RECORD OF POST-MESOZOIC EROSION 251 



Evidence of climatic variation in very recent times has been observed 

 by Mr King in the Sierra Nevada.* About 1860 an increase of precip- 

 itation began, marked by forcing of the timber line downward by en- 

 croaching snow and by a rise in lake Mono. In Great Salt lake consid- 

 erable historic oscillation has been chronicled. f 



General leveling tendency. — Daring the Tertiary, as a whole, materials 

 washed from the Great Basin ranges largely found their way into lakes 

 or dry basins of the same region, so that erosion contributed to level the 

 rugged topography instead of further differentiating it. The repeated 

 Tertiary uplifts and foldings antagonized this tendency, sometimes up- 

 lifting portions of the detrital deposits so that they lay on the flanks 

 of the ranges; but with little or no drainage to exterior regions these 

 movements could result in no permanent commensurate effect onthe 

 topography. During all the disturbances and after their close mate- 

 rials were steadily stripped off from the ranges and deposited in the 

 valleys. 



Active erosion in moister climatic intervals. — Quite as important as defor- 

 mation, as an offset to the degrading and leveling tendency, has been the 

 drainage instituted in the alternating intervals of moister climate. That 

 during the Eocene there were periods of active erosion is shown by the 

 enormous bulk of sediments derived from the Central Nevada land-mass 

 which were laid down near the Wasatch and in southeastern California 

 and adjacent Nevada. The same conclusion applies in a less degree to 

 the Miocene and in a still less degree to the Pliocene Shoshone Lake 

 epoch. 



Erosion in drainage basins of Pleistocene lakes. — The lakes of the Pleisto- 

 cene were probably of less extent than those of the earlier periods ; they 

 seem to have formed a relatively insignificant quantity of deposits, and 

 so their feeding streams are to be credited with correspondingly little 

 erosion. Yet Professor Russell measured a thickness of 375 feet of lake 

 Lahontan sediments, while Mr Gilbert found 150 feet of Bonneville beds. 

 In neither case was the bottom of the section reached, and the total 

 amount is probably very greatly in excess of these figures. Estimating 

 on the basis of 375 feet as the average thickness of sediment in each lake, 

 the total bulk is 589 cubic miles for lake Lahontan and 1,312 cubic miles 

 for lake Bonneville. When we consider the relatively small drainage 

 basins of the lakes J and remember that the material, which is rather 

 uniformly spread out on deposition, has been derived chiefly from more 

 restricted areas along the drainage lines, we perceive that even the rela- 



* Explorations of the Fortieth Parallel Survey, vol. i, p. 527. 



t G. K. Gilbert : Monograph i, U. S. Geol. Survey, p. 230, etc. I 



X Monograph xi, U. S. Geol. Sui'vey, map, p. 30. 



