20 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1918. 



ered upper zone. In the more nearly level upland areas, weathering 

 under conditions of imperfect drainage has resulted in a mottled lower 

 subsoil. The accumulation and retention of the black organic matter 

 that imparts the dark color to these soils has been favored by their 

 prairie condition, topography, abundant vegetation, and probably by 

 an adequate supply of lime. The light-colored soils derived from the 

 loess were origmally wooded, and owing to their more rollmg surface, 

 which made erosion, leaching, and oxidation more active, conditions 

 for the accumulation of organic matter were much less favorable. 



In this area the darker colored loessial soils are divided into three 

 series, the Grundy, Tama, and Muscatine. ' The light-colored loessial 

 soils are classed with the Clinton and the Knox series. The Putnam 

 series has a rather dark surface soil but has a lighter gray subsurface 

 layer. 



Beneath the loess and overlying the bedrock are beds of glacial 

 drift composed of rock debris left by successive invasions of the con- 

 tinental ice sheets. Three beds of different age have been identified 

 in this area, but the two older are covered by the later or lowan 

 sheet. The drift is exposed only along deep stream valleys, and the 

 soil derived from it is of small extent. It is correlated as the Lindley 

 silt loam. 



The bedrock of the area, consisting of shale and limestone, is ex- 

 posed only to a very slight extent and its weathered products affect 

 the composition of the soils only over very small areas, none of suffi- 

 cient extent to indicate on the soil map. 



The alluvial or stream-deposited soils fall into two broad groups, 

 terrace soils, covermg the benches or second bottoms, and first- 

 bottom or flood-plain soils. The terraces lie above the present 

 limit of overflow from the streams, but the first bottoms are subject 

 to occasional or frequent inundations except where artificial protec- 

 tion has been provided. The more thorough weathering, aeration, 

 and leaching of the older terrace soils justifies their separation from 

 the newer soils of the present flood plains. 



The Grundy soils are dark brown to black and underlain by a 

 rather heavy, mottled dark-brown, drab, yellowish-brown, and 

 rusty-brown sul^soil. Frequently the lower part of the surface 

 layer is slightly lighter colored than the upper part, suggesting a 

 gray subsurface layer. The topography varies from level to rolling, 

 but is predominantly gently rolling. 



The Muscatine soils are dark brown to black, overlying dark-brown, 

 Hght brownish gray, grayish-brown, and yellowish-brown subsoils. 

 Below a depth of 24 inches the subsoil is heavier in texture and brown 

 or yellow mottled with gray. This material extends to a depth of 

 3 feet or more, though sometimes the lower 6 inches of the 3-foot 

 section gradually becomes looser than that immediately above. 



