THE WORK OF STREAMS 



91 



The rate of cutting of the 

 4.5 feet a year since 1842, 



'SHALE==^f 



Fig. 66. — An ideal section of Niagara 

 Falls, showing how the soft shales are 

 being worn away, leaving the limestone 

 above unsupported. (Gilbert.) 



about 300 feet deep in places. 

 Canadian Falls has been about 

 while that of the American Falls, 

 because of the smaller volume 

 of water, is as small as 0.2 foot a 

 year. 



A natural bridge may be formed 

 when the water above a fall per- 

 colates through a joint or crack 

 athwart the stream and thence 

 along a bedding plane or approxi- 

 mately horizontal crack, emerging 

 under the fall as a spring. If the 

 cracks are enlarged by solution 

 and erosion, a tunnel large enough 

 to accommodate the entire volume 

 of the stream may be formed, and a natural bridge result (Fig. 67). 



(2) When the fall of a river in working up stream passes the 

 mouths of tributaries falls develop in them also. The beautiful Min- 

 nehaha Falls of Minnesota are an example of falls formed in this way. 



(3) In mountainous regions, where the main streams have 

 deepened their valleys rapidly their tributaries are often unable, 

 a t* because of their 



smaller volume, to 

 keep pace with them 

 and therefore flow 

 into them over falls 

 or rapids. To this 

 cause the " roaring 

 brooks " of New 

 England are for the 

 most part due. 



(4) The Atlantic 

 coast, from New 

 York southward, is 

 bordered by a low- 

 lying plain (Coastal 

 Plain, p, 224) composed of soft, unconsolidated sands and clays. 

 To the westward, this belt joins a belt of older and harder rocks 

 (Piedmont. Plateau) along a line roughly parallel with the coast. 



1 1 1 \ c 



11 1 1 £ 



«™»«1 



A B 



S :-i 



I, I 



_--i L_1_A 





m 



Fig. 67. — Diagrams illustrating the formation of a 

 natural bridge by the widening of a joint or other crack 

 B athwart the stream, through the solution of the lime- 

 stone by water which reappeared as a spring under the 

 fall at C. In the process of time a tunnel sufficient to 

 carry a large part of the volume of the stream was ex- 

 cavated, and finally the entire volume of the stream. 

 When this was accomplished a natural bridge (shown in 

 the cross section) spanned the valley. 



