4 SOILS OF THE EASTERN UNITED STATES. 



lacking. Not infrequently, however, there are found within the sub- 

 soil concretions and accumulations of lime carbonate, and occasionally 

 the limy remains of shells, principally of land forms of organic life. 



The Marshall silt loam is derived from the extensive body of fine 

 silty rock powder wliich overspreads a considerable proportion of the 

 central prairie States, mantling the older rock formations and cover- 

 ing the underlying glacial till to varying depths. This silty material 

 doubtless originated from the outpouring of turbid water through the 

 melting of the glacial ice during one of the later stages of its recession. 

 It was widely distributed over the central portion of the Mississippi 

 drainage region, and there is good evidence to show that when the 

 surface of tliis material became dried and powdery it was taken up by 

 the winds and thus distributed even over the higher elevations of that 

 region hi the form of a thin mantle of loess. In fact the Marshall silt 

 loam is one of several important soil types derived directly from the 

 partial weathering of this loess mantle. It constitutes the brown 

 prairie areas, stone free, and gently rolling to undulating in its charac- 

 teristic surface features. 



The Marshall silt loam and other soils of the Marshall series are 

 thus distinguished from the soils of the Miami series, which are light 

 colored and derived from the weathering of the glacial till, and also from 

 the black soils of the Carrington series, which resemble it in color, but 

 have also been derived principally from ice-laid materials. It is 

 separable from the soils of the Knox series, wliich also owe their origin 

 to the surface layer of loess, in that the latter are distinguished by 

 light-colored surface soils and are found within the timbered areas as 

 contrasted with the prairie areas in which the darker Marshall soils 

 occur. The black soils of the Waukesha series usually occur to the 

 northward of the regions occupied by the Marshall silt loam and its 

 associates, and the Waukesha soils are derived from the coarser out- 

 wash laid down directly by the action of moving water. 



The Marshall silt loam may be briefly characterized as the most 

 important soil of that series, and as the brown to black silty prairie 

 soil formed from the modification of the surface materials of the loess. 



SURFACE FEATURES AND DRAINAGE. 



Throughout its entire extent the Marshall silt loam is marked by 

 nearly level, slightly undulating, or gently rolling surface topography. 

 Only where the larger streams have cut deep trenches are sloping 

 areas found within this type. These, even, are infrequent since the 

 eroded and sloping bluffs of the loess along the stream drainage ways 

 are most frequently timbered and possess the lighter colored surface 

 soils, thus falling within the Knox series as contrasted with the Mar- 

 shall. This fact of nearly level surface, or at the most of slight undu- 

 lations, over large extents of territory has rendered the Marshall silt 



