. 223.—Ideal section across the rising mountain wall
surrounding a desert and a part of the neighboring slope (after R. W.
Pumpelly).]
As the mountains upon seacoasts are generally rising with reference to
the neighboring sea bottom, so the mountains which hem in the deserts
are generally growing upward with reference to the inclosed desert
floor. The marginal dislocations which separate the two are often in
evidence at the foot of the steep slope (Fig. 223), and these may even
appear as visible earthquake faults to indicate that the uplift is
more accelerated than the deposition along the mountain front.
[Illustration:
FIG. 224.—Dry delta or alluvial fan at the foot of a mountain range
upon the borders of a desert.]
=The zone of the dwindling river.=—The rapid uplift so generally
characteristic of desert margins gives to the torrential streams which
develop after each cloudburst such an unusual velocity that when they
emerge from the mountain valleys on to the desert floor, the current
is suddenly checked and the burden of sediment in large part deposited
at the mouth of the valley so as to form a coarse delta deposit which
is described as a _dry delta_ (Fig. 224). Dependent upon its steepness
of slope, this delta is variously referred to as an _alluvial fan_
or _apron_, or as an _alluvial cone_. Over the conical slopes of the
delta surface the stream is broken up into numerous distributaries
which divide and subdivide as do the roots of a tree. In the Mohammedan
countries described as _wadi_, these distributaries upon dry deltas are
on the Pacific coast of the United States referred to as “washes” (Fig.
225).
[Illustration: FIG. 225.—Map of the distributaries of neighboring
streams which emerge at the western base of the Sierra Nevadas in
California (after W. D. Johnson).]
Fast losing their velocity after emerging from the mountains, the
various distributaries drop first of all the heavy bowlders, then
the large pebbles and the sand, so that only the finer sand and the
silt are carried to the margin of the delta. As they enlarge their
boundaries, the neighboring deltas eventually coalesce and so form an
_alluvial bench_ or “gravel piedmont” at the foot of the range. Only
the larger streams are able to entirely cross this bench of parched
deposits with its coarsely porous structure, for the water is soon
sucked up by the thirsty materials. Encountering in its descent more
clayey layers, this water is conducted to the surface near the margin
of the bench and may there appear as a line of springs. At this level
there develops, therefore, a zone of vegetation, though there is no
local rain.
The alluvial bench grows upward by accretion of layers which are
thickest at the mountain end, so that the steepness of the bench
increases with time.
=Erosion in and about the desert.=—The violent cloudburst that is
characteristic of the arid lands is a most potent agent in modeling
the surface of the ground wherever the rock materials are not too
firmly coherent. Under the dash of the rain a peculiar type of “bad
land” topography is developed (plate 5 B and Fig. 226). Such a
rain-cut surface is a veritable maze of alternating gully and ridge, a
country worthless for agricultural purposes and offering the greatest
difficulty in the way of penetrating it. When composed of stiff clay
with scattered pebbles and bowlders, the effect of the “rain erosion”
is to fashion steep clay pillars each capped by a pebble and described
as “demoiselles” (Fig. 226).
[Illustration:
FIG. 226.—A group of “demoiselles” in the “bad lands” (after a
photograph by Fairbanks).]
Behind the mountain front the valleys out of which the torrents are
discharged are usually short with steep side walls and a relatively
flat bottom, ending headward in an amphitheater with precipitous walls
(Fig. 227). In the western United States such valleys are referred to
as “box cañons”, but in Mohammedan countries the name “wed” applies to
the river valley within the mountains and to the distributaries as well.
=Characteristic features of the arid lands.=—It is characteristic of
erosion and deposition within humid regions that all outlines become
softened into flowing curves, due to the protective mat of vegetation.
In arid lands those massive rocks which are without structural planes
of separation, partly as a consequence of exfoliation, develop broad
domes which are projected upon the horizon as great semicircles,
broken in half it may be by displacement. The same massive rocks where
intersected by vertical joint planes yield, on the contrary, sharp
granite needles like those of Harney Peak (plate 8 A). Similarly,
schistose or bedded rocks, when tilted at a high angle, may yield forms
which are almost identical. Examples of such needles are found in the
Garden of the Gods in Colorado.
At lower levels, where the flying sand becomes effective as an eroding
agent, flat bedded rocks become etched into shelves and cornices, and
if intersected by joints, the shelves and cornices are transformed
into groups of castellated towers and pinnacles of a high degree of
ornamentation. These fantastic erosion remnants are usually referred to
as “chimneys” and may be seen in numbers in the bad lands of Dakota, as
they may in Colorado either in Monument Park or in the new Monolithic
National Park (plate 8 B).
