landforms that originate under the action of wind, primarily in regions with arid climates (deserts and semideserts). Eolian landforms are also found along the shores of seas, lakes, and rivers with sparse vegetation, which is not capable of protecting unconsolidated and weathered substrate rocks against the action of the wind. The most widespread forms are the aggradational and aggradational-defiational forms, which occur as a result of the transport and deposition of sand particles by the wind, and the deflational eolian landforms, which occur as a result of the deflation of unconsolidated products of weathering and the destruction of rocks by the action of the dynamic force of the wind itself and especially by abrasion by small particles carried by the wind.
The shape and size of aggradational and aggradational-defiational formations depend on the wind regime (force, frequency, direction, and structure of the wind stream) predominating in a given locale in the past and present, the degree of saturation of the wind-sand stream with sand particles, the extent to which vegetation holds the unconsolidated substrate in place, moisturization, and other factors, as well as on the nature of the underlying relief. In sandy deserts, the eolian landforms were carved out by strong winds, acting like a stream of water with turbulent motion of the medium close to the solid surface. For medium- and fine-grain dry sand (grain diameters of 0.5–0.25 mm), the minimum velocity of a strong wind is 4 m/sec. Aggradational and deflational-aggradational forms generally shift in the direction of seasonal prevailing winds: advancing where strong winds blow from one or several closely aligned directions year-round and fluctuating or changing direction of progression if the wind directions change significantly in the course of the year (for example, in the opposite direction or perpendicularly). Denuded sandy aggradational forms are shifted especially fast, up to several dozen meters a year.
Aggradational and deflational-aggradational eolian landforms in deserts are characterized by the simultaneous presence of forms of several levels of magnitudes superimposed on one another: first level—wind ripples, measuring from fractions of 1 mm to 0.5 m in height, with a distance ranging from a few millimeters to 2.5 m between crests; second level—shield-shaped accumulations, measuring at least 40 cm in height; third level—barchans, measuring up to 2–3 m in height and connected into a ridge extending parallel to the wind or a barchan chain extending cross-wind; fourth level—barchan landforms, measuring 10–30 m in height; and fifth and sixth levels—large forms, measuring up to 500 m in height and created chiefly by ascending air currents. In the deserts of the temperate zone, where vegetation plays a large role by retarding the work of the wind, the relief develops more slowly and the largest landforms are no higher than 60–70 m. The most typical forms in these regions are small tongues in lee of shrubs, hillock-spits, and small mounds formed near brush, ranging in height from a few decimeters to 10–20 m.
Since the prevailing wind regime (trade winds, monsoon breezes, cyclonic winds) and the degree of cementation of the un-consolidated substrate are determined mainly by zonal geographic factors, aggradational and aggradational-deflational eolian landforms are on the whole distributed by zones. According to the classification proposed by the Soviet geographer B. A. Fedorovich (1964), barren, highly mobile sandy forms are mainly characteristic of the tropical extra-arid deserts (the Sahara, the Takla Makan, and the deserts of the Arabian Peninsula, Iran, and Afghanistan); the relatively immobile scrub-growth forms are found primarily in the extratropical deserts of Middle Asia, Kazakhstan, Dzungaria, Mongolia, and Australia; and the overgrown dune forms, which are virtually immobile, occur in non-desert regions, primarily in the ancient glaciated regions of Europe, Western Siberia, and North America. (A detailed classification of aggradational and deflational-aggradational landforms that are dependent on wind regime are given in DUNES and BARCHANS. Table 1 gives a similar classification of aggradational and aggradational-deflational eolian landforms for scrub-growth sandy deserts.)
|Table 1. Classification of desert scrub-growth landforms and types of wind regimes and movements of sand forms1|
|1Compiled by B. A. Fedorovich|
|(I) Classification of desert scrub-growth landforms according to wind regime|
|(A) Simple forms of the lowest levels of magnitude (predominantly small, young, single, and group forms with some entire fields)|
|(B) Complex forms of various levels of magnitude (with large old forms and a prevalence of entire fields)|
|(II) Types of wind regimes and movements of sandy landforms|
|(A) Steady trade-wind regime with marked prevalence of one wind direction or several close directions and advancing movement of sand or landforms parallel to the wind|
|(1) Linear forms|
|(a) Tongues in lee of shrubs|
|(b) Single ridge or a group of ridges|
|(c) Small ridges|
|(d) Small and large ridge relief|
|(2) Forms not strictly linear|
|(b) Single ridge or group of ridges|
|(c) Small ridges|
|(d) Small and large ridge (patterned) relief|
|(B) Seasonal trade-wind regime with alternation of winds that are close to mutually perpendicular and advancing movement of landforms along the resulting line of force of the winds|
|(C) Monsoon breeze (prefrontal) regime, with seasonal alternation of winds in opposite directions and fluctuating movement of landforms perpendicularly to the winds|
|(D) Interference regime with seasonal alternation of mutually perpendicular winds and movement of sands|
|(E) Convection-cyclonic regime with alternation of winds of different directions and similar intensity, strong action by descending and ascending air currents, and development of forms in place|
|(1) Circumshrub hillocks|
|(2) Circumshrub hills|
|(3) Small cellular forms|
|(4) Small-large cellular forms|
|(5) Small pyramidal forms|
|(6) Large, complexly pyramidal forms|
The most widespread deflational microforms (up to a few dozen centimeters across) are fluted or honeycombed stones, formed primarily by terrigenous rocks and dreikanters. Among the landforms of medium size (measuring from a few meters to dozens of meters across) are yardangs, ravines, deflation kettles and niches, and rock prominences of fantastical shapes (mushroom-shaped, circular), which are sometimes grouped into eolian “cities.” The large eolian landforms (measuring several kilometers across) include deflation basins and solonchak-deflation depressions, which form as a result of the combined action of intensive physicochemical (salt) weathering processes and deflation. Some of these basins are enormous, measuring hundreds of square kilometers in area. An example is the Karagie depression in Western Kazakhstan.
A comprehensive study of eolian landforms and their morphology, origin, and dynamics is important for the economic development of desert regions.
REFERENCESAristarkhova, L. B. Protsessy aridnogo rel’efoobrazovaniia. Moscow, 1971.
Petrov, M. P. Pustyni zemnogo shara. Leningrad, 1973.
Fedorovich, B. A. “Zonal’nost’ eolovogo rel’efoobrazovaniia.” In the collection Razvitie i preobrazovanie geograficheskoi sredy. Moscow, 1964.
Fedorovich, B. A. “Aridnye protsessy i morfoskul’ptury v SSSR.” In the collection Morfoskul’ptura i ekzogennye protsessy na territorii SSSR. Moscow, 1975.
B. A. FEDOROVICH and L. A. ARISTARKHOVA