] 36 GEOLOGICAL HISTORY OF LAKE LAHOXTAK 



deposits are apparently due to their having been formed in agitated waters ; 

 just how the intiicate folds and contortions were produced, however, it is 

 extremely difficult to explain. 



In a few localities the lacustral clays, especially below the medial 

 gravels, are faulted ; and at times the strata on one side of a fault have 

 been thrust over the beds forming the opposite wall, as indicated at the left 

 on Plate XXV. In this instance the projecting strata seem to have been 

 removed by erosion previous to the deposition of the superimposed clays. 



The medial gravels in the Truckee section vary from 2f^ to 100 feet In 

 thickness, and exhibit great diversity both in composition find structure, 

 thus indicating many variations in their mode of formation. Examples of 

 ci'oss-bedding are abundant, and the presence of arched strata and lines of 

 unconformability, as well as the irregularity of the beds and the manner in 

 which they wedge-out and are replaced by others of a somewhat different 

 nature, all tend to show that the entire middle member of the Lahontan 

 series here exposed was deposited in shallow, current-swept waters. The 

 arches seen in the canon walls are, apparently, cross-sections of current- 

 built embankments, while the irregular layers of fine sediment are proof, on 

 the other hand, of more quiet condition during which the fine silt held in 

 suspension was allowed to subside. The general conclusion that the medial 

 gravels were formed during a time of low water, separating two periods when 

 the lake was broad and deep, cannot be questioned by any one who lias 

 examined the records exposed in the Truckee section, which, as previously 

 stated, are in harmony with the similar evidence furnished elsewhere in the 

 basin of the ancient lake. 



On following the Truckee River from the Agency Bridge to its mouth, 

 one finds its banks becoming low, and exposing, for the most part, only por- 

 tions of the upper clays ; in a few localities, however, limited sections of 

 the medial gravels may be seen, thus showing that the valley could not have 

 held a lake much, if any, larger than that of the present day during the time 

 that the medial member of the Lahontan series was being deposited. 