[Illustration:
FIG. 227.—Amphitheater at the head of the Wed Beni Sur (after
Walther).]
Where wind erosion plays a smaller rôle in the sculpture, but where
after an uplift a river has made its way, horizontally bedded rocks
are apt to be carved into broad _rock terraces_, nowhere shown upon so
grand a scale as about the Grand Cañon of the Colorado. Each harder
layer has here produced a floor or terrace which ends in a vertical
escarpment, and this is separated from the next lower layer of more
resistant rock by a slope of talus which largely hides the softer
intermediate beds. The great Desert of Sahara is shaped in a series of
rock terraces or steppes which descend toward the interior of the basin.
A single harder layer of resistant rock comes often to form the
flat capping of a plateau, and is then known as a _mesa_, or table
mountain. Along its front, detached outliers usually stand like
sentinels before the larger mass, and according to their relative
proportions, these are referred to either as small mesas or as the
smaller _buttes_ (Fig. 228).
[Illustration: FIG. 228.—Mesa and outlying butte in the Leucite Hills
of Wyoming (after Whitman Cross, U. S. G. S.).]
=The war of dune and oasis.=—In every desert the deposits are arranged
in consecutive belts or zones which are alternately the work of wind
and water. Surrounding the desert and upon the flanks of the mountain
wall there is found (1) the deposit of loess derived from the dust that
is carried out of the desert by the wind. Immediately within the desert
border at the base of the mountains is (2) the zone of the dwindling
river with its sloping bench of coarse rubble and gravel.
[Illustration: FIG. 229.—Flat-bottomed basin separating dunes—_bajir_
or _takyr_ (after Ellsworth Huntington).]
Next in order is (3) the belt of the flying sand, a zone of dune
ridges often separated by narrow, flat-bottomed basins (Fig. 229) into
which the strongest streams bring the finer sands and silt from the
mountains. Lastly, there is (4) the central sink or sinks, into which
all water not at once absorbed within the zone of alluviation or in
the zone of dunes is finally collected. Here are the true lacustrine
deposits of clay and separated salts (Fig. 230 and Fig. 207, p. 201).
The lake deposits fill in all the original irregularities of the desert
floor, out of which the tops of isolated ranges of mountains now
project like islands out of the surface of the sea. The several zones
of deposits in their order from the margin to the center of the desert
are given schematically in Fig. 231.
┌─────────────────────────────────────────────────────────────────────┐
│ PLATE 8. │
│ │
│ [Illustration: _A._ The granite needles of Harney Peak in the Black │
│ Hills of South Dakota (after Darton).] │
│ │
│ [Illustration: _B._ Castellated erosion chimneys in El Cobra Cañon, │
│ New Mexico. │
│ (_Photograph by E. C. Case._)] │
└─────────────────────────────────────────────────────────────────────┘
[Illustration:
FIG. 230.—Billowy surface of the salt crust on the central sink in the
Lop Desert of central Asia (after Ellsworth Huntington).]
The zone of vegetation, as already stated, lies near the foot of
the alluvial bench, so that here are found the oases about which
have clustered the cities of the desert from the earliest records of
antiquity until now. Just without the line of oases is the wall of
dunes held back from further advance only by the vegetation which in
turn is dependent upon the rains in the neighboring mountains. With
every diminution in the water supply, the dunes advance and encroach
upon the oases (plate 7 B); while with every considerable increase in
this supply of moisture the alluvial bench advances over the dunes and
acquires a strip of their territory. Thus with varying fortunes a war
is continually waged between the withering river and the flying sand,
and the alternations of climate are later recorded in the dovetailing
together of the eolian and alluvial deposits at their common junction
(Fig. 231).
[Illustration: FIG. 231.—Schematic diagram to show the zones of
deposition in their order from the margin to the center of a desert.]
[Illustration:
FIG. 232.—Mounds upon the site of the buried city of Nippur (after the
cast by Muret).]
