Geography of Ohio 203 



Former higher altitude of continent. Consequently, the explana- 

 tion most universally offered for a glacial period rests upon a 

 high altitude of the continent. Those who advocate this urge that, 

 preceding the Pleistocene period, northern North America must 

 have had a much greater altitude than at present. They do not 

 urge that this altitude was maintained throughout the period, but 

 that it existed long enough to make extensive snow fields, from 

 which ice moved over adjacent regions. 



Altitude alone cannot make an ice sheet. In many of our con- 

 tinents to-day there are extensive areas of great altitude, but no 

 large snowfields. More essential than altitude, in inducing glacia- 

 tion, is precipitation. A region may be but slightly above sea 

 level ; if its winters are long enough and a heavy snowfall occurs, 

 which the succeeding summers do not waste, eventually ice will 

 be formed. Therefore, the relation that an area bears to winds 

 and the consequent precipitation, and the relative length of its 

 summers and winters, are important elements in the probability 

 of glaciation. 



Change in position of the earth^s poles. It was once thought that 

 the earth's interior was in a hot, liquid, condition, and that 

 man and other organisms lived on the cooled, solidified crust. 

 On these premises, it was urged that tidal influence might shift 

 the polar areas into lower latitudes, the ''crust" slipping on the 

 molten or plastic interior, thus changing the geographical loca- 

 tion of the the glaciated regions, and inducing the development 

 of ice fields in regions which were shifted to the polar positions. 

 This liquid interior and thin crust theory is no longer accepted; 

 physicists have taught us its impossibilit3^ If the premises were 

 correct, and if glaciation were always developed at the poles, the 

 explanation would certainly account for temperate zone, and even 

 tropical ice fields. 



Change in eccentricity of the earth's orbit. This theory was ad- 

 vanced by James Croll.^ That the eccentricitj' of the earth's 

 orbit is not constant, is an accepted fact. The present difference 

 in the earth's distance from the sun between the perihelion and 

 aphelion positions is about three million miles. The direct heat 

 received from the sun varies inversely with the square of the dis- 

 tance. When the eccentricity is at its maximum, the difference 



^ Climate and Time in their Geological Relations (1890), pp. 312-328. 



