50 SOILS: PROPERTIES AND MANAGEMENT 



that doubling the amount now present would bring about 

 tropical climate in the temperate zones, while halving 

 it would cause frigid conditions and a probable return 

 of the great ice fields. 



37. The extension of the ice sheet. — While the 

 advancing ice in general exhibited well-defined viscosity, 

 certain parts were more or less rigid. This was especially 

 true of the parts near the edges of the sheet. These 

 parts had become filled in their advance, particularly 

 near the bottom, with earthy and stony material, which 

 aided the erosive processes to a very great degree. The 

 eroding and denuding power of the glaciers is shown 

 everywhere by the gouged-out valleys and by the scratches, 

 or striae, on exposed rocks. As this sheet of ice slowly 

 advanced a few inches or a few feet a day, the mantle of 

 residual soil was carried away or mixed with the rock flour 

 constantly formed by the moving ice. The original soil 

 was really, an instrument for more effective ice action. 

 The scouring effect is observed now to the best advantage 

 in valleys which lay longitudinally to the ice movement, 

 as did the valleys of the Finger Lakes of central New 

 York. Valleys lying at right angles to the ice were very 

 often partially or wholly filled with debris, and the en- 

 tire topography was altered. Rivers flowing under the 

 ice often left large amounts of materials designated now 

 as eskers and kames. The mixing, grinding, trans- 

 porting, and stratification that went on emphasizes 

 again the great influence of glaciation on general topog- 

 raphy and soils. 



The greatest southward extension of the ice in the United 

 States is marked by a great terminal moraine (Fig. 8). 

 It is supposed that the margin of the sheet was stationary 

 at this point for a sufficiently long period to allow this 



