18 AGRONOMY 



rocks that are not exposed, but the great banks of irregular 

 fragments at the base of cliffs show how rapidly the work of 

 tearing down the solid rock goes on under favorable condi- 

 tions. When water is present in the rocks the lowering of 

 the temperature below the freezing point causes weathering to 

 progress still more rapidly, since the expansive force of water 

 in freezing is equal to the weight of a column of ice a mile 



Photograph by E. Vickers 



FIG. 6. A hard portion of a rocky ledge 

 The softer parts have been carried away by the stream 



high, or about 150 tons to the square foot. Most rocks go to 

 pieces rapidly under alternate freezing and thawing. Even 

 polished granite soon deteriorates in severe climates through 

 the freezing of water that finds its way between the crystals 

 in the rock. The obelisk known as Cleopatra's Needle, which 

 resisted the atmosphere of Egypt for many centuries, began 

 to deteriorate at once when removed to the latitude of New 

 York, and had to be protected. When pieces of the rock 

 have been chipped off by the cold and carried to the base 

 of a cliff by gravity, running water may carry them for 



