C. K. Wentworth— Wedge Work of Pehhles. 315 



pebble would ultimately wedge the rock apart. When 

 the pebble is cold and contracted it will fall until its 

 weight is supported. When the air becomes warmer the 

 pebble is heated more rajDidly than the general mass of 

 the rock and its expansion exerts pressure on the walls of 

 the crack or fissure. If the pressure is completely met 

 by an elastic yield of the rock mass and the rock recoils 



Fig. 2. 



as the pebble becomes cooler again the latter will not fall 

 and the process will merely be an alternate growth and 

 relief of stress with little or no cumulative rupturing 

 effects. If however the response to expansive stresses is 

 only partially elastic the rock will not recoil completely 

 and the pebble mil fall on cooling to a new and more 



