LAND WATERS STREAMS 



157 



ween 1827 and 1905, the American fall receded less than three 

 Inches a year. 1 



Rapids and falls sometimes occasion the development of pot- 

 loles (Fig. 116), a peculiar rather than important feature of erosion. 

 The holes are excavated in part by the falling and eddying of silt- 

 charged water, but chiefly by stones which the eddies move. Pot- 



Fig. 116. Pot-holes in granite. Upper Tuolumne River, Cal. 



oles which are not now in immediate association with rapids or 

 alls often point to the former existence of rapids or falls. 



Rock terraces. Where a hard layer outcrops in the side of a 

 -alley above its bottom, the side slopes of the valley become gentle 

 ust above the hard layer, and steep, or even vertical, at and below 

 ts outcrop, as illustrated by Fig. 117. The hard layer then stands 

 ut as a rock terrace on either side of the valley. Such terraces 

 ire not rare, and are popularly believed to be old "water-lines"; 



1 Gilbert. Bull. 306, U. S. Geol. Surv. 



