PENEPLANATION 75 



surfaces were ascribed to peneplanation effects modified greatly on 

 account of the great distance from the sea, the arid climate, and recent 

 orogenic uprisings. The possible conditions were briefly outlined.^^ 

 Among other statements, it was mentioned that the peculiar alternation 

 of narrow mountain ranges and broad plains presents many features 

 which are not easily imderstood until the country both to the eastward 

 and to the westward is taken into account. In both directions from the 

 central highland, or plateau, the basin character of the plains is soon lost. 

 The different intermont plains become confluent and more continuous, 

 and the mountain ranges become more distant from one another until 

 finally they are widely isolated. Still beyond, the plain alone persists 

 without noteworthy mountains. This condition continues on the one 

 hand to the gulf of California and on the other to the gulf of Mexico. 



A large portion of the plains part of the region is worn on the beveled 

 edges of Cretacic and older strata. The Las Vegas plateau, the Llano 

 Estacado, the bolsons of central New Mexico, and perhaps some of the 

 broken plains of eastern Arizona seem to belong genetically together. 

 Tight's^- arguments to the contrary are clearly untenable, since in no 

 uncertain terras he accounts for the physical features of the region on the 

 basis of a humid climate. 



That the general and dominant surface of eastern New Mexico may 

 have been, in the main, originally essentially a peneplain appears to be 

 indicated by old planation surfaces which are known to exist in the 

 northeastern part of the territory. One of these old planation levels, the 

 great Mesa de Maya, now only a relatively narrow belt, truncates the 

 Raton range for a distance of over 100 miles. It lies 3,500 feet above 

 the present plains surface. The range itself is all that is left of the 

 immediate substructure of a once vast plain. 



Three thousand feet below the level of the Mesa de Maya is a second 

 great plain, the Ocate mesa, which covers thousands of square miles. 

 Far beyond the borders of what now remains of this central body of the 

 mesa are many large and small outliers of the same plain. Still lower, 

 about 500 feet, is the general plains surface of tlie region, the Las Vegas 

 plateau, which is on a level with, and at no distant date was probably 

 continuous with, the Llano Estacado, the Estancia, the Jornada, and 

 other large plains. Below the last-mentioned level the Canadian river, 

 the Eio Pecos, and the Eio Grande have cut narrow valleys 1,500 to 2,000 

 feet and more. 



" American Geologist, vol. xxxiv, 1904, p. 161. 

 "American Geologist, vol. xxxvl, 1905, pp. 271-284. 



