THE TOPOGRAPHIC INFLUENCE OF ARIDITY. 17 



iiess and cb-yness of the surface allow the wind to etch out the soft rocks in a fashion quite 

 unknown in moist lands. Consequently, where strong contrasts of hardness exist in a dry 

 climate the soft rocks may be worn back for miles, leaving the underlying hard rocks to form 

 broad plains of erosion, while the renmants of the overlying hard rocks form mesas. Where 

 the climate is moist the sharp contrast between the hard rocks and the soft is diminished, 

 as we have seen. Moreover, the number of residual hills of hard rock is likely to be large 

 because of the abundance of streams and consequent minute dissection. Thus the plains 

 of erosion are apt to be more broken by hills than in dry regions, while the slopes are 

 gentler because more masked by talus. 



(3) Cliffs bordering Mesas and Canyons. — The origin of the steep cliffs of the plateau 

 country is evident from what has just been said. The uplifting of the plateaus has caused 

 rapid erosion and the swift deepening of valleys. The differences between hard and soft 

 strata have resulted in a benched topography; the hard layers form cliffs while the soft 

 wear back so as to form benches on top of the hard. Wliere the cliffs wear back far from 

 the streams, leaving plains, the hard formations may still retain their steepness, and thus 

 mesas and buttes arise. For our present purpose this is important, partly because such 

 topography is characteristic of arid regions, and still more because of its relation to human 

 occupation. The ancient cliff-dwellers, who figure so largely in American archeology, 

 made most of their dwellings in narrow canyons just at the point where the lowest soft 

 layer makes a hollow under the overlying hard layer. Starting, probably, with no shelter 

 except that of the cliffs overhanging their wind-scoured caves, they gradually learned to 

 dig caves in soft formations such as the volcanic tuff of the Pajarito Plateau near Sante Fe 

 in northern New Mexico, while later they developed the art of building walls in front of 

 the caves, and these in turn led them to build entire rooms, sometimes three or four rows 

 deep, at the base of the chffs. Others among the ancient Americans utilized these same 

 cliffs for protection, building their houses of stone on the tops of great steep-sided mesas, 

 of which the Mesa Verde is the best known. Many of the ancient inhabitants, as may 

 be seen near the remarkable ruins of the Chaco Canyon in northwestern New Mexico, 

 dwelt at the base of the cliffs, but apparently cultivated the plains of erosion high above 

 their heads. This last matter is still in dispute, but there can be no question that the 

 peculiar topography characteristic of arid plateaus was the warp upon which was woven 

 one of the most interesting of all the phases of pre-Columbian American civihzation. 



THE BASIN REGIONS. 



(1) The Mounlain Slopes. — Going down from the plateaus to the basin regions of the 

 south, we find a country where, during the most ancient times, men dwelt as numerously 

 as in the plateaus, although the remaining ruins are less conspicuous. Here, as there, 

 three chief elements of physiographic form dominate the landscape: (1) rough, rocky 

 mountain slopes, usually of steep ascent; (2) gently sloping piedmont deposits of gravel 

 merging imperceptibly into smooth plains and playas of fine silt; and (3) terraces com- 

 posed of alluvium, chiefly in the form of gravel. For convenience I shall not attempt a 

 general description of these elements, but shall describe them as they occur in the region 

 of Tucson, where much of our future investigation will center. This will serve as well as 

 a more general description, for in all essential matters there is little difference between the 

 various parts of the basin region. 



Near Tucson the mountain slopes, the first of our three physiographic elements, form the 

 sides of irregular ranges scattered here and there like islands in the midst of a sea of gravel 

 and silt. In general the mountains run northwest and southeast. They vary in height 

 from 4,000 to 9,000 feet, while the plains he at an altitude of 2,000 to 3,000 feet, dimin- 

 ishing to the west and increasing to the east in New Mexico. Some ranges, such as the 

 Santa Catalinas northeast of Tucson, the Tortolitas farther to the north, and the Sierritas 



3 



