GEOLOGICAL CLASSIFICATION OF SOILS 53 



of till may alternate. This great mantle of material, 

 varying in thickness and constitution according to the 

 underlying rocks and the strength of glaciation, gives a 

 great soil province in northern United States which 

 may be designated as glacial till soil. 



39. Glacial till soils. — The glacial till soils may be 

 characterized physically as heavy or relatively heavy 

 soils. The tremendous force of the grinding has pro- 

 duced fine particles, and as a consequence clay loams 

 and silt loams predominate. Such soils usually have a 

 subsoil which is finer than the surface material and may 

 be so impervious to water as to produce bad drainage 

 conditions. The individual particles of a glacial soil 

 are found to be unweathered to a great degree unless the 

 soil is mixed with some of the old mantle of residual 

 material which once overspread all our glaciated areas. 

 The particles are jagged and unrounded ; the feldspars 

 retain all of their luster, and the iron stains so common 

 in residual soils are almost absent. As the glacial soils 

 are young soils, their colors are seldom made up of reds 

 and yellows, but grays and browns prevail. Red may 

 occur, however, where red sandstones have been glaciated 

 or where red residual soil has become incorporated in 

 the till. Where considerable organic matter has accu- 

 mulated the soil is usually very black. The subsoils 

 in the glaciated areas usually present colors ranging 

 from light grays to light browns. Blue or mottled clay 

 or clay loam is often found, due to a lack of aeration in 

 the soil ; to the soil expert such a condition near the sur- 

 face indicates a need of drainage. 



40. Composition of glacial soils. — The chemical com- 

 position of glacial soil approaches more nearly than that 

 of any other soil the composition of the original rock. 



