WATER AS A MECHANICAL AGENT. 



231 



and thus carries on a process of displacement and destruction. It tears to 

 pieces rifted, jointed, and laminated rocks, often separating large masses ; and 

 as most rocks absorb moisture at the surface, if not also through the mass, 

 few escape disintegration by this means when exposed to icy weather. 

 Hence rocky bluffs in cold latitudes have usually a talus of broken stone, 

 while, in the tropics, this source of fragments fails. This kind of degra- 

 dation has produced much of the earth and coarser loose material of the 



globe. 



The divellent effect of freezing in fissures may be increased by an 

 addition to the ice first formed in the fissure through water taken in 

 between the ice and the rock. The same interstitial process often goes on 

 beneath the stones of a pebbly soil, and ends in lifting them out of the 

 ground, to a height of an inch or two, each on its own ice-column. The 

 process serves to bring the stones to the surface, and thus has an assorting 

 effect. 



As a body of water 35 feet wide will make a volume of ice a foot thick 

 and 36 feet wide, the freezing of the surface of small ponds brings pressure 

 against the sides, or their rocks, and shoves loose stones up the shore, making 

 a low rampart. It also causes fractures and ridges over the surface of the 

 ice. Freezing usually begins about the shores, and in its expansion this 

 littoral belt of ice slips over the water, and only the central portion, which 

 becomes frozen later, is thrown into a strain. 



2. Doionward creeping of soils through freezing. — A displacement or 

 creeping downward of the earth or loose material on inclined surfaces 

 is a common effect of successive freezings and thawings, as well as of 

 changing temperature and other causes. Interesting examples have been 

 described from North Carolina by W. C. Kerr. 



201. 



202. 



203. 



Displacement by the action of frost. Kerr, '81. 



Fragments of quartz veins are here represented as traveling down the 

 slope after becoming detached. In the first of the figures, the veins have 

 received a bend downward through the decomposition of the rock, a mica 

 schist, and the slipping movement also includes the soil. According to 

 the experiments of C. Davison (1889), each freezing produces a slight 

 upward movement, normal to the inclined surface, and the thawing, a 

 vertical settling, and thereby a displacement downward. The deeper the 

 freezing in any case, the greater would be the displacement. 



