140 



GEOLOGY. 



proximately fixed. By surveys executed in 1842 and 1890 it has 

 been determined that its average rate of recession between those dates 

 was something hke 4 J feet per year, or about 150 times as great as 

 the highest estimate stated above. It is to be noted that this is the 

 average rate of recession, for all parts of the ledge over which the water 

 falls are not receding at the same rate. The point of the ''Horseshoe" 

 has, during the same time, gone back at more than twice this 

 rate.^ 



Rapids and falls sometimes occasion the development of pot-holes 

 (Fig. 119), a pecuhar rather than important erosion feature. The 

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

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

 are not now in immediate association with rapids or falls often point 

 to the former existence of rapids or falls. 



Rock terraces. — The tendency to sapping shown in many waterfalls 

 is also shown in the weathering and erosion of the sides of a valley 

 where a hard layer outcrops above the bottom, and the profile of the 

 side slopes of the valley simulates that of the stream: that is, the slope 

 b'ecames gentle just above the hard layer, and steep, or even vertical, 

 at and below its outcrop. This is illustrated by Fig. 120, where the 



MW^; 



X 



Fig. 120. — Diagram to illustrate the development of rock terraces. 



hard layer through which the stream has sunk its valley 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-lines''; that is, to 

 represent the height at which the water once stood. In one sense 

 this interpretation is correct, since a river has stood at all levels be- 

 tween that of the surface in which its valley started, and its present 

 channel, but the shelf of hard rock does not mean that the river, after 

 attaining its present channel, was ever so large as to fill the valley 

 to the level of the terrace. Rock terraces may also result from changes 

 of level. 



1 Gilbert, Science, Vol. VIII, p. 205, 1886. 



