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or Plateau Table-Land

feet, peneplain, strata, elevation, surface, level, mountain and plateaus

TABLE-LAND, or PLATEAU, an ele vated flat tract of country of considerable area. In ordinary usage the term is applied to such flat areas elevated above 1,000 feet from sea level. The level character of the plateau may be due to the horizontality of the strata com posing it, the surface being formed by a re sistant stratum, or it may be due to subaerial or marine denudation of a flexed and folded mountain region, the erosion having proceeded so far as to reduce the region to a nearly level tract or peneplain, which is then elevated bodily. Any portion of the ancient mountain system not worn away will rise as a peak or mountain range above the peneplain surface, and consti tute a monadnock. A plateau of this type— that is, an elevated peneplain — may be readily recognized by the disagreement of the slope of the strata composing it with the level surface of the peneplain. the two not infrequently mak ing an angle of 90 degrees with each other. When a young plain of deposition with horizon tal strata is elevated into a plateau, and the rivers begin to cut their channels down into it, it is in the beginning of its first geographic cycle. If it remains stationary at the altitude to it was raised, the rivers will eventually incise their channels to such a depth that their bed from ocean to head is almost perfectly graded. Then lateral erosion will widen the valley bottoms, and reduce the portions of the plateau between the river valleys until finally these separating remnants of the plateau have dwindled to such an extent that they appear as ridges or peaks rising from a level plain, which latter is the result of the confluence of all the river valleys cut into the former plateau. This is the beginning of the peneplain stage, and continued erosion will bring the surface nearer to a perfect plain not far above sea-level. If this is reached, the first cycle of geographic de velopment is complete. The second cycle is in augurated by a re-elevation of the land into a new plateau, when the whole process will be repeated. In like manner a folded mountain region may be worn down to a peneplain, and re-elevated to enter upon the next cycle of erosion. Most plateaus are probably elevated peneplains, either of horizontal strata or of more or less strongly folded and truncated strata. Into these the present drainage has in cised itself more or less successfully. Thus the New England region is a peneplain plateau, of inclined strata, whose surface, moderately re dissected by streams, slopes gently toward the coast. Southern New York, western Pennsyl vania, Ohio and other districts are part of a plateau in which the strata are neatly horizon tal and which has been more or less strongly dissected. This extends southward into Ten

nessee where it is known as the Cumberland plateau. It is probably past its first cycle of erosion, and appears to have been a peneplain.

Among other noteworthy plateaus of North America are the "High Plateaus" of southern Utah, which range in elevations from 7,000 to 9,500 feet above the sea, their elevation being connected with that of the Wasatch Range of mountains. Between the Rocky and Sierra mountains extends a broad plateau from Mex ico northward through British America. It averages from 3,000 to 5,000 feet in elevation, and is deeply dissected by the canons of the Colorado, the Columbia and other rivers. The portion included between the two rivers named is called the Great Basin. It is bounded on the east by the Wasatch Mountains, and on the west by the Sierra Nevada and Cascade ranges, and has a width of nearly 500 miles, with an elevation of 4,000 to 5,000 feet. It has no out side drainage, and hence the streams are short, and the water bodies saline. Great Salt Lake and Mono Lake in California are respectively on the eastern and western side of this pla teau, and the surface between is arid and more or less desert. Similar conditions exist in the Mexican extension of this plateau. A higher type of table-land—to which the name inter mont plateau has been applied — is found in the great plateau of Tibet between the Himalayas and the Kuen-lun Mountains. Its altitude is about 13,000 feet, while the enclosing moun tains rise from 25,000 to 29,000 feet in height. It is 1,200 miles long from east to west, and half as wide. The plateau of Quito is 10,000 feet above sea-level and surrounded by lofty peaks rising 20,000 feet or more. That of Bo livia has an elevation of 12,900 feet with Lake Titicaca at 12,830 feet, and the city of Potosi at 13,330 feet elevation.

The plateau of Spain averages 2,250 feet in elevation, that of Auvergne, in France, about 1,100 feet. Bavaria is a plateau rising 1,660 feet. Persia is another from 2,000 to 4,000 feet above the sea. The Abyssinian plateau in Africa averages 7,000 feet above the sea, while much of the Sahara region is about 1,500 feet above sea-level. The table-land character of all these regions appears only in a general view. Owing to the dissection of the plateaus they ap pear as a rule anything but level to the traveler unless a comprehensive View from a summit is obtained. Many of the larger plateaus are also traversed by mountain ridges. _