GEOLOGICAL HISTORY OF THE EARTH 43 



terials were taken from less than a hundred miles ; how- 

 ever, geologists believe that in Europe, as in America, 

 some materials can be identified as belonging to ledges 

 five or six hundred miles northward. The southern two- 

 fifths of Minnesota, or that part south of the line drawn 

 east and west through St. Cloud, Minnesota, is an area 

 where the surface is made up of bowlder clay, more 

 technically called " till," into which is mixed clay, sand, 

 gravel and stones. This same kind of excellent soil- 

 making material was left on the surface through Iowa 

 and as far as the glacier proceeded into Missouri, through 

 eastern Dakota, Nebraska, Illinois, Wisconsin and other 

 states eastward adjoining the Great Lakes over whicb- 

 the great glacier moved. Northeast of St. Cloud, Min- 

 nesota, there is not such a happy rruxture, and the soils 

 are assorted, sand, gravel and clay often appearing in 

 separate areas. 



Soils formed in place. Only in the northern states, 

 however, do we find these mixed soils of glacial origin. 

 In most districts, as in the southern part of the United 

 States, the soils have been formed in situ (in place). 

 In the Piedmont Plateau region of the South Atlantic 

 states the soil is the remains of rocks which have gradu- 

 ally decayed, only those particles remaining to form the 

 soil which longest resisted decay. In some cases a 

 limestone rock has decayed, leaving a limestone soil ; in 

 other cases a granite rock has rotted, the more soluble 

 particles being washed out, leaving particles forming a 

 granitic soil. In some places more than one layer of 

 rock has been dissolved, the remains of the upper rock 

 being mixed with the remains of the lower layer. In 

 many cases the fine particles resulting from this soil 

 weathering have been mixed by the agency of water 

 and wind, especially in the lower places, resulting in a 

 mixed soil, or more frequently in a stratified soil. In 

 case of young soils, as where rocks have recently been 



