Atlas
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a mountainous area in northwestern Africa. It stretches east from the Atlantic Ocean along the coast of the Mediterranean Sea for nearly 2,000 km, passing through Morocco, Algeria, and Tunisia. It constitutes a separate natural region of Africa, with sharply contrasting landscapes because of its mountainous relief, its different exposures and climates, and its location at the juncture of subtropical and tropical geographic belts.
The Atlas attains its greatest elevation in Morocco, in the El Rif, Middle, and High Atlas ranges (Mount Toubkal, 4,165 m, the highest peak of the Atlas). To the west of its highest section, the Moroccan meseta descends by degrees from altitudes of 1,000–800 m. The Tell Atlas extends to the east, along the Mediterranean coast; the Saharan Atlas— elevations 1,200–1,500 m—stretches along the southern outskirts. The Oran-Algiers plain lies between these two ranges at elevations of 1,000–1,200 m. Spurs of the northern and southern ranges divide them into separated hollows with large salt lakes—sebkhas (Chott ech Chergui, for example). In the east, the northern and southern ranges run together and are separated by a band of meridional foothills from the coastal depression of Tunisia.
The northern coastal part of the Atlas constitutes an alpine folded structure; it has outcrops in the core (Kabyle massifs) of ancient (Precambrian) metamorphic formations with a sheath of shallow Paleozoic and Carbonaceous Triassic and Jurassic. However, Cretaceous-Paleocene deposits—flysch, to a considerable extent—play the primary role in the structure of this zone: they form a system of tectonic sheaths, dislocated from north to south and partially overlapping the forward downwarp (pre-Rif, pre-Tell) realized by the molasse Miocene. In the structure of the Atlas further south, a layer of geosynclinal Paleozoic, which experienced intensive Her-zinian tectogenesis, played a significant role in the west (the Moroccan meseta). Further east in this same belt (the zone of the Hauts Plateaux, including the Oran meseta) relatively thin, weakly deformed shallow marine deposits of the Cretaceous and Paleocene and continental deposits of the Neocene are found on a more ancient, probably Late Precambrian foundation. Still further south, in the zone of the High and Saharan Atlas, and also in the Middle Atlas, the thickness of the Mesozoic grows and its folding increases perceptibly. In the extreme east (in Tunisia) the fold structure is to a considerable extent determined by highly plastic, salt-bearing rocks of the Triassic. In the south, the Atlas is separated by a great break (the South Atlas Break) from the African platform. Another break with a subsidence of the central part of the Atlas structure passes along the coast of the Mediterranean Sea; manifestations of recent volcanism and earthquake activity are connected with it. Deposits of iron ore and polymetals are known in the Atlas.
Variegated lithology, fluctuations of climate in the Pleistocene, and contemporary climatic differences are the bases of the diversity of exogenous forms of the relief of the Atlas: on the highest peaks, traces of ancient glaciation have survived (peaks, cirques, glacial troughs, and moraines); chains have dense, deep ancient erosional dissection. Interior regions are occupied by denuded and accumulated plains, cues-tian chains, and residual plateaus. In the south, the slopes of the mountains are covered with gravelly screes, and physical weathering proceeds actively. Karst is extensively developed in regions of widespread limestone rocks.
The climate is subtropical Mediterranean in the north and semidesert in other regions. Precipitation falls primarily in autumn and winter with the passage of polar-front cyclones over the Mediterranean Sea. The greatest quantity of precipitation (1,000–1,800 mm per year) falls on the slopes of northern and western exposure in the Tell Atlas east of 2° E long and in the High Atlas at elevations of 2,000–2,500 m. Most of the Atlas Range receives 400–600 mm of precipitation; southern regions receive 300 mm and less per year. In the lower belt of the mountains, the average January temperature is 10–12° C in the north and 4–6° C in interior regions. Snow lies on the ground four to five months at altitudes of 1,500 m in the mountains. Summers are dry and hot. The average July temperature is approximately 25° C; the absolute maximum is 40° C on the interior plains and 49° C in the south.
The rivers of the Atlas—called wadis—are fed primarily by rains. Floods occur in the winter; in the summer, almost all the rivers dry up. The deepest rivers are of the Atlantic Ocean basins (Oum er Rbia, Sebou) and the Mediterranean Sea (Moulouya, Chéliff). In the rainy season, their discharge increases to several hundreds and thousands of cubic meters per second. Wadis of the internal and southern regions have occasional flow.
The soil and vegetative cover of the Atlas reflects its location in two physical-geographic zones: in the north and west, on the coast, and in the mountains up to 800 m, landscapes of the zone of arid forests and scrubs of typically Mediterranean climates are prevalent. Thickets of evergreen hard-leaf scrub (maquis) are similar in their flora to those of southern Europe. Forests are composed primarily of cork oak on brown soils. In arid interior regions and in the south, in the subtropical semidesert zone, there is sparse grassy vegetation (feather grass, alfa), wormwood scrub, and esparto on grey-brown, extremely gravelly soils. Altitudinal zonation is displayed in the mountains. It is most fully developed in El Rif and the Tell Atlas, on windward slopes: the belt of evergreen forests made up of cork and stone oak reaches up to 1,200 m; the belt of mixed forests with evergreen, summer-green, broad-leaf (oaks, maples) and coniferous trees ends at 1,700 m. The belt of coniferous forests (primarily composed of Atlas cedar), which supply valuable building lumber, is located in the coolest and most humid climate up to 2,200 m. Mountain brown-leached and, primarily, brown forest soils are developed under the forests. There are patches of mountain-meadow and mountain-steppe vegetation and soils on the peaks.
Animal life has been greatly depleted over a long period of time. There are representatives both of African and of some European animals (hares). Monkeys have survived in the north; there are jackals everywhere; and in the south there are hyenas and some hoofed animals. There are many migratory birds. Reptiles are particularly numerous.
The landscapes of the Atlas have been modified greatly by man. Natural vegetation has been reduced in northern areas; large tracts of cultivated lands are concentrated in the river valleys (grains, citrus fruits, grapes, and vegetables are cultivated). The reduction of forests has brought much soil erosion. Primitive terraced irrigated agriculture, and nomadic and seminomadic livestock raising (primarily goats and sheep) are found in the valleys of the southern sections.
REFERENCES
Bernard, O. Severnaia i Zapadnaia Afrika. Moscow, 1949. (Translated from French.)Gornung, M. B. Alzhiriia. Moscow, 1958.
Birot, P., and J. Dreche. Sredizemnomor’e, vol. 1. Moscow, 1960. (Translated from French.)
Atlas
the name of a series of American launch vehicles used as the first two stages of the Atlas interplanetary ballistic rocket. The initial mass is between 125 and 135 tons, and the overall length is between 31 and 36 m. The first stage has three liquid-propellant rocket engines having a total thrust of 1.6 meganewtons (163 tons) fueled with liquid oxygen and kerosene. In the Atlas-Agena version the third stage is an Agena-D. In the Atlas-Centaur the third stage is a Centaur. The payload mass placed into a low satellite orbit around the earth ranges from 2.2 to 4.5 tons for the different versions. The Atlas is used to launch the satellites Midas and Samos, secret satellites, and the unmanned interplanetary stations Ranger, Mariner, and others.
atlas
[′at·ləs]Atlas
[′at·ləs]atlas, pl. atlantes
