60 Physics of the Soil. 



land and in parts of Alaska today that they move over the 

 face of the country much as a broad river would move, 

 except at a much slower rate. The same type of phenom- 

 ena occur, too, in the elevated mountain districts of Europe 

 and in the Sierras of this country, the ice streams con- 

 verging and flowing into the lower valleys in the form of 

 glaciers. As these ice streams move over the uneven sur- 

 face of their valleys and crowd against .their sides, the 

 rocks, gravel and sand taken up by the moving ice act with 

 great effectiveness to abraid into soil the rigid rock surfaces 

 over which they move. 



FIG. 18, Showing rock surface over which glaciers have passed, scratching and 



polishing it. 



In a recent geological epoch the whole of the North 

 American continent north of the Ohio and Missouri rivers 

 and much of northern Europe and Siberia were under enor- 

 mous moving ice sheets which resulted in the formation 

 of the extensive glacial soils of these countries ; consisting 

 largely of a rock flour ground to varying degrees of fine- 

 ness, and naturally very fertile where the materials have 

 not been sorted by the waters from the melting ice in such 

 a way as to form siliceous sandy plains. Figs. 16, 17, 18 

 and 19 are views illustrating different phases of soil forma- 

 tion by glacial action. 