In addition to the smaller periodic alternations of pluvial and
interpluvial climate—the “pulse of Asia”—the record of the Asiatic
deserts indicates a progressive desiccation of the entire region, which
has now given the victory to the dune. The ancient history of the
cities of the plains supplies the records of many that have been buried
in the dunes. To-day, where once were prosperous cities, nothing is to
be seen at the surface but a group of mounds (Fig. 232). Exhumed after
much painstaking labor, the walls and palaces of these ancient cities
have once more been brought to the light of day, and much has thus been
learned of the civilization of these early times (Fig. 233). Quite
recently the mounds which cover between one and two hundred buried
villages have been found upon the borders of the Tarim basin of central
Asia, where they were lost to history when they were overwhelmed in the
early centuries of the Christian Era.
[Illustration: FIG. 233.—Exhumed structures in the buried city of
Nippur (after Hilprecht).]
=The origin of the high plains which front the Rocky Mountains.=—To
the eastward of the great backbone of the North American continent
stretches a vast plain gently inclined away from the range and
generally known as the High Plains region (plate 9). The tourist who
travels westward by train ascends this slope so gradually that when he
has reached the mountain front it is difficult to realize that he has
climbed to an altitude of five thousand feet above the level of the
sea. That he has also passed through several climatic zones—a humid, a
semiarid, and an arid—and has now entered a semiarid district, is more
easily appreciated from study of the vegetation (Fig. 234). The surface
of the High Plains, where not cut into by rivers, is remarkably even,
so that it might be compared to the quiet surface of a great lake.
[Illustration: FIG. 234.—Section across the High Plains, showing the
position of the terrace and the climatic zones above it (after W. D.
Johnson).]
The materials which compose the surface veneer of these plains are
coarse conglomerates, gravels, and sands, and the so-called “mortar
beds”, which are nothing but sands cemented into sandstone by carbonate
of lime. The pebbles in all these deposits are far-traveled and appear
to have been derived from erosion of those crystalline rocks which
compose the eastern front of the Rocky Mountains. These different
materials are not arranged in strictly parallel beds, as are the
deposits of a lake or sea; but the beds are made up of long threads of
lenticular cross section which are interlaced in the most intricate
fashion and which extend down the slope, or outward from the mountain
front (Fig. 235). It is thus shown that the High Plains are a bench or
plain of alluviation formed at the front of the Rocky Mountains during
an earlier series of pluvial periods, and that subsequent uplift has
produced the modern river valleys which are cut out of the plain. The
plexus of long threads of the coarser materials are the courses of
dwindling rivers which interlaced over the former plain, and which in
time were buried under other channel deposits of the same nature but in
different positions (Fig. 236). The pluvial periods in which this bench
was formed produced in other latitudes the great continental glaciers
which wrought such marvelous changes in northern North America and in
northern Europe.
[Illustration: FIG. 235.—Section across the great lenticular threads
of alluvial deposits which compose the veneer of the High Plains (after
W. D. Johnson).]
[Illustration: FIG. 236.—Distributaries of the foothills superimposed
upon an earlier series (after W. D. Johnson).]
=Character profiles.=—In contrast with the profiles in the landscapes
of humid regions (see Fig. 187, p. 177), those of arid lands are
marked by straighter elements (Fig. 237). Almost the only exception
of importance is furnished by the domes of massive granite monoliths,
which are sometimes broken in half by great displacements. Below
the horizon the secondary lines in the landscape betray the same
straightness of the component elements by the gabled slopes of talus
which are many times repeated so as almost to reproduce the lines in
a house of cards, since the sloping lines are maintained at the angle
of repose of the materials (Fig. 482, p. 443). Wherever the waves of
desert lakes have made an attack upon the rocks and have retired the
projecting spurs, other gables characterized by slightly different
slopes are introduced into the landscape.
[Illustration: FIG. 237.—Character profiles in the landscapes of arid
lands.]
┌─────────────────────────────────┐
│ PLATE 9. │
│ │
│ [Illustration: THE HIGH PLAINS] │
└─────────────────────────────────┘
READING REFERENCES FOR CHAPTERS XV AND XVI
General:—
JOHANNES WALTHER. Das Gesetz der Wüstenbildung in Gegenwart und
Vorzeit. Berlin, 1900, pp. 175, many plates. (This extremely valuable
work is now out of print, but both a revised edition and an English
translation are promised for 1912.)
SIEGFRIED PASSARGE. Die Kalihari. Berlin, 1904, pp. 662.
W. M. DAVIS. The Geographic Cycle in an Arid Climate, Jour. Geol.,
vol. 13, 1905, pp. 381-407.
ELLSWORTH HUNTINGTON. The Pulse of Asia. New York and Boston, 1907,
pp. 415.
