166 THE ICE AGE IN NORTH AMERICA, 



narrower portions of the rock gorges through which the stream 

 runs, was met by the sagacity of Professor E. H. Williams, 

 whose intimate knowledge of the subject obtained as a hy- 

 draulic and mining engineer has enabled him to make the 

 whole process clear. This is checked by the fact that the 

 planes of sedimentation uniformly follow the contours of 

 the underlying rocks and show that, first, those contours 

 were carved out before the deposition began and, second, 

 that the later sediments poured over them as in the case of 

 ordinary bars and fans and varied from coarse to fine with 

 the velocity of the current. 



As tidal currents scour out and keep clear certain channels 

 where their action is concentrated, so a powerful river tor- 

 rent scours out its main channel to a great depth, while it 

 throws up and deposits its sediment both coarse and fine upon 

 the higher bordering levels whenever there is a bend in the 

 trough changing the course of the current, or wherever there 

 is a strong tributary coming in at a broad angle. The forces 

 involved are nothing other than those which form an ordinary 

 flood plain, except that they are extremely vigorous. On 

 following down the Allegheny and Ohio valleys all the high 

 level gravel terraces can be accounted for by the action of 

 the tumultuous torrents from the melting ice of the closing 

 stages of the Glacial period as they poured through the 

 tortuous channel as it now exists. 



