DEBRIS-COVERED MESAS OF BOULDER, COL. 5 1 1 



rationally explained by such processes of erosion and deposi- 

 tion as may be witnessed in the same region at the present time. 

 Conclusions. — From the phenomena observed in this region 

 four conclusions may be drawn: (i) That the accumulations of 

 debris forming the protecting surface of the mesas and the 

 mesa-terrace are of fluviatile origin, and that in their mode of 

 accumulation they differ in no essential respect from that in 

 action over the valley bottoms in that vicinity at the present 

 day. (2) That the mesa tops mark the level of a grade formed 

 by the young streams soon after the adjacent mountains had 

 assumed something like their present attitude. (3) That the 

 three grades represented by [a) the mesa tops, (d) the mesa- 

 terrace, and (c) the present valley bottoms do not seem 

 necessarily to require the assumption of any change in the atti- 

 tude of the land subsequent to the elevation of the mountains, 

 but are the natural sequences of erosion as influenced by the 

 local distribution and difference in hardness of the formations 

 involved. (4) That the grades were formed and covered with 

 debris by the streams at essentially the same time, and that it is 

 contrary to the observed phenomena to assume that the grading 

 was done at one period and the debris accumulated during some 

 other period. 



Willis T. Lee. 



