HOW ROCK BECOMES SOIL 281 



water runs over the surface of the earth, it follows the depres- 

 sions until it finally comes to a place of so low a level that 

 it no longer flows. When the difference in level is great 

 enough between two places which are near one another, 

 water runs rapidly from one to the other. If the difference 

 is abrupt, a waterfall is formed. Falling water wears away 

 the hardest rock if the falling continues long enough. Not 

 only does the water itself wear the rock upon which it falls 



Fio. 137. Freezing water breaks the rod 

 Note that last year's plants grew in crevices of the rock 



but many large and small pieces of rock are carried by it, 

 and these add greatly to its cutting power. 



282. The work of freezing water. Water may readily enter 

 crevices hi the rock. When it freezes it expands, and its 

 expansive force is very great. Alternate freezing and thaw- 

 ing may break the rock into smaller and smaller pieces. An 

 interesting experiment to show the effect of freezing water 

 may be made by filling a strong metal cylinder with water, 

 plugging the ends of the cylinder tightly, then placing it 

 where the water will freeze and carefully noting the results. 



