HENDERSON] GEOLOGY AND TOPOGRAPHY 39 



is both requisite and adequate to account for the existence of the 

 chff on one side and its absence on the other. That force is doubtless 

 frost. 



The importance of geologic and topogi*aphic phenomena and pro- 

 cesses in the euAdronment of both the ancient and the present inhab- 

 itants may be covered by the following summary: 



The mountains influence temperature and precipitation, giving 

 the region somewhat more rain than the arid region to tlie southwest 

 receives. From the mountains flow several streams which afford 

 water for the irrigation of tillable lands in their valleys. The Rio 

 Grande, which is the principal stream, derives much of its water from 

 the greater mountain ranges north of this region. The general slope 

 of the country from the mountains to the river and the thorough 

 dissection of these slopes by parallel canyons and their tributaries 

 cause complete drainage and consequent absence of lakes^ ponds, and 

 swamps, which, if they existed, would harbor fish and waterfowl 

 and add to the permanent water supply. This complete drainage 

 makes the mesas very dry and the confinement of the streams to 

 deep canyons makes the artificial irrigation of the mesas imprac- 

 ticable. The dry mesas seem nnfavorable to the existence of game 

 in large quantities, thus forcing the inhabitants to have recourse to 

 agriculture. The topography vitally affects vegetation, certain trees 

 and plants being confined to the valleys, others to. the mesas. The 

 varying altitude of the general slope from the mountains to the river 

 marks off the region into somewhat definite plant-life zones, each 

 with its characteristic species. The canyons afford many village 

 sites easily defended from enemies. Villages built at the bases of 

 vertical cliffs could be held indefinitely against a direct hand-to-hand 

 assault, the chief danger being from the rolling of rocks from the tops 

 of the cliffs. The easily excavated material of the cliffs made possi- 

 ble the many cave dwellings ; even some of the ceremonial chambere 

 and estufas occupy artificial or natural caves. The tufa furnished 

 easily worked building material for the walls of the cliff dwellings, 

 pueblos, and small houses. The weathering of the tufa furnishes 

 the soil which is necessary for the existence of vegetation, and because 

 of its origin the soil is very sandy. As the weathering is continuous, 

 it constantly replenishes the soil, which is slowly carried away by 

 storms. Such an environment would be apt to develop or to attract 

 a rather sedentary people, agriculturally inchned, seeking seclusion 

 from wandering warlike tribes and not dependent on the chase for 

 sustenance. 



