A CLOCK OF RECENT GEOLOGICAL TIME 



355 



of the gorge with increasing radius of rotary motion with each in- 

 crease in volume of the falling water. Under this rotary churning 

 the soft shales are torn out near the bottom and in sue- 



?.< w *^v^> 







FIG." 380. Ideal section to show 

 the nature of the drilling process 

 beneath the cataract. 



cession the harder layers 

 above until the capping is 

 reached (Fig. 380) . The con- 

 ditions appear now to be such 

 that the effective work is 

 largely concentrated, as it 

 usually has been, near the 

 middle of the channel, and 

 so the gorge recedes with a 

 margin of the earlier river 

 bed remaining as a terrace on 

 either side and extending to 

 the former river bank (Fig. 

 377). 



As must have been noted, 

 one peculiarity of the opera- 

 tion of the churn drill beneath 

 the cataract is that the depth 

 of the gorge will bear a direct 

 proportion to its width, and 



FIG. 381. Plan and section of the Niagara 

 gorge, showing how in each section the 

 depth is proportional to the width, except 

 in the lowest sec^n where subsequent river 

 action of the normal type has modified the 

 bed of the channel (plan after Taylor and 

 section after Gilbert). 



