RECORD OF POST-MESOZOIC EROSION 255 



paratively fresh. The closeness of the comparison indicates that Meadow 

 Valley canyon was cut during the Bonneville and Lahontan epoch. 



In an unpublished study the writer has concluded that the Pliocene 

 lake Shoshone came to an end in the late Pliocene or early Pleistocene 

 through the destruction of its basin by the relative depression of south- 

 ern Nevada and California. He suggested that this southerly tilting 

 was the same as that which produced the acceleration of the Colorado 

 and the consequent beginning of the inner gorge of the Grand canyon 

 described by Major Button as occurring at the close of the Pliocene. 

 In western Nevada the disappearance of the Shoshone depression was 

 closely followed by the formation of the smaller basin in which, after a 

 period of dryness, lake Lahontan formed. 



NET RESULT OF LEVELING AND DIFFERENTIATION TENDENCIES 



But in spite of the attempts instituted during the intervals of moister 

 climate the net result of the contending processes has been to fill the 

 valleys at nearly the same pace as the lowering of the mountains. Ma- 

 terials removed by erosion go at once toward smoothing out topographic 

 irregularities. The process is the reverse of that by which the moun- 

 tains were originally differentiated, and if it continues at the rate it has 

 maintained since late Tertiary it will end by reducing the region to a 

 great waste without mountains or valleys and covered by drifting sands. 



As yet, however, the work is only half finished. The crests of half- 

 buried mountains stand out from intervening stretches of desert plain 

 which occupy the position of the old valleys. 



Thus the general topography, although striking, does not indicate ex- 

 traordinary structure, but only exceptional conditions of erosion and 

 deposition. Suppose the Appalachians, which likewise consist of parallel 

 ridges eroded along lines of folding, should become arid, so that the rivers 

 were unable to remove the detritus and the valleys become choked. 

 There would develop in course of time exactly what exists in the Basin 

 region, namely, a nearly level desert, containing a series of parallel, 

 synclinal, and anticlinal ranges. 



ARIDITY AS A PROMOTER OF STEEP SLOPES 



The desert ranges generally terminate in the plains with a fairly mod- 

 erate slope. Steep slopes or scarps are rather the exception, and when 

 present they are not more abrupt than in the Rocky mountains or in the 

 Alaskan ranges, though by rising from level deserts they often acquire a 

 borrowed emphasis. Such scarps have been assumed to be directly due 

 to faulting. It has been and will be repeatedly shown in this essay that 



