316 C, K. Wentivorth— Wedge Work of Pebbles. 



effective position and the process will be cumulative in its 

 results. 



Assuming now a perfectly elastic rock mass but several 

 pebbles in place of the one. Let these pebbles be of dif- 

 ferent rocks having different thermal constants and be 

 differently exposed to the air and to the rays of the sun. 

 When periodic temperature changes take place not all 



Fig. 3. 



these pebbles will reach their maximum expansion at the 

 same time. Recoil of the rock will follow the progress 

 of the combined stresses rather than that of any one 

 pebble and some pebbles will be free to fall ever so slightly 

 while others hold the load. The pebbles thus act both at 

 the time of greatest heat and similarly at the time of 

 greatest cold to each other as pawls on a ratchet and the 

 process becomes cumulative even with a strictly elastic 



