CLIMATE AND TERRESTRIAL DEPOSITS 167 



and a maximum of naked rock surface ; second, mature mountainous 

 topography, marked by broadly V-shaped valleys, long slopes covered 

 with soil and talus, and lower rounded summits ; third, old topography, 

 with widely opened, flattened valleys holding meandering rivers and 

 with the interstream spaces no longer mountainous, but reduced to 

 rolling hills. The four kinds of climate to be considered have already 

 been mentioned as combinations of pluvial and arid, warm and cold. 



YOUNG TOPOGRAPHY AND VARIOUS CLIMATES 



Young and mountainous topography, as is well known, produces 

 an intensification of climatic effects and a heightening of the climatic 

 contrasts of nearby regions, inducing an extreme precipitation, often 

 largely as snow, upon the windward slopes and to a lesser extent upon 

 the leeward slopes, and resulting in semiarid or truly arid tracts 

 farther to leeward. Rapid erosion will prevail in both regions, but 

 the results are of essentially different characters. Where the pre- 

 cipitation is regular and not violent, that is, not as snowfall or irregu- 

 lar cloud-bursts, the rivers and valley walls tend to become graded, 

 vegetation holds the soil to the slopes, and interstream erosion over 

 the soil-covered regions is diminished, and the material carried away 

 by the rivers is finer ^ and more thoroughly leached of its soluble 

 elements.^ Over the steeper slopes and along the streams, however, 

 erosion may go forward with immense rapidity. The waste which 

 is held within the region is, therefore, largely held as soil, on the 

 moderate slopes, and not as detritus, and upon any climatic change 

 which shall diminish the effectiveness of the vegetative covering is 

 liable to be swept away with geological suddenness as a flood of clay, 

 sand, and gravel. 



On the side of the mountains possessing a subarid or arid climate, 

 the waste is swept away as soon as disintegration frees it from the 

 rock, and is largely stored in interior basins or on piedmont slopes. 

 This material adjacent to the mountains is but little decomposed, 

 has less true clay, and on the whole is coarser than the similarly 

 situated waste of milder or more pluvial climates. As examples 



I E. Huntington, Explorations in Turkestan, with an Account of the Basin oj 

 Eastern Persia and Sisten, Carnegie Institution Publications, No. 26, 1905, p. 269. 

 " E. W. Hilgard, Soils of the Humid and Arid Regions, p. 413, 1906. 



