34 THE PRINCIPLES OF AGRICULTURE. 



and thaw several times, there will be found a perceptible 

 quantity of fine particles at the bottom. 



But the greatest effect of ice in forming soil, and 

 changing the face of nature, has been through glaciers. 



In high, mountainous regions, and in cold latitudes, 

 snow steadily accumulates, forming immense masses of 

 ice in the deep valleys. These are steadily, but very 

 slowly, pushed along by their own weight until they 

 reach a lower and warmer region, where they melt away. 

 These huge rivers of moving snow and ice are called 

 glaciers. Rocks in the course of the glacier are torn up 

 and borne along, grinding upon one another, and grind- 

 ing paths through other solid beds of rock, until they 

 are deposited as bowlders and soil at the point where 

 the glacier melts. 



It is believed that, at some time in the history of the 

 earth, the regions to the north of the equator became 

 for a time much colder than at present, causing perpet- 

 ual snow to fall upon large portions of North America 

 and Europe, deep enough to bury most of the hills and 

 mountains beneath vast, continuous glaciers. 



These glaciers, moving toward the equator, ground 

 enormous quantities of rock into soil, and deposited it 

 over a large extent of country, together with the rocks 

 which remained unground. Huge bowlders, as well as 

 smaller rocks, scattered over the country, may be traced 

 back northward many miles, to their original bed. In 

 New England they have been carried two or three hun- 

 dred miles, and in the Mississippi Valley one thousand 

 miles. Soil and rocks which have been transported in 

 this way are called drift. 



5. Winds. Winds have also taken some part in form- 

 ing soil, and especially in changing its location. In 



