In the Basin region of Western America there exists a remarkable type of intermontane valleys to which the Spanish name Bolson has been given. The geological substructure of these plains has been recently described at some length. In the present connection some additional features are discussed

Keyes, Charles R.

English

University of Northern Iowa

Proceedings of the Iowa Academy of Science Volume 11 Annual Issue Article 32 1903 Certain Basin Features of the High Plateau Region of Southwestern United States Charles R. Keyes Copyright ©1903 Iowa Academy of Science, Inc. Follow this and additional works at: https://scholarworks.uni.edu/pias Recommended Citation Keyes, Charles R. (1903) "Certain Basin Features of the High Plateau Region of Southwestern United States," Proceedings of the Iowa Academy of Science, 11(1), 254-257. Available at: https://scholarworks.uni.edu/pias/vol11/iss1/32 This Research is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Proceedings of the Iowa Academy of Science by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. 254 IOWA ACADEMY OF SCIENCES. CERTAIN BASIN 1'EATU1-ms OF THE HIGH PLATEAU REGION OF SOUTHWESTERN UNITED STATES. BY CHARLES R. KEYES. [ABSTRACT.] In the Basin region of W ostern America there exists a remarkable type of intermontane valleys to -vvhich the Spanish name Bolson has been given. The geological sub-structure of these plains has been recently described at some length.* In the present connection some additional features are discussed. The bolson plains of New Mexico, for example, are found only in that part of the region which belongs to the geographic subdivision known as the basin region. This includes the southern two-thirds of New Mexico, or the portion lying south of the Ifocky mountains, which abruptly terminate 100 miles south of 'the Colorado line. South-ward, from this latitude, the bolson plains occur~long level strips of plains country, separated from one another by high but narrow mountain ranges. Far beyond the New Mexican boundaries the same type of physiography prevails, nearly as far as the city of Mexico. The peculiar alternation of narrow mountain ranges and · broad plains presents many features which are not easily understood until the country both to the eastward and to the westward is taken into account. In both directions from the central highland the "perso" character of the basin plains is soon lost. The different plains become confluent and more contin-uous, and the mountain ranges more disconnected and *Am. Jour. Sci., (4), Vol. XV, pp. 207-~12, 1903. 1Keyes: Certain Basin Features of the High Plateau Region of SouthwesternPublished by UNI ScholarWorks, 1903IOWA ACADEMY OF SCIENCES. 255 finally isolated altogether. Still beyond, the plain alone persists without notable mountains. This condition con-tinues on the one hand to the Gulf of Ca.lifornia and on the other to the Gulf of Mexico. At the beginning of rrertiary time the region between the two great gulfs north to the present Colorado line must have been a vast lowland plain, with but faint relief feat-ures. A large part of this plain was on the bevelled edges of Cretaceous and older strata as is shown now in its rem-nants s.till clearly discernible. 'I'he Las Vegas plateau, the Llano Estacado, the bolson plains of central New Mex-ico and some of the less broken plains of eastern Arizona·. seem to belong genetically together. To the east and west of the vast area thus outlined a brm1,d submarint;i platform was formed from the sediments derived from the planing off of the central land area. When the general bowing up of the region took place later in Tertiary brnes the grea,t plain formed was partly a peneplane of destructional land origin and partly a, constructional plain of marine origin. After the period of the main uprising", after the whole surface of the country had attained somewhat more than its present elevation above sea-level, normal faulting on a vast scale gave rise to numerous rnonoclinal block moun-tains, with a trend of north and south. There were num-erous halts in general movement and the ,Mesozoic and youngest Paleozoic beds bore are completely stripped off the mountain summits. Several times the staying process has enabled partial peneplantation to take place. But the mountain blocks have become more and more tilted. Between 'fertiary time and the present, enormous ero-sion has taken place. The vast plain has been deeply dis-sected by such old mountain-born streams as the Cana-dia,n, Pecos, Rio Grande, and Colorado. The valleys of the"e water courses are very wide and deep. On the east the Canadian flows 4,000 feet below the level of the old plain. The Pecos perhaps 2,500 feet. The Hio Grande about 1,500 feet. While the Colorado canyon is a mile in depth. 