526 LIFE AS RELATED TO PHYSICAL CONDITIONS 



spread toward the south with extreme slowness and with 

 many halts, the plants of colder latitudes found conditions 

 suitable for their growth ever opening toward the south. 

 They were thus induced to spread in that direction, so 

 that at the time of the greatest extension of the ice the 

 plants suitable to a cold climate had penetrated far to the 

 south of their former habitat. 



As the ice receded, these cold-loving plants were forced 

 to follow its retreat or to climb the mountains in order to 

 obtain the climate they needed. They did both, so that 

 in areas once covered by the ice, plants similar to those of 

 far northern regions are found on the tops of the mountains 

 in middle latitudes. What was true of the plants was true 

 also of the animals. 



Waterfalls Due to Glaciation. As the ice spread over the 

 country it filled the river valleys in many places with debris. 

 When the ice melted away, some rivers could no longer 

 find their old courses and were forced to seek new ones. 

 In its new course a stream might fall over a cliff. 



The Merrimac furnishes a fine example of water power 

 due to glaciation. The great manufacturing cities of Lowell, 

 Lawrence, and Haverhill would not exist had not the river 

 been displaced from its previous channel by the glacial ice, 

 and in developing its new valley come upon ledges. The 

 Niagara is another notable example of vast water power 

 due to the displacement of drainage by the ice. It is probable 

 that in pre-glacial time there was a river which carried off 

 the drainage of the area now drained by the Niagara, but 

 it did not flow where the Niagara now flows. 



Thus we see that the hum of the spindle and the lathe 

 are often but the modulated whispers of those ancient forces 



