354 



EARTH FEATURES AND THEIR MEANING 



rectly from above, selecting for the purpose the lines of jointing 

 of the rock which it widens by solution and corrasion until the 

 included blocks are in so far separated that they are torn out and 

 go over the brink of the Falls (Fig. 378). This process of over- 

 head attack in the powerful currents just above a cataract is even 



FIG. 379. Falls of St. Anthony, looking westward from Hennepin Island in 1851 

 (after N. H. Winchell, daguerreotype by Hessler of Chicago). 



better illustrated by the Falls of St. Anthony near Minneapolis, 

 which have had a similar history of recession to that of the Niagara 

 Falls (Fig. 379). 



The blocks of the capping limestone at Niagara Falls are to 

 some extent fixed in size by the joint planes present in them, and 

 as they fall to the bottom of the gorge, they promote or retard the 

 further recession of the Falls according as they can or cannot be 

 moved about by the churning currents beneath the cataract. Of 

 the retarding effect there is an illustration in the accumulation of 

 the blocks below the American and the intermediate Luna Falls 

 (plate 23 A), which the weaker currents upon the American side 

 find too heavy to handle. The Canadian Fall, with its much greater 

 power, is an example of the promotion of recession through the 

 churning about of the blocks at the base of the cataract. We have- 

 here to do with a churn drill which bores its way into the bottom 



