GREAT RIVERS OF THE UNITED STATES 357 



epitome of the geographical history of our continent. It 

 winds its masterful way over the oldest and youngest 

 rocks. For part of its course it follows a valley built long 

 before the Glacial Period shrouded the northern part of 

 the continent in ice. In the northern part of its course 

 the blanket of debris left by this vanished ice choked its 

 path arid forced it to seek a new channel. For the south- 



THE COLUMBIA HIVER AND ITS OLD FLOOD PLAIN. 



ern part of its course its mighty sediment-laden waters 

 built new lands that it might extend its dominion. 



At times in its history its might has been greater than 

 now and at times less. But through all its history it has 

 borne to the ocean the ceaseless current flowing from the 

 heart of our continent. To it the modern geographer 

 turns again and again as an inexhaustible record of geo- 

 graphical development. The geographical, political, in- 

 dustrial and commercial history of this continent are 

 closely connected with this, its mightiest artery. 



