SOIL FORMATION 73 



Cleopatra's Needle is a good example of this disintegration of 

 rock by freezing. This granite obelisk had stood for centuries 

 without injury in the warm equable climate of Egypt. On being 

 removed to New York, however, it was found necessary to coat 

 the surface with paraffin to keep out the water 5 otherwise, the 

 inscriptions would soon have been completely obliterated. 



In the temperate regions the wide daily fluctuations of tem- 

 perature are potent factors in the disintegration of rock. More- 

 over, it keeps the soil and often the subsoil open so that both air 

 and water may enter. These accelerate the chemical and biologi- 

 cal factors going on within the soil. 



The superiority of fall plowing is due in a large measure to 

 freezing. The freezing disintegrates the rock and liberates more 

 plant- food, and the heaving of the soil leaves it more open, thus 

 improving tilth. 



Wind. — One must live in a dry-windy climate to fully appre- 

 ciate the importance of wind in soil formation. The wind free 

 from sand accomplishes little, but when laden with the sharp 

 angular sand of arid districts it becomes a powerful engine for 

 modifying the surface of the earth. The sharp grains of sand 

 while in the air are ground against each other and blown against 

 the surface of rocks with such force that cliffs are slowly worn 

 away. We often see in mountainous districts fantastic shapes 

 which have been carved by the continuous impact of the sand. 

 The effectiveness of wind decreases as the distance from the 

 surface of the earth increases. Hence, we often find huge pear- 

 shaped bowlders which have been carved by the wind. In time 

 they topple over, and the work of undermining is repeated. 



The effectiveness of wind laden with sand is well illustrated 

 in the fact that telegraph wires along the Trans-Caspian Rail- 

 way have had to be renewed after eleven years, for the continu- 

 ous impact of the sand had reduced the wire to one-half its 

 original diameter. It is also found necessary to protect by means 

 of piles of rock or short additional posts the telegraph poles along 

 the Southern Pacific Railway through the San Bernardino Pass 

 in Southern California in order to save them from the driving 



