270 



/. P. GO ODE 



shelving ledge upon which the water is ceaselessly pounding, as 

 shown in longitudinal section in Fig. 5. So this fall may be said 

 to be showing signs of old age — that is, the rapids phase of 

 development has already begun. 



With the lake outlet approaching 

 this barrier at the contour of 7800 feet 

 a current was formed at the Thistle 

 Creek narrows, and two separate lakes 

 resulted, with a short river between. 

 The lower lake, covering Havden Val- 

 ley we may provisionally call Hayden 

 Lake. The river at the narrows had 

 glacial drift only to work on and was 

 competent to cut this out widely as 

 Hayden Lake level followed the lower- 

 ing brow of the falls. 



The problem was made more com- 

 plex when the river discovered another 

 wall of firm rhyolite at the site of the 

 Upper Fall. This wall is much thicker 

 than the lower one, and the process of cutting is proportionally 

 slower. It was the lowering of this barrier which determined 

 the lowering of Hayden Lake level. When the wall was cut 

 somewhat below the 7700-foot contour, Hayden Lake was 

 drained, and this has only very recently been accomplished, as is 

 shown by the flat and sinuous course of Trout Creek. 



The wearing down of the barrier at the Upper Fall has always 

 lagged behind that of the lower. It could not be touched at all, 

 until the lower barrier was reduced below its level, and the 

 height of the Upper Fall has always been limited at its lower 

 level, by the brow of the Lower Fall. The Upper Fall has 

 increased in height almost uniformly with the decrease in 

 height of the Lower Fall, and it is plain to be seen, that when 

 the Lower Fall has finally sawed thru its barrier, the river 

 will carry the canyon gradient back to the Upper Fall which will 

 then be perhaps four hundred feet high. 



Fig. 5. — Longitudinal diagram 

 matic section of the great fall. 



