GEOLOGICAL CLASSIFICATION OF SOILS 49 



great sheet of ice possessing all the properties described 

 above. Accumulation seems to have occurred in three 

 well-defined centers, from which, over long geologic 

 periods, the ice slowly moved southward, encroaching 

 upon and covering thousands of square miles. The 

 ice cap of Greenland is a very good example of the con- 

 ditions then existing in the northern part of the United 

 States. The area covered by glaciers in North America 

 at the time of the greatest extension of the ice is estimated 

 as 4,000,000 square miles. The thickness of the sheet 

 was probably very great, ranging from a few feet at the 

 margin to probably a mile or more toward the centers; 

 at least it was thick enough to override some of the 

 highest mountains of the New England ranges. Local 

 glaciation also occurred on the hill and mountain tops, 

 which tended to increase the apparent thickness of the 

 ice mantle. 



36. Cause of the ice age. — The ice age was not one 

 unbroken invasion and retreat of the ice cap, but was, 

 as is conceded by *all authorities on glaciation, really 

 divided into epochs. Five great invasions appear to 

 affect at least the central part of the United States, 

 possibly without bringing about a disappearance of the 

 ice across the Canadian border. These interglacial 

 periods are shown by forest beds, accumulations of 

 organic matter, and evidences of erosion between the 

 drift deposited by the successive ice sheets. Some of 

 the interglacial periods evidently were times of warm, 

 and even semitropical, climate. Just exactly what was 

 the cause of the ice age is still under dispute. The most 

 probable theory, both as to its occurrence and as to its 

 disappearance, is that a change in the carbon dioxide 

 content of the atmosphere took place. It is believed 



