DESTRUCTION OF ROCK BY FROST 



8l 



original bulk of the water, takes place with irresistible power, 

 bursting thick iron vessels like egg-shells. 



Excepting loose, incoherent masses, like sand and gravel, no 

 rocks are formed of continuous sheets of material, but are rather 

 to be considered as masses of blocks, divided by the joints. (See 

 pp. 48 and 200.) In addition to these visible clefts, the blocks 

 are traversed by minute crevices, rifts, and pores, all of which 

 openings take up and retain quantities of water, as may readily be 



Fig. 25. — Cliff and talus slope, Delaware Water Gap, Pa. 



seen by examining freshly quarried stone. When exposed to a 

 low temperature, the water freezes and forces out the large blocks 

 and shatters them into pieces of smaller and smaller size. The 

 fragments thus formed are called talus, and great accumulations of 

 such blocks are found at the foot of cliffs in all regions where the 

 winters are at all severe. Talus accumulations are also formed by 

 other agencies, as will be seen in the sequel. Alternate freezings 

 and thawings not only break up rocks, but cause the broken 

 fragments and soil to work their way down slopes. Each freezing 