2Proceedings of the Iowa Academy of Science, Vol. 11 [1903], No. 1, Art. 32https://scholarworks.uni.edu/pias/vol11/iss1/32256 IOWA ACADEMY OF SCIENCES. In the Llano Estacado the remnant of the great plain contains 50,000 square miles. The bolson plains are al-ready beginning to give way to erosion agencies. In the valley of the Rio Grande nearly all traces of the old pbin are already destroyed. The displaced intermontane basins, like the J ornada del Muerto, which adjoin the long Rio Grande valley are beginning to be deeply dissected wher-ever the great river touches the borders. In its broader fea;tures the surface of New Mexico may be regarded as a ribbed tableland. Both north and south valleys alternate with long narrow more or less continuous. mouritain ridges. The most important of the long basin plains and valleys are the Pocos, Huerco, Estancia, .Tor-nada, Rio Grande, San Augustine, and Mimbres. Over such a surface from the southern end of the Rockies three great streams diverge. These are the Canadian river, the Rio Pecos, and the Hio Grande. The first of these after leaving the mountains flows eastward to the Arkansas in Indian r:I.1erritory and thence its waters find their way to 1 the Mississippi. Hio Pecos trends southeastwardly, enter-ing Texas nea.r the southeast corner of New Mexico. ] 1rom the San Luis Valley in Colorado the H.io Grande Hows. slightly west of south to El Paso. Of these the last two streams mentioned flow in broad valleys between lines of block mountains. Comparison of physiographic features of basin valleys which the groat mountain-born streams traverse and of those which are not so occupied, quickly demonstrates that the bolson owes its existence merely to lack of erosion agencies. The Rio Grande no doubt at first passed through a series of bolsons identical with those at present found on either side of its present valley. It has cut down its channel often 2,000 feet below the surface of the ancient bolsons. Within the valley nearly all traces of the bolson characters are now lost. No waters are received by the great stream after it emerges from the Rockies. The work of this river has been confined to cutting its canyon. Little additional work of side streams has imposed upon it. 3Keyes: Certain Basin Features of the High Plateau Region of SouthwesternPublished by UNI ScholarWorks, 1903IOWA ACADEMY OF SCIENCES. 257' A still grander example of its kind is found in the Colorado River of the West. Its great valley, so far below the level of the table land, exists merely because the drainage-way has its source in a region of abundant moisture. Between the Rio Pecos and Rio Grande valleys at the south end of the Hockies there is a small mountain stream, the Rio Galisteo, which crosses the Estancia bolson, and which soon falls into the Rio Grande. This little stream has carved out a remarkable valley. It is an illustration of hovv wonderfully effective is even a small, often dry. rivulet in corrading the high plains. The bolson plains may be r,onsidered as sections of an upraised peneplaned surface in its earliest infancy, at a stage in which they iue as yet untouched by stream action. They could not exist under present hyprometric conditions except in an arid region, which snow-fed perennial rivers do not traverse. The bolsons are only apparently lake-like basins. They have a marked slope in at least one direction of their major axis, <ts in the case of the J ornada del Muerto, where the slant is twenty feet to the mile and greater than the gradient of the parallel Rio Grande. Had tbe latter stream entered at Santa Fe the Estancia-Huerco, or JjJstancia-J ornada line of bols.ons instead of the line to the westward, a vast canyon would have occupied these basins. The bolson plains of central New Mexico hang high above the great channels of the Rio Pecos and Rio Grande ;on either side. Were the rainfall of the region sufficient to produce perennial streams the plains would soon be as deeply carved' out as the great adjoining valleys of the. rivers just mentioned. 17 4Proceedings of the Iowa Academy of Science, Vol. 11 [1903], No. 1, Art. 32https://scholarworks.uni.edu/pias/vol11/iss1/32

Certain Basin Features of the High Plateau Region of Southwestern United States

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Certain Basin Features of the High Plateau Region of Southwestern United States

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