100 



PHYSICAL GEOLOGY 



by streams, but the more typical upland loess was probably 

 deposited by the wind. Loess soils are of great fertility 

 where moisture is sufficient. 



THE ATMOSPHERE AND ROCK BREAKING 



Since changes in temperature are conditioned by the at- 

 mosphere, their effects upon rocks are considered here. 

 When water freezes, it expands about one tenth of its volume. 

 In doing so it exerts great force, as shown by the bursting of 

 strong pipes by water freezing in them. When water which 

 nearly fills rock cavities freezes, it may pry and break off 

 pieces, and it may form and enlarge cracks. All unobtrusive 

 processes by which exposed rock surfaces break up or decay 

 are processes of weathering. Weathering by the wedge work 

 of ice is obviously favored by many thawings and freezings. 

 For the maximum of weathering by this process, it is desirable 

 that the ice, having exerted great pressure upon the rocks in 



; forming, should melt 



promptly, so that the 

 water may penetrate far- 

 ther into the enlarged 

 cavities, and then freeze 

 again. Accordingly, the 

 wedge work of ice is in 

 general most important 

 in early and late winter, 

 in moist regions that are 

 situated in high middle latitudes. In very low latitudes tem- 

 perature changes never involve the freezing point, except 

 at high altitudes, while in very high latitudes the temperature 

 may be below the freezing point for weeks or months, continu- 

 ously. 



Changes in temperature help to break rocks in another 

 way. Rocks expand when heated and contract when cooled. 

 Since they are poor conductors of heat, it is their surfaces 

 which are not only first, but most affected by changes in 



FIG. 93. Exfoliation of a 

 bowlder. (Hole.) 



granite 



