WEATHERING 



15 



vegetation. Very porous rocks are frequently disintegrated rapidly 

 by freezing, especially when the rocks approach saturation. Kocks 

 possessing vertical joints or made up of inclined strata of different 

 material will weather rapidly because of greater absorption of water. 

 This action does not occur in tropical or subtropical climates, but in 

 temperate regions it is very important in breaking down rocks and 

 in keeping the subsoil open so that both air and water may enter the 

 soil much more readily and carry on their work to a greater extent 

 upon the underlying rocks. 



Fio. 8. Front of Columbia Glacier in 1910 compared in height to Bunker Hill Monument. 

 The pinnacle fell a few minutes after the picture was taken. (Lawrence Martin.) 



(c) Glaciers. At the present time the work of glaciers is lim- 

 ited to a comparatively small area of the earth's surface (Figs. 7 

 and 8). During the glacial period about half of North America 

 and Europe were covered with an ice sheet, and the work of this 

 agent was very important in that it leveled hills and filled valleys, 

 ground up and deposited large amounts of fine soil-forming mate- 

 rial. This deposit is found not only on the glaciated areas, but was 

 carried far beyond the ice sheet by water and further distributed by 

 the wind. (Jlacial areas are now confined to polar and a few moun- 

 tainous regions. (Jreenland with an area of 500,000 square miles is 



