132 PHYSIOGRAPHY 



Great ice-sheets are not known to have developed in other 

 continents during the glacial period, but their mountain glaciers 

 were very large. 



The history of the continental glaciers was complex, both in 

 Europe and in North America. In North America the history 

 was somewhat as follows: After the grow r th of the first great ice- 

 sheet, it shrank to small size, or disappeared altogether. Then 

 followed a relatively warm period, when plants and animals lived 

 in the region abandoned by the ice. Another continental ice- 

 sheet then developed, spreading over the region from which the 

 first had melted, and extending still farther south. As it advanced, 

 the second ice-sheet occasionally buried the soil on the top of the 

 drift left by the ice of the first epoch. Such soils, sometimes with 

 the remains of the plants which grew in them, are one of the means 

 by which it is known that there was more than one ice-sheet. A 

 third, fourth, and fifth ice-sheet, each somewhat smaller than its 

 predecessor, developed and disappeared. In other words, there 

 were at least five epochs when ice-sheets were extensive, separated 

 by epochs when the ice was greatly diminished, or when it dis- 

 appeared altogether. The ice-sheets of Europe had a similar 



history. 



Cause of the Glacial Epochs 



The development of the great ice-sheets was doubtless due to a 

 change in climate, and especially to a reduction of temperature. 

 The cause of the cold is not certainly known, though many explana- 

 tions have been suggested. One is that the northern lands were 

 raised to great heights. Another is based on changes in the shape 

 of the earth's orbit, and on changes in the direction of the earth's 

 axis. But the hypothesis which seems most likely to prove to be 

 true is that the change of climate was due to some change in the 

 atmosphere. An increase in the amount of carbonic-acid gas and 

 water vapor would make the climate warmer, while a decrease in 

 these elements would make it colder. Good reasons have been 

 suggested for variations in the amounts of these substances in the 

 air, and also for the heavy precipitation, which is as necessary as 

 low temperature for extensive glaciation, in the regions where the 

 ice -sheets existed. 



