Science, Geology and Physics 

 Recession of the Cataract. ,, 1 ? 01 , 



Hitchcock 



Everyone has heard how the cataract recedes and the gorge 

 elongates. The water pours over a cliff of hard limestone, from 

 1 40 to 1 70 feet deep, and pounds upon the soft rocks beneath. 

 As these give way, the higher, harder strata project like cornices, 

 and when they have assumed considerable prominence, being 

 unsupported they fall down. As seen from Horseshoe Fall, 

 erosion is more effective in the center of the current. The stream 

 rushes more and more furiously into the middle, and tends to 

 abstract some water from the sides — perhaps leaving large sur- 

 faces of rock bare. The undermining of the cliffs cause these 

 *' table-rocks " to fall, eventually, and thus the width of the 

 gorge is kept essentially uniform; and in the long run, the breadth 

 keeps pace with the extreme points of erosion. 



The piles of talus upon both flanks of the gorge throughout 

 its seven miles of extent, testify to the wide extent and con- 

 stancy of this movement. The fragments at the base of the 

 cataract will be broken quite rapidly, as nothing can resist the 

 force of the fall. The depth of the water here is greater below 

 the level of the stream than the height of the rock above. The 

 limestone is from 60 to 80 feet thick ; the shale, 82 ; the Clinton 

 limestone, 30 feet. Beneath the water level there may be 200 

 feet of the Medina terrane. 



By comparing the best views of the falls taken at different 

 dates, it is very easy to note the fact of recession. Precision has 

 been given to the rate by careful measurements, with the most 

 delicate instruments, of the positions of every part of the cataracts 

 at stated intervals. It is usual to compare the positions deter- 

 mined trigonometrically by Prof. Hall in 1842, and in 1890 

 by Engineer A. S. Kibbe. During this period of 48 years, about 

 220 feet in length of the limestone has been removed, or about 

 4|/2 feet each year. This means that the gorge is lengthened 

 by this amount each year. Although the uppermost part of the 



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