GEOLOGICAL CLASSIFICATION OF SOILS 47 



It is the important soil of these regions. Glaciation was 

 instrumental, then, either directly or indirectly, in the 

 formation of three general classes of materials — glacial 

 drift soils, glacial lake soils, and a certain class of iEolian 

 materials designated as loess and adobe. 



34. The ice sheet. 1 — If in any region, but more likely 

 one of some elevation, the temperature and the snowfall 

 stand in such relationship that the heat of summer does 

 not offset the winter accumulation of snow, great snow 

 fields form. As this condition persists year after year, 

 and the snow becomes deeper and more widely spread, 

 the temperature is reduced to such an extent as to in- 

 crease the proportion of the precipitation which persists 

 through the summer's heat. The pressure of the over- 

 lying snow, and the water from the melting surface, bring 

 about a change of the snow into ice. Often a recrystalli- 

 zation appears to occur without a melting and refreezing. 

 As the depth of ice increases, the phenomenon of move- 

 ment is inaugurated as the thickness of the ice at the 

 center develops strong lateral pressure. Ice, when under 

 great stress, exhibits a plasticity which it does not or- 

 dinarily possess. As it moves slowly forward under 

 this tremendous pressure, and with a thickness of develop- 

 ment almost incredible, it conforms itself to every un- 

 evenness of the surface it may be invading. It rises over 

 hills, or shapes itself to valleys and even small depressions, 

 with surprising ease. The rate of advance or retreat of 

 a glacier is determined by the rate at which its edges 

 are wasting or melting away. If the melting is slow, the 

 ice front advances ; if it just balances the advance, the 



1 For a complete discussion of glaciers and glaciation, see 

 Salisbury, R. D. The Glacial Geology of New Jersey. GeoL 

 Survey of New Jersey, Vol. 5. 1902. 



