SOILS AND FERTILIZERS 71 



Note. Many thousand years ago the northern part of North 

 America was covered by a great accumulation of ice and snow, called 

 an ice sheet. The 

 period of the for- 

 mation of this large 

 glacier is called the 

 Glacial Period. At 

 that time conditions 

 were favorable for 

 the accumulation of 

 a great depth of ice 

 and snow. The 

 pressure became so 

 great at the point 

 of greatest depth 

 that the ice was 

 forced out at the 

 bottom as a vast 

 stream. It ex- 

 tended south as far 

 as the central part of 

 the United States. 

 Such great force had 

 this ice stream in its 

 motion that it planed 

 off large areas of 

 rock, plowed up 

 hills of clay, gravel, and bowlders, and transported the mixture hun- 

 dreds of miles from its source. When the climate changed and the 

 ice melted, the transported material was left in places in the form of 

 lines of hills called moraines, and in other places the surface was left 

 comparatively level, forming rolling prairies. 



FIG. 15. Map showing the Area of North America 

 covered by Ice in the Glacial Period. 



(Salisbury. Geological Survey of New Jersey.) 



Moisture in Soils. -The soil is the great storehouse 

 of moisture. Through this conservation of water 

 which the plant needs to hold its food in solution, the 



