VERTICAL FARMING 3 



stretches so severely as to smooth off the rough places on brick 

 walls, and to scar the glass in windows. 



u. s. G. s. 



LIME STONE AFTER EROSION 



The Part Played by Water. Much rock material is slowly 

 dissolved out and carried away by rain water. This is usually 

 carried long distances before being thrown back into a solid 

 state by the evaporation of the water, or by coming into con- 

 tact with some other substance that causes it to be precipitated. 

 Often this reforms into another rock that may be harder than 

 the original one. 



A much greater effect of water, however, is in the formation 

 of ice, which expands and acts as a powerful wedge in splitting 

 off small fragments. You will often notice along the foot of a 

 cliff, or at the base of a large rock, a mass of small splinters of 

 stone that have been pried off the parent rock in this way. 

 Running water also slowly wears away even the hardest rocks, 

 reducing their close material into finer particles. 



