10 Practical Farming 



floods came, and nature began washing down the moun- 

 tains to form the soil of the lower levels. On this soil 

 and in a purer atmosphere, other forms of vegetation 

 appeared, and by their decay added to the mineral soil. 



Later on there came a great accumulation 

 The Work of ^ . ^^^ ^^^^^ ^^ ^^^ ^^^^^ p^j^ ^^^^^ 

 Glaciers ' 



gradually extended southward and by its 



weight so shifted the earth that a great change came in the 



climate. Plants of warm climates retreated southward. 



Fossil palm leaves of immense size have been dug up on 



the elevated Laramie plains, and give evidence of the 



climate that once prevailed there. As the great ice pack 



moved south, carrying with it the rocks and vegetation 



and grinding the rocks over which it passed, it finally 



made great deposits as its lower edge reached a warmer 



climate, and left through its whole course the evidence of 



its passage in the grooves plowed in the soHd rocks. The 



glaciers not only deposited their burden of rocks and soil 



at their termination, which can now be traced by the range 



of hills thus formed across the country from southern 



Pennsylvania westward, but deposited dams in the valleys 



which caused the formation of lakes in all the sections of 



the country exposed to the glacial action. Of all the 



agencies in reducing the mineral elements of the rocks to 



soil, the glaciers were probably the greatest. But, like all 



other ages in the growth of the earth, the glacial period 



came to an end, and the melting ice carried the ground-up 



^ soil southward and covered the rocks with the clays of 



to-day. In the sand and clays in the track of the glaciers 



the rock bowlders were dropped after being pounded and 



shaped by their journey. Thus are found in the clays of 



