MAN'S DISORDER OF NATURE'S DESIGN — ALBERTSON 365 



the middle west. These grasses advanced slowly toward the east and 

 northeast, proceeding as a wedge-shaped extension between the conif- 

 erous vegetation on the north and the deciduous forests on the south. 

 The grasses apparently displaced the deciduous forests in the drier 

 locations as far east as Ohio (Woodard, 1924). One explanation of 

 this unusual phenomenon of prairie succeeding the forest is that a 

 xerothermic period began during the Wisconsin glaciation and per- 

 sisted through the post-Wisconsin glacial retreat. Because of the 

 dry period, the advance of the deciduous forest from the south was 

 delayed, but the more humid grasses and their associates moved north- 

 ward and came in contact with the prairie vegetation that moved in 

 from the west. Thus the bluestems, the Indian grass, and the panic 

 grasses came to be associated with buffalo grass, the grama grasses, 

 and other xeric forms from the west. This association evidently 

 represents the farthest eastward general advance of the prairie vege- 

 tation of which we have any record. 



At a later period amelioration of the climate occurred which 

 gi'adually ended the xerothermic period. As a consequence, the 

 oaks, hickories, elms, ashes, cottonwoods, maples, etc., of the deciduous 

 forests followed the retreating grasses in a westward direction. As 

 the short grasses retreated westward, they took with them their 

 "cousins" from the south, and upon their return to the high plains 

 the more xeric grasses came to occupy the drier positions, whereas 

 the grasses of the more humid south became established on the eastern 

 border of the grassland formation and along streams and more favored 

 positions westward. 



There is no attempt here made to discuss in any detail the source 

 of the material that went into the formation of the soils of the 

 Great Plains except to mention in passing that some of the material 

 was brought in by glaciers, some by winds, some by water, and some 

 of the soils were formed in situ from existing rocks. Soil is not just a 

 mass of inert mineral and organic material. It must have both of 

 these materials, but in addition, if it is a good soil, it is necessary to 

 have soil solution, soil atmosphere, and an abundance of soil organ- 

 isms. The interaction of all these constituents working through cen- 

 turies of time has resulted in a soil that is one of the most fertile known 

 to mankind. It was the interaction of climate, plants, and soils that 

 brought plants and soils to their present native state of development. 



The prairie vegetation is particularly well adapted to the production 

 and protection of a deep fertile soil. The roots of many of our 

 grasses penetrate the soil to a depth of 5 to 8 feet depending in part 

 upon species of grass and in part upon the type of soil in which they 

 grow. Many of the broad-leaved herbaceous plants, such as wild 

 alfalfa, extend their roots somewhat deeper than do the grasses. 



