PLATEAU, pla-to' (Fr., diminutive of plat, plate). An elevated area of fairly level land with approximately horizontal strata. The chief difference between a plain and a plateau is that the plateau is higher: but this very difference in altitude gives the streams a chance to cut deeper valleys than in a plain. Plateaus are al most uniformly associated with mountains, being great bloeks of strata uplifted during mountain folding with little disturbance of the layers. Thus the uplift that formed the Western Cordil leras raised the plateau known as the Great Plains, which rises gently from the :Mississippi to an elevation of -two 0000 the the Rockies; the Colorado Plateau, between the Rockies and the Sierra Nevada, 13000-9000 feet high; and the Columbia Plateau of Oregon and Washington. \Vest of the Appalachians, and up lifted with them, is the Allegheny Plateau, 2000 feet or more in height. whieh extends from east (rn New York to Tennessee and includes hilly central New York. western Pennsylvania, West Virginia, eastern Kentucky, and Tennessee. The uplift of the Himalayas has formed the lofty plateau of Central Asia, which rises from 10,000 to 13,000 feet. Plateaus are frequently so sculptured by denudation as to resemble moun tains in form as well as in height, as in the case of the Catskills, a part of the Allegheny Plateau. Their tops, however, form an even sky line, their strata are nearly horizontal, and it, is only the excessive denudation, carving deep valleys, that has given them the mountainous appearance. For such dissected plateaus the name pseudo mountains has been proposed. Where sculptured plateaus are adjacent to mountains it is often difficult to tell where the plateau leaves off and the true mountain begins.
Boca use of their elevation, plateau streams have great eutting power and often form deep gorges, and, in arid regions, canons, of which the best example is the Grand Cafion of the Colo rado. In arid regions the plateau topography is typically angular. The walls of the cations consist of alternate precipiees and talus slopes, as resistant and weak rock layers are encoun tered, and between the streams are tlat-topped areas. capped by resistant strata, and known in Western America as mesas if large, and as buttes if small. It is this flat-topped condition of the plateau. Loyd by steep slopes, that has given rise
to the name tableland as a synonym for many plateaus in arid lands. Plateaus, in moist mates, where the work of weathering and erosion is greater, are far less rugged and angular.
By reason of their height, plateaus are generally eool, and, because of their association with moun tains, they are often arid, having rain-hearing winds cut off by the higher land near by. Plateaus are not infrequently deserts, for other reasons. The high plateau region of Africa and Arabia, for example, is traversed by the drying trade winds the year round and has little or no vegetation. Often the only parts of a desert plateau capable of supporting human life are the alluvial fans, on which streams flow from the adjacent mountains for a short distance before losing themselves in the sand. Portions of many plateaus, for example that of the Columbia, would he capable of cultivation if the water could be led out of the cation valleys. In temperature there is often great range, front the warm base to the cool or cold top. This is well illustrated in the plateau of Alexico, which rises from a tropi cal climate to that of the cool temperate zone. Difference in temperature is aecompanied by a decrease in evaporation and an increase in pre eipitation, so that the tops of high plateaus are often forest-eovered, as in the ease of the Colo rado Plateau. which rises from the desert region of western Arizona.
When the strata of deeply dissected plateaus contain mineral deposits, like the coal of western Pennsylvania and \Vest Virginia, they are im portant mineral-produeing regions. because the mineral deposits are exposed where the deep valleys cut into the horizontal strata. ln the Allegheny Plateau such valleys have cut down fully 1000 feet, while in the Colorado Cation the horizontal layers are revealed for over 6000 feet. Agriculture is not usually highly developed on plateaus, partly because of the climate, partly because of the ruggedness. Grazing is the most widespread industry on arid plateaus, as in the Western United States, where large areas are oc cupied only by scattered ranchmen. Consult: Davis, Physical Geography (Boston, 1898) ; Tarr and AlcMurry, Physical Geography (New York, 1897). See