36 FARM DEVELOPMENT 



sand and gravel are not suitable soils from which 

 plants can secure food. 



The glacial period. Beginning tens of thousands of 

 years ago, and continuing, perhaps, ten thousand years, 

 a most important occurrence of vast proportions hap- 

 pened to the north temperate zone of the earth. At 

 least this occurrence was important for the welfare of 

 man in his present state, because it served as a stu- 

 pendous motor to provide immense areas of finely pul- 

 verized mixed soils suited to growing valuable crops. 

 Owing to some astronomical or geological phenomenon 

 not well understood, the sun failed to heat this zone as 

 had been its custom before, or as it does at present, and 

 the climate became as cold in Maine or Minnesota as 

 it is at the present time near the north pole. From 

 Missouri northward in America, and in a similar zone 

 in Eurasia, was a region of perpetual snow. The rain 

 and snowfall of each season was added to the layer 

 which fell the year before, and thus there gradually 

 accumulated a sheet of snow and ice hundreds of feet 

 thick, extending southward far into the Mississippi 

 valley basin in America and another sheet extending 

 into the central part of Eurasia. This sheet of accu- 

 mulated snow and ice was called the great glacier. In 

 the beginning the zone of perpetual snow of the arctic 

 circle gradually extended southward as the cold con- 

 tinued to increase from age to age. The cold crept 

 southward and extended the southern edge of the region 

 of perpetual snow. The annual fall of ice and snow 

 continued to accumulate and finally extended southward 

 in the United States, as shown by the map in Figure 7, 

 reaching to the Missouri and the Ohio rivers. It is not 

 thought that this movement took place rapidly or even 

 regularly, as during some ages the cold would increase 

 more rapidly than at other times, then again, the cold 

 would not increase but would even decrease. 



