9 o 



PHYSICAL GEOLOGY 



falls and their verticality are due to the fact that the strata which 

 compose the higher land consist of massive limestone, about 80 feet 

 thick at the falls, which are underlain by soft and easily weathered 



FlG. 64. — Falls of the Genesee River, Rochester, New York. 

 (Photo. C. R. Dryer.) 



and eroded shale. When the water plunges over the limestone, it 

 wears away the soft rock beneath more rapidly than the hard capping 

 stratum, leaving the latter projecting. Fragments are continually 

 falling from this overhanging ledge and are used by the water as tools 



Fig. 65. — Diagram illustrating the recession of a waterfall formed by a resistant 

 bed that dips up the stream. (Modified after Salisbury.) 



to excavate the shale further. This erosion is also aided materially by 

 blocks of ice in winter. The height of the falls is about 165 feet, and 

 tin gorge which has been excavated is from 200 to 400 yards wide and 



