A MOUNTAIN STREAM 155 



The same sort of thing is familiar at the upstream 

 and downstream ends of the middle pier of a bridge 

 spanning a rapidly flowing river. 



There are many other physiographical features to 

 be studied in our stream besides the S -shaped wind- 

 ings in the meadow, the cutting back of the waterfall, 

 and the little examples of " crag and tail " ; but these 

 may serve as a beginning. There is a pleasure in 

 looking at the pot-holes, where a hard stone shut up 

 in a corner has been whirled round and round so 

 many millions of times that it has worn out a bowl- 

 shaped cavity in the rock. Many of the pot-holes 

 have lost their stones were they worn small and 

 swept away in a flood, or were they lifted away by ice ? 

 Other pot-holes have been left on one side by a change 

 in the line of the stream, and in one of them there is a 

 Golden Rod (Solidago officinalis) growing. 



When the earth's surface is folded like a tablecloth 

 forced into ridges, the heights form mountains and 

 the troughs form valleys. But it is very seldom that 

 this is the explanation of a valley. Sometimes, indeed, 

 the valley of to-day corresponds to the line of the 

 original ridge and not to the line of the original 

 trough. 



When two parallel breakages or faults occur in the 

 earth's crust and a strip of country falls in, if one 

 may so say, a long trough may be formed; and the 

 Jordan Valley was, to begin with, a depression of this 

 sort, and is known as a "rift valley." But it is not 

 very often that this is the explanation of a valley. 



The earth's crust is made up of heterogeneous 

 materials, different in hardness, chemical composition, 

 and other qualities, and the slow weathering effected 



