THE WORK OF WATERS UNDERGROUND 115 



a day) and at any given point, in small volume. By making 

 the rocks which it saturates heavier and sometimes slippery, 

 ground water often assists 

 gravity in moving mate- 

 rial, sometimes masses of 

 great size, down slopes. 

 When the movement is 

 too slow to be seen, it is 

 called creep (Figs. 106 and 

 107), when sudden, a land- 

 slide or slump (Figs. 108 

 and 109). When slopes 

 are of unprotected clay, 

 ground water influences 

 creep as follows. As the 

 surface clay dries after 

 rains, it shrinks and cracks, 

 forming sun cracks. The 

 opening of a horizontal 

 crack is largely the result 

 of the down-slope move- 

 ment of the clay below, 

 rather than the up-slope 

 movement of that above, 

 for gravity assists downward movement, while it opposes 

 movement up-slope. With the first shower the clay swells 

 and the crack is closed. Under the influence of gravity 

 it is closed chiefly from above rather than from below. 

 Repeated shrinking and swelling' mean very slow move- 

 ment down slope. Other factors besides ground water 

 are involved with gravity in creep. For example, rock 

 fragments on a slope expand under the heat of day, and, be- 

 cause of the influence of gravity, the expansion is chiefly 

 downward. When the fragments cool, they contract, and 

 largely from their up-slope ends, since this again involves 

 movement with, rather than against, gravity. The result of 



FIG. 107. The side of a ravine near 

 Crawfordsville, Ind., showing trees 

 leaning down-slope, in part because 

 of creep. The surface material creeps 

 faster than that at a slight depth, tip- 

 ping the trees toward the axis of the 

 ravine. (Forestry and Irrigation.) 



