SOIL COMPOSITION 79 



surface washing during many centuries and collected in lower 

 lying flat areas. The exposure of the surface after the annual 

 prairie fires permitted some slight surface washing which other- 

 wise would not have occurred. 



The flat prairie soils occupy the lower lying level areas that were 

 naturally poorly surface-drained and inclined to be swampy, espe- 

 cially during the wet season of the year. This soil has been formed 

 in part from deposits of fine earth and vegetable matter washed in 

 from the surrounding higher land. The rank growing swamp 

 grasses have, from the partial decay of their roots (and of more or 

 less of their tops), added much organic matter to this soil. 



The undulating prairie soils vary from a gray silt loam on tight 

 clay in the older areas, to a dark brown silt loam, in the later forma- 

 tions, and the common flat prairie soils vary with age from drab 

 silt loam to black clay loam. 



Many other less extensive soil types occur here and there on the 

 prairies, including, as extremes, sand dunes formed of wind-blown 

 material from old shallow lake beds, gravel points, or exposed glacial 

 till, bogs of peat or muck, and sometimes adjoining strips of plastic 

 clay. Some intermediate types include deep silt loam, sandy loam, 

 silt on clay, etc. These are of small importance compared with 

 the very extensive and most common prairie soils reported in 

 Tables 15, 16, and 17. 



There are three principal types of upland timber soils in most of 

 the great loess-covered areas. One, a light gray silt loam, occupies 

 the flat areas ; a second type, yellow-gray silt loam, covers the 

 undulating or gently sloping lands; and the third (yellow silt loam) 

 is hilly or steeply sloping and consequently subject to serious ero- 

 sion, or surface washing, especially when under cultivation. In 

 addition, there are the areas of deep loess (yellow fine sandy loam) 

 covering the bluffs in many places along the larger river valleys; 

 and other less extensive types are sometimes found. In northern 

 Illinois and southern Wisconsin, in what is termed the lowan 

 glaciation, considerable areas are found of a brown sandy loam, 

 occupying in the main the undulating uplands. The top soil con- 

 sists of brown sandy loam, containing some gravel in places and 

 occasionally pieces of stone. The subsoil at a depth of three feet 

 or more frequently contains much stone, the proportion increasing 



