164 THE OCEAN RIVER 



but after its radius has reached about six hundred miles, the 

 growth of the icecap becomes rapid because the ice itself has 

 a coohng effect on surrounding areas and the rate of summer 

 melting is reduced. Growth continues until the ice extends 

 so far from polar areas that its margins encounter tempera- 

 tures sufficiently high to stop further extension. Rising tem- 

 peratures cause retreat of the ice and the modifications of 

 climatic patterns. The cooling effect of the icecap is so great 

 that retreat is slow until the ice has diminished in area to its 

 critical point, a radius of about 600 miles, after which the ice 

 disappears very rapidly/' 



This kind of a retreat from a maximum extension set in 

 just before the time of historic man; that is, before the time 

 of factual records of man's existence in a civilized community. 

 We know that since this retreat the level of the seas and the 

 level of land areas has risen and fallen, sometimes in re- 

 sponse to the release of the pressure caused by the mass of the 

 icecap. Today the Gulf of Bothnia is measurably rising in a 

 release of pressure and Greenland appears to be sinking. Just 

 what combination of conditions actually existed at the ad- 

 vance or retreat of the icecap has never been determined. But 

 three main factors, singly or together, are probably the cause, 

 namely: the variation of the orbit of the earth in its distance 

 from the sun; the transmission of tropical heat by winds and 

 ocean currents to the north; and sudden radical geological 

 changes in the topography of the earth's crust. Without get- 

 ting caught in the lively scientific battle of theories we can 

 look at the evidence in this order. 



When the earth, traveling in its orbit around the sun, is 

 nearest the sun it is called in perihelion; when most distant, 

 in aphelion. At perihelion the earth travels along its orbit 

 more rapidly than at aphelion; therefore, the season that coin- 

 cides with perihelion will be shorter and warmer than the 

 season that coincides with aphelion. At present the northern 

 hemisphere has its winter in this shorter season. Because of 



