112 



PHYSICAL GEOLOGY 



deep, meandering rock gorges. When a stream has thus intrenched 

 its meanders, the evidence is strong that it has been rejuvenated. 

 Many examples of intrenched meanders are to be seen in Europe and 

 in the United States. In the latter, Pennsylvania, Kentucky, and 

 Utah furnish excellent and striking examples. The great natural 

 bridges of Utah, one of which has a height of 305 feet and a span of 





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Fig. 99. — Map showing the course of the Ardfrrhe River, France. The origin of 

 the natural bridge by the perforation of the neck of the meander is evident. 



273 feet, were formed by the perforation of the necks of intrenched 

 meanders, as was also that of the Ardeche River, France (Fig. 99). 



If a region is uplifted before the erosion cycle is completed, the 

 rivers will deepen their courses, leaving their former broad flood 

 plains (p. 128) standing as terraces or " benches." A section of such 

 a valley will show a valley within a valley. If a region is more ele- 

 vated near the ocean than further inland, the upper courses of the 

 streams will be " ponded," unlessthey are able to deepen their valleys 

 as rapidly as the land is elevated. This differential movement of 

 the earth's surface is called warping. Streams which hold their 