SVEN HEDIN. Scientific Results of a Journey through Central Asia,
1899-1900. Stockholm, 1904-1905, vols. 1 and 2, pp. 523 and 717, pls.
56 and 76.
JOSEPH BARRELL. Relative Geological Importance of Continental,
Littoral and Marine Sedimentation, Jour. Geol., vol. 14, 1906, pp.
316-356, 429-457, 524-568.
E. F. GAUTIER. Études sahariennes, Ann. de Géogr., vol. 16, 1907, pp.
46-69, 117-138.
The self-registering gauge of past climates:—
G. K. GILBERT. Lake Bonneville, Mon. I, U. S. Geol. Surv., Chapter vi,
pp. 214-318.
T. F. JAMIESON. The Inland Seas and Salt Lakes of the Glacial Period,
Geol. Mag. decade III, vol. 2, 1885, pp. 193-200.
J. E. TALMAGE. The Great Salt Lake, Present and Past. Salt Lake City,
1900, pp. 116, plates.
E. HUNTINGTON. Some Characteristics of the Glacial Period in
Non-glaciated Regions, Bull. Geol. Soc. Am., vol. 18, 1907, pp.
351-388, pls. 31-39.
T. C. CHAMBERLIN. The Future Habitability of the Earth, Rept.
Smithson. Inst., 1910, pp. 371-389.
The red and brown desert varnish:—
I. C. RUSSELL. Subaërial Decay of Rocks and Origin of the Red Color of
Certain Formations. Bull. 52, U. S. Geol. Surv., 1889, pp. 65, pls. 5.
Erosion in the desert:—
J. A. UDDEN. Erosion, Transportation, and Sedimentation performed by
the Atmosphere, Jour. Geol., vol. 2, 1894, pp. 318-331.
S. PASSARGE. Die pfannenförmigen Hohlformen der südafrikanischen
Steppen, Pet. Mitt., vol. 57, 1911, pp. 57-61, 130-135.
The dust carried out of the desert:—
F. VON RICHTOFEN. China, Ergebnisse eigene Reisen und darauf
gegründeten Studien, Berlin, 1877, vol. 1, pp. 56-125.
E. HILGARD. The Loess of the Mississippi Valley, Am. Jour. Sci., (3),
vol. 18, 1879, pp. 106-112.
T. C. CHAMBERLIN and R. D. SALISBURY. Preliminary Paper on the
Driftless Area of the Upper Mississippi Valley, 6th Ann. Rept. U. S.
Geol. Surv., 1885, pp. 278-307.
E. E. FREE. The movement of soil material by the wind, with a
bibliography of eolian geology by S. C. Stuntz and E. E. Free, Bull.
68, U. S. Bureau of Soils, 1911, pp. 272, pls. 5.
M. NEUMAYR. Erdgeschichte, vol. 1, pp. 510-514.
E. DE MARTONNE. Géographie physique, pp. 663-668.
Dunes:—
VAUGHAN CORNISH. On the Formation of Sand-dunes, Geogr. Jour., vol. 9,
1897, pp. 278-309 (a most important paper).
F. SOLGER and Others. Dünenbuch. Enke, Stuttgart, 1910, pp. 373.
The zone of the dwindling river:—
E. HUNTINGTON. The Border Belts of the Tarim Basin, Bull. Am. Geogr.
Soc., vol. 38, 1906, pp. 91-96; The Pulse of Asia, pp. 210-222,
262-279.
The war of dune and oasis:—
R. PUMPELLY. Explorations in Turkestan, Expedition of 1904, etc., Pub.
73, Carneg. Inst., Washington, vol. 1, pp. 1-13.
E. HUNTINGTON. The Oasis of Kharga, Bull. Am. Geogr. Soc., vol. 42.
1910, pp. 641-661.
TH. H. KEARNEY. The Country of the Ant Men, Nat. Geogr. Mag., vol. 22,
1911, pp. 367-382.
Features of the arid lands:—
C. E. DUTTON. Tertiary History of the Grand Cañon District, with
Atlas, Mon. II, U. S. Geol. Surv., 1882, pp. 264, pls. 42, maps 23.
G. SWEINFURTH. Map Sheets of the Eastern Egyptian Desert. Berlin,
1901-1902, 8 sheets.
The origin of the high plains:—
W. D. JOHNSON. The High Plains and their Utilization, 21st Ann. Rept.
U. S. Geol. Surv., Pt. iv, 1901, pp. 601-741.