CHAPTER II. 

 THE TOPOGRAPHIC INFLUENCE OF ARIDITY. 



In the modern science of physiography as developed in recent years, under the leader- 

 ship of Professor Davis, one of the most interesting features is the connection between 

 the form of the earth's surface and the climate of anj^ given region. Not only do glaciated 

 areas possess their own peculiar topography, but so do humid and dry regions. The scenery 

 of Arizona and New Mexico is stamped indelibly with the impress of an aridity which 

 has lasted hundreds of thousands of years. Just when it began we can not tell, but 

 certainly far back in the Tertiary era, and possibly earlier, for deposits characteristic of 

 aridity not only attain a great thickness superficially, but are interbeddcd with marine 

 strata in formations dating far back in geological time. A full discussion of the effects of 

 aridity upon the form of the land in all parts of New Mexico and Arizona would require a 

 volume and would demand an amount of field work far greater than I have been able to 

 give to the matter. Accordingly, in the following pages I shall limit myself to a few salient 

 features which clearly show evidences of aridity, or are of special importance in relation 

 to changes of climate and the ancient human occupation of the country. 



Topographically Arizona and New Mexico consist of two chief parts, plateaus of nearly 

 horizontal strata 5,000 to 7,000 feet high and basin regions where mountain ranges, due to 

 faulting or to rapid uplift of relatively small areas, alternate with more or less completely 

 inclosed basins filled with alluvial waste. In Arizona the plateaus and the basin ranges are 

 sharply separated by the Mogollon Escarpment, a line of southward-facing cUffs which 

 extend approximately northwest and southeast across nearly the whole State and pass 

 almost through its center. North of the escarpment lies a high plateau broken in places 

 by fault scarps running north and south, diversified by extinct volcanoes and cut by deep 

 canyons, like that of the Colorado, but preserving almost uniformly the practically level 

 position of its rock formations in spite of thousands of feet of uplift since their original 

 deposition. South of the escarpment the basin-range region lies at a general elevation 

 3,000 or 4,000 feet less than that of the plateaus. Here the strata by no means lie hori- 

 zontal, but have been tipped this way and that, chiefly by means of block faulting along 

 lines running more or less closely north and south. The spaces intervening between the 

 uplifted blocks form basins which have been filled with alluvium. Thus to the eye of the 

 traveler the difference between the plateaus and the plains may be briefly summed up by 

 saying that the plateaus are a region of great plains cut by deep canyons, while the basin- 

 range country is composed of great plains broken by narrow mountain ranges. In New 

 Mexico the separation between the plateaus and the basin ranges is not so distinct as in 

 Arizona. In the elevated regions of the northwest, however, and in the Staked Plains of 

 the eastern part of the State the plateau quality is as well marked as in Arizona, while 

 basins and ranges of mountains due to faulting are almost as characteristic a feature in 

 the south as in the neighboring State to the west. In the center, especially toward the 

 north near Colorado, the main chain of the Rocky Mountains extends down into New 

 Mexico and adds a distinct type of topography. The mountains, however, soon break up 

 into isolated ranges rising from the plateau or bordering waste-filled basins, so that most 

 of the country may fairly be said to belong to one of the two main types with which we are 

 dealing. Inasmuch as the main chain of the Rockies has little to do either with the early 

 inhabitants or with the other evidences of chmatic changes with which we are here con- 

 cerned, it will not be further discussed. 



15 



