254 J. E. SPURR — ORIGIN AND STRUCTURE OF THE BASIN RANGES 



the Colorado Plateau drainage S3^stem. The Meadow Valley canyon, 

 however, is of the same t3''pe and age as the inner gorge, and in that it 

 shows evidence of having been cut during a short and relatively recent 

 humid period, succeeded by the i)resent i)eriod of aridity, it belongs to 

 the class of gorges whicli Major Dutton believed to have been formed 

 during that Pleistocene moist epoch which he correlated with the Glacial 

 period. 



Considering the relation of the dry southeastern Nevada valleys to the 

 associated deposits, we note that Meadow Valle}'' can3''on cuts nearly 

 horizontal lake sediments, which, on account of their relation to the gen- 

 eral crustal deformation, have been ])rovisionally classed as very late 

 Pliocene. It also cuts a series of volcanics, the youngest of which is 

 believed, frojn a comparative study of Great Basin vulcanism,* to be 

 very late Pliocene or very early Pleistocene. When tlie can3''on was 

 nearl3'' completed scant basalts and rhyolites were poured out, whicli, 

 by the above method, are classed as Pleistocene. But the late Plio- 

 cene sediments and lavas were deposited in the bottom of the older 

 valleys. 



Considering the relation of the can3"on to the present time, as illus- 

 trated b3'^ the freshness of the to[)ogra])h3'', we observe that the gorge is 

 not encinnbered with Pleistocene alluvium like that which has accumu- 

 lated in the wider valleys. Where the can3n)n widens to a basin near 

 Panaca, the terraces suggest a lake which was drained by the down- 

 cutting. These features point to a comparatively recent portion of the 

 Pleistocene as the epoch of can3"on erosion. 



The assembled deductions from these three relations lead to the con- 

 clusion that Meadow Valley canyon originated in an intermediate epoch 

 of the Pleistocene, which was a period of humidity as compared with 

 the preceding and succeeding ones. The wider detritus-floored valleys 

 tributary to the Colorado experienced no erosion during the greater part 

 of the Pleistocene, but were, perhaps, deepened and widened during the 

 earlier Pliocene. Tiieir origin as erosion features probably dates back 

 as far as the Eocene. 



We may test these conclusions by comparison with related phenomena 

 in other parts of the Great basin. The ancient lakes, Lahontan and 

 Bonneville, originated in a moist interval of the Pleistocene, preceded 

 and succeeded b3'' arid epochs. This interval was believed by Gilbert, 

 Russell, and others to have been contem[)oraneous with the Glacial 

 period. During their existence limited quantities of basalt were ex- 

 truded. The topographic records left by these water bodies are com- 



* J. E. Spun- : Journal of Geology, vol viii, no. 7, pp. (130, G37, G42. 



