EFFECTS OF UNEQUAL HARDNESS 



The fall of the Niagara l is one of the most remarkable known 

 both because of its large volume of water and its great descent, 

 between 160 and 170 feet. This fall is 

 divided into two parts, separated by 

 Goat Island, the Horseshoe fall (Fig. 79) 

 on the west, and the American fall on 

 the east. Between 1842 and 1905, the 

 Horseshoe fall receded about five feet 

 a year, while between 1827 and 1905, 

 the American fall receded less than 

 three inches a year. 1 



Rock terraces. Where a hard layer 

 outcrops in the side of, a valley above 

 its bottom, the side slopes of the valley 

 become gentle just above the hard layer, 

 and steep, or even vertical, at and be- 

 low its outcrops, as illustrated by F-ig. 

 80. The hard layer, H, then stands out 

 as a rock terrace on either side of the 

 valley. Such terraces are not rare, and 

 are popularly believed to be old "water- 



Fig. 78. Twin Falls, Yoho 

 Valley, British Columbia. 



lines"; that is, to represent the height at which the water once 

 stood. In one sense this interpretation is correct, since a river has 



l"\. 71). Niagara. I-';ills. I S. lu-ol. Surv.) 

 1 Gilbert, in Physiography of the United States, and Bull. 306, U. S. Geol. Surv. 



