CHAPTEE XXIX. 



DRAINAGE BEFORE THE ICE AGE. 



We have already said that during the ice 

 age river-beds were changed, valleys were filled 

 up, new lakes were made, and waterfalls 

 created. Great as were the changes made by 

 the carrying power of moving ice, still greater 

 were those made in preglacial times ; not, how- 

 ever, from the action of moving ice, but from 

 running water. Erosion caused by running 

 water has, probably, during the life of the 

 world, transported more material from place 

 to place, from mountain to valley, and from 

 valley to ocean, than any other agency ; chiefly 

 for the reason that it has been so much longer 

 doing its work. 



The valley of the Ohio River, a thousand 

 miles or more in length, together with the 

 great number of feeders that empty into it, is 

 an instance of the wonderful erosive power of 

 running water. The valley of the Ohio River 

 will probably average a mile in width at its 

 upper level and, deep as it is to-day, it was 

 much deeper in preglacial times. There is 

 evidence that the whole bed of the river was 

 from 100 to 150 feet deeper than it is at pres- 



