25 



Discussion. The average percentages of coarser fragments (Table 

 13) are higher in the soils on the Kansan than in those of the Late Wis- 

 consin, with exception of the third foot-section of the Carrington loam. 

 The explanation of this exception appears to lie in the fact that on three 

 fields erosion has removed the surface material and thus left the less- 

 weather portion nearer to the surface than it is ordinarily found. The 

 larger amount of coarser particles on the younger formation may be 

 due partially to a less advanced degree of weathering than on the older 

 drift. This also accounts for the different amounts of softer rocks 

 (limestones, shales and cherty material) found on the two drifts, 

 these on the older formation having practically entirely gfiven way to 

 the processes of weathering to a depth greater than three feet. 



Glacial material was encountered in the third foot on two fields of 

 Marshall silt loam, but on the other three the thickness of the loess 

 deposit was sufficient to he characteristic. 



Table 13. — Coarse gravel iu the diifcrciii sections. Tin- data are the aver- 

 ages for the file fields on each type refortcd iu the last coluiiin of tables 5, 7, 

 9 and II. 



The average weight of the gravel particles (Table 14) found in 

 the three-foot section was much the same from type to type and from 

 drift to drift, that of those found on the Kansan being slightly higher 

 than that of those on the Late Wisconsin, the difference being greatest 

 on the Fargo silt loam. 



Fable 14. — Average weight of coarse gravel particles in the ditferent sections. 

 The data are the averages for the five fields on each type reported in the last 

 column of tables 6, 8, 10 and 12. 



