RECORD OP^ POST-MESOZOIC EROSION 253 



is their relation to the present as illustrated by the freshness of the river- 

 cut topograpliy. 



According to Major Button* the Colorado drainage sytem was in- 

 augurated in much its present form at the close of the Eocene, when the 

 lakes of that period were drained. During the Miocene erosion went on 

 rapidly, so that the streams cut down through Tertiary and Mesozoic 

 strata till they reached the Carboniferous. At a period conjecturally 

 placed at the beginning of the Pliocene the plateau region was uplifted 

 and an arid climate succeeded to the preceding moister one. Many of 

 the lateral streams dried up, and the cutting of the outer and broader 

 gorge of the Grand canyon began. Soon after the uplift the rivers 

 reached their baselevel and occupied themselves in widening their val- 

 leys. This epoch was closed by a new upheaval, assumed to have been 

 near the close of the Pliocene. Consequent and subsequent was the 

 cutting of the inner gorge of the Grand canyon. During this process 

 came a time of moister climate, intervening between the two arid 

 periods of the Pliocene and the present. This Dutton believed to repre- 

 sent the Glacial period. During this relatively short epoch new ravines 

 were begun, while some of the older canyons were probably deepened. 

 Most of these comparatively recent minor gorges are now dry and are 

 being rapidly filled with alluvium. " The recurrence of a climate suffi- 

 ciently moist to sustain a vigorous perennial stream would probably 

 sweep out all this unconsolidated alluvium and return the valley to its 

 former condition of an ordinary canyon." f 



The features of the dr}^ valleys of southeastern Nevada are not all 

 equally ancient. Las Vegas valley is upward of 10 miles broad in its 

 middle portion. It is cut in Paleozoic limestones and is floored with 

 thick subaerial wash, which doubtless hides Tertiary beds. Sierra val- 

 ley, through which White river runs, and Pahranagat valley, in which 

 lies another portion of the drainage channel between White river and 

 Muddy creek, are of the same type as Las Vegas valley, though nar- 

 rower. But Meadow canyon is cut in the bottom of an older valley, 

 which is like that just mentioned, and it has exposed the Tertiaries 

 which are hidden in the others. 



Comparing these features with the general topographic stages adduced 

 by Major Dutton, we find that the older wide valleys have not partici- 

 pated in the active erosion which produced the inner gorge of the Grand 

 canyon, but judging from their considerable depth and from their being 

 cut in Paleozoic rocks they may have been somewhat eroded during the 

 excavation of the outer gorge. They probably originated as early as did 



* Monograph ii, U. S. Geol. Survey, p. 219 et seq. 

 fOp. cit., p. 229. 



