FALLS AND RAPIDS 331 



new course the stream may fall over a cliff, as did Niagara 

 River which used to fall into Lake Ontario over the cliff 

 near where Lewiston is now located. Here was developed 

 a great fall which, owing to the kind and position of the 

 rocks over which the river flowed, has moved back, leav- 

 ing a gorge about seven miles long. 



The rock layers are nearly horizontal with a hard layer 

 at the top and softer layers below. As the water strikes 

 the foot of the falls it drives rebounding currents against 

 the rock wall behind it, and wearing away the softer rock 

 undermines the harder rock at the top, which breaks off in 

 great blocks. Thus the falls maintain an almost vertical 

 wall behind them. These falls are about 160 feet high, 

 one of the grandest of nature's wonders and one of the 

 greatest sources of water power in the world. 



Falls or rapids may also be formed in a stream where it 

 passes from harder to softer material, as from the old land 

 to the coastal plain. The softer material is worn away 

 faster than the hard material and the stream bed lowered 

 more rapidly, thus forming a precipitous descent. Falls 

 of this kind were also formed where the glacial ice forced 

 the streams to make new channels for themselves across 

 the upturned edges of layers varying in hardness. 



The falls in the northern part of the United States were 

 most of them formed by the rearrangement of the drain- 

 age lines at the close of the Glacial Period, and those in the 

 southern part of the country by the more rapid wearing 

 away of the softer rocks of the coastal plain. Thus we 

 see that the hum of the spindle and the lathe are often 

 but the modulated whispers of those ancient forces which 

 thousands of years ago sorted the rock materials and built 

 the vast continental ice palaces of the Glacial Period. 



Streams which have falls and rapids have not flowed in 

 their present channels a long time, as time is reckoned 



