9 o 



ELEMENTARY SCIENCE 



them to contract, the expansion of water into ice being a 

 conspicuous exception to a general rule. Now where rocks 

 are exposed, as on the sides of cliffs and in deserts, and 

 especially where there are considerable changes in tempera- 

 ture between day and night, the alternating contraction 

 and expansion causes the outer layers of rocks to break 

 and scale off (Fig. 38). The surface of the rock, when the 

 sun is shining on it, becomes hotter and more expanded 



FIG. 38. Photograph showing how rocks may crack and 

 scale off, due to changes in temperature. 



than the interior. This causes a strain in the rock, and 

 starts the process of breaking. Then, at night, the surface 

 may become colder and more contracted than the part 

 just below it. This causes more strain and more break- 

 ing, so that as a result it is common to find that cliffs 

 have much loose rock on their surfaces, and the fallen 

 pieces accumulate in great heaps at the bottom. Such 

 accumulations of broken rock at the foot of cliffs are 

 called talus (see Figs. 39, 40). The talus at the bases 

 of large mountains is sometimes hundreds of feet deep. 

 This shattering of rock by alternate heating and cooling 



