62 WARREN N. THAYER 



closely folded Mesozoic sediments. The elevation of the peneplain 

 made the great Mexican Plateau, which in its present condition 

 may be described as a great tilted block, the up-tilted end over- 

 looking the isthmus of Tehuantepec and the down-tilted end lying 

 under the scarp of the Colorado Plateau and along the course of the 

 Rio Grande. 



On this plateau erosion and vulcanism have carved and molded 

 a variety of topographic forms. The products of one or both of 

 these factors characterize definite areas, and applying the definition 

 that a physiographic province is an "area which is characterized 

 throughout by similar or closely related surface features, and which 

 is contrasted in these respects with neighboring areas," 1 we find 

 that the prominent topographic divisions of Mexico become its 

 true physiographic provinces. 



These provinces may be gathered for convenience in discussion 

 into two groups: one, the northern, having a general north-south 

 alignment, and the other, the southern, having a general east-west 

 alignment (see accompanying map, Fig. i). In order from east to 

 west the members of the northern group are the Gulf Coastal Plain, 

 the Anahuac Desert Plateau, the Sierra Madre Occidental, and the 

 Sonoran Desert. Below these, in succession southward, come the 

 Volcanic, the Sierra del Sur, and the Tehuantepecan provinces. 

 Where province names are not self-explanatory, reasons for them 

 will be given in the text to follow. In our discussion of the prov- 

 inces we will begin with the Sierra Madre Occidental, for the reason 

 that it is the oldest feature of Mexican topography and will aid 

 in orienting us, so to speak, when we come to the discussion of 

 younger features with more complex histories. 



THE SIERRA MADRE OCCIDENTAL PROVINCE 



Definition and boundaries. — The Sierra Madre has been defined 

 as "a vast area of circumdenudation," "a higher island of summit 

 survival," 2 "a great plateau, fringed by mountains on the east, 

 trenched by deep canyons in the center, and bordered by a wild and 

 rugged complex of mountains on the west." 3 "It is not to be 



1 Fenneman, op. cit., p. 87. 2 Hill, Eng. and Min. Jour., LXXX, 633. 



3 W. H. Weed, Trans. A.I.M.E., XXXII, 444. 



