RECORDS LEFT BY RIVERS 



steps) it is possible for the river as it foams in a waterfall 

 over the hard step at the top to eat its way into the 

 lower softer step. The lower softer step will gradually 

 disappear, and then the waterfall, still eating its way in, 

 will begin to undermine the hard top step, and when that 

 has gone on long enough the hard top step will fall down 

 and the waterfall will have to begin a little farther up the 

 stream. In this way a waterfall, gorge, or ravine can be 

 constructed by a river. 



The Falls of Niagara are an illustration of this method. 

 The river flows from Lake Erie through a level country 

 for a few miles, then begins to go faster as the path 

 becomes steeper, and finally plunges over a hard limestone 

 precipice. Beneath the hard limestone (the top step) are 

 softer beds of shale and sandstone. As the water eats 

 into them and removes them, large portions of the face of 

 the limestone precipice on the top fall into the stream below. 

 Thus gradually the Falls of Niagara are eating their way 

 back to Lake Erie, and have been doing so for hundreds 

 of thousands of years. In the process of doing so the 

 Niagara River has cut out below the Falls a gorge which 

 is not less than seven miles long, from two hundred to 

 four hundred yards wide, and from two hundred to three 

 hundred feet deep. There is no reason to doubt that the 

 Niagara gorge has been entirely cut out in this way, and 

 that at first the river fell over cliffs seven miles farther 

 down its course at Queenstown. The amount of rock 

 thus tunnelled would make a rampart about twelve feet 

 high and six feet thick going round the world at the 

 Equator. Still more gigantic are the gorges or caverns of 



53 



