IV. Soils of the semi-arid or arid regions with a short grass or 

 desert vegetation. 



The content of organic matter is lower than in the preceding 

 group. Little or no percolation occurs. The movement of mineral 

 constituents is confined to a concentration of carbonates and soluble 

 salts just beyond or near the root tips in the drier regions and to the 

 removal of only the soluble salts in the moister. This condition is 

 found on the Great Plains of the United States and Canada and on the 

 Steppes of Russia (16, p. 132). 



V. Soils formed under excess of moisture, the upper soil horizon 

 being saturated with water the most of the time. 



Organic matter being unable to decay accumulates, iron is present 

 largely as ferrous salts, and pyrites is formed. Where there is no 

 outlet for the water evaporation will cause an accumulation of gypsum 

 and carbonates near the surface if these are present in surrounding 

 higher-lying soils. Peat, muck and marsh-meadow soils belong to this 

 group. 



VI. Alkali soils. 



These form in both the third and fourth zones in places where 

 water accumulates periodically and then escapes by evaporation, there 

 being little or no loss by seepage or run-oflf (18, p. 103), or where 

 water rises from a water table close to the surface and evaporates so 

 rapidly that the percolation following rains does not counter-balance 

 the ascent of soluble salts. 



Glacial History of Minnesota. 



The whole of the state of Minnesota, with the exception of a very 

 small area in the extreme southeastern corner, has, at one time or 

 another been glaciated (23 p. 16), the mantle of glacial material left 

 behind constituting the parent rock of most of its soils. The study 

 of the deposits left behind by the receding ice has shown that they 

 are the result of successive glaciations, some of the ice sheets hav- 

 ing radiated from centers east of James Bay and being designated 

 the Labradorian, and the others from north or northwest of Minnesota 

 and being referred to as the Keewatin. While some of the ice sheets 

 from different centers may have been contemporaneous, or the one may 

 have followed almost immediately after the other, in most cases each of 

 the advances of ice after the first was so widely separated in time from 

 its immediate predecessor that under the milder climate prevailing the 

 earlier deposit had had soils developed upon it and drainage channels 

 fully formed. The succeeding invasion then planed oflf the hilltops, 

 filled up the valleys and buried the remaining portion of the earlier 

 drift sheet. In those cases v.'here one of the later ice sheets did not 

 extend as far as the earlier, there is offered an opportunity for a com- 

 parison of the soils formed on the two. 



