16 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



THE TOPOGRAPHIC FEATURES OF THE PLATEAUS. 



(1) Mature Uplands. — Where most typically developed the plateaus present three 

 chief types of topographic form, which may be described as mature uplands of ancient 

 origin, young plains of erosion upon soft strata, and young cliffs composed of hard strata 

 and forming the borders either of mesas or can3'ons. Other features, such as volcanic 

 cones or fault scarps, for example, may be omitted as of secondary importance in spite of 

 their great interest. The plateaus, it is needless to saj^, were formed by the slow upUfting 

 of large areas of the earth's surface without any pronounced tilting or bending of the rocks. 

 In all such cases an old topography, brought to a greater or less degree of maturity, must 

 have been carried up to a height far above that under which it was originally developed. 

 In some cases — for example, the Kaibab Plateau in northern Arizona just north of the 

 part of the Grand Canyon most commonly visited — this ancient topography is still pre- 

 served. On the edges it is being rapidly dissected and removed by the rapid streams 

 which are the normal result of uplift. The Mescalero Plateau, east of the Otero Basin 

 in the south central part of New Mexico, is another good example. Here a steep fault 

 scarp, gashed by precipitous young canyons, rises on the east side of the basin to a height 

 of about 9,000 feet, nearly 5,000 feet above the basin floor. At the top one emerges from 

 the narrow valleys formed since the last uplift and finds himself in a wooded region of 

 open, mature topography. Gentle slopes rise from broad valleys to round-topped hills of 

 nearly uniform height. Everywhere the soil is deep, and outcrops of naked rock are rare. 

 Often the valleys converge into flat sink-holes, where the water stands for a while before 

 it can seep away tlirough underground passages in the soluble limestone. Everything 

 indicates that the region was subjected to extensive erosion long before it was slowly 

 upheaved to its present situation. Its topography was formed under conditions quite 

 different from those of to-day, and we can as yet draw no satisfactory conclusion as to the 

 climate prevalent during the long ages required for its erosion. 



(2) Young Plains due to Erosion. — The mature uplands are in most cases so elevated 

 as to be too cold for extensive habitation or agriculture. On their borders, however, the 

 processes of erosion have in many cases given rise to broad and relatively youthful plains 

 of subaerial denudation at altitudes of 6,000 or 7,000 feet. These would be habitable 

 if provided with more water, and many of them seem to have been cultivated in former 

 times. The plains are rarely smooth for any great distance. At frequent intervals they are 

 interrupted by steep-sided mesas, hues of clifTs, or canyons, the product of the same pro- 

 cess of erosion which has produced the plains. It is unnecessary here to enter into any 

 detailed description of this well-known process. I would merely call attention to the 

 fact that it reaches a high state of development only in arid regions. Where strata of 

 unequal hardness are exposed to erosion, such soft materials as shales are worn back 

 much faster than hard formations, such as massive sandstones or limestones. If the strata 

 are horizontal the weathering of the soft formation tends to carry it away from under 

 the hard formation wherever a vertical surface is exposed by erosion. The hard rocks of 

 course break oS as soon as they are undermined, and thus steep chffs are formed. This 

 process takes place in a moist chmate quite as much as in a dry, but it can not go so far. 

 In the moist chmate two things tend to check it. In the first place, the action of frost, 

 rain, snow, and vegetation tends to cause the weathering of the hard rocks to go on at a 

 rate which approximates that of the soft rocks more nearly than in dry regions. Hence rela- 

 tively more talus falls from the cliffs of moist regions than from those of dry regions, and the 

 tops of the chffs are worn back, while the soft strata at the base are protected by the accu- 

 mulation of debris. Hence steep chffs are not common. In the second place, erosion is less 

 hindered in dry regions than in wet. The torrential character of the rains and the absence 

 of vegetation allow the talus to be carried rapidly away in arid countries, while the barren- 



