628 JOHN LYON RICH 



west as far as one can see, there is still the dune strip three or four 

 miles wide, but all is covered with vegetation. The irregular dune 

 topography still remains and in places is as well developed as in the 

 area of actively moving sand. Sage brush, often as high as a man's 

 head, covers the dunes. 



It is evident that at the time these dunes were forming, the 

 climatic conditions must have been different than at present. A 

 change, either to greater precipitation or lowered mean tempera- 

 ture, or both, with consequent lessened evaporation, would allow 

 vegetation to gain a foothold on the dunes. 



This, taken in connection with the fact that the streams are now 

 dissecting the valley-filling formed during a previous dry period, is 

 strong evidence of recent increased precipitation, or, what amounts 

 to the same thing, a cooler climate with lessened evaporation. 



It might perhaps be argued that the series of terraces found 

 along the streams could be due to changes in level of the land rather 

 than to changes in climatic conditions. To account for the terraces 

 on this basis would necessitate the following series of crustal move- 

 ments: First, elevation during the first great erosion; second, 

 depression to account for the first period of aggradation; third, 

 elevation to account for the second period of erosion below the 

 bottoms of the present streams; fourth, depression to account for 

 the second period of aggradation and the formation of the "valley- 

 fill"; and fifth, another slight elevation giving rise to the present 

 stream-trenching. Such an explanation necessitates too many 

 unproved crustal movements. 



Relative lengths of the periods of terrace-formation. — It must not 

 be supposed that all these different periods of aggradation and 

 dissection were of equal duration or importance. By far the most 

 important event since the deposition of the high-level gravels was 

 the long cycle of erosion which produced the dominant features 

 of the present topography. All the later terraces are minor 

 features developed within the larger valleys resulting from this 

 great erosion, and represent comparatively recent climatic fluctua- 

 tions. Fluctuations of equal or greater magnitude might have 

 occurred while the great erosion was in progress without leaving 

 any surviving record. 



