QUATERNARY HISTORY 339 



in the district studied are now at about 4,000 feet above sealevel. The 

 tops of the lowest depressed portions of the relatively sunken blocks are 

 probably not more than a few hundred feet below this. 



The result of these movements was the production of the Great Basin 

 province as an interior basin, the formation of the ranges under discus- 

 sion, and of the intermontane landlocked valleys. 



Following the production of these ranges and valleys, probably also 

 during their production, there was a rather long period of erosion. Dur- 

 ing this period the alluvial cones were built up almost to their present 

 size. It is known that the larger rivers, such as the Humboldt and 

 Truckee, were flowing at that time, but no deposits corresponding to 

 their corrasive activities have been found. They probably underlie the 

 Lahontan beds in the deeper portions of the basin. 



With the advent of more humid conditions the valleys were flooded 

 by lake Lahontan, which, as Russell has shown,* passed through two 

 stages of high water, separated by an interlake period of desiccation. 

 During Lahontan time the lower parts of the valleys were filled with 

 sediment up to several hundred feet,f and the mountain flanks were 

 scored with the shorelines, which still form such striking features of the 

 topography. 



It has been shown by Russell that the period of the Quaternary lakes 

 of the Western basin corresponds closely to the period of glaciation in 

 the Sierra Nevada. J The pre-Lahontan uplift and erosion correspond, 

 then, to the Sierran period, and we may consider that the orogenic dis- 

 turbances described mark the opening of the Quaternary era for the 

 Humboldt region. 



RECENT 



Soon after the close of the western Glacial period arid conditions again 

 set in and caused the gradual desiccation of the lakes. Lake Lahontan 

 slowly passed away, and left, among other Recent remnants, Humboldt 

 lake, just west of the southern part of the Humboldt Lake range; but 

 even this has disappeared within the last few years — since the introduc- 

 tion of irrigation along the river — and the Humboldt river is entirely 

 dissipated before reaching the old " sink." 



*U. S. Geol. Survey, Monograph xi. 



fit appears that a number of geologists are unaware that the "Humboldt Pliocene" deposits 

 of "Shoshone" lake described by King for the Humboldt and several other western Nevada val- 

 leys, and the Lahontan Quaternary deposits of Russell, are identical. They are occasionally re- 

 ferred to as two distinct formations deposited in two different epochs. They are thus described 

 in duplicate in the paper, " Origin and structure of the basin ranges," already cited. The Quater- 

 nary age of these beds seems to have been well established. 



% U. S. Geol. Survey, Monograph xi, and also U. S. Geol. Survey, Eighth Annual Report, pp. 

 369-370. 



