314 C. K. Wenhvorth— Wedge Work of Pebbles. 



tightly that it is impossible to remove them without the 

 use of a hammer. In other words many of the pebbles are 

 much more tightly wedged than would result from the 

 impact of falling alone. In figures 1, 2, and 3 are shown 

 several cracks in which pebbles were tightly lodged. 

 None of the pebbles shown could be removed with the 

 unaided hands. 



Fig. 1. 



The explanation seems to be that the pebbles are 

 wedged in place by the combined action of gravity and 

 the expansion and contraction due to changes in tem- 

 perature. In the case of a crack offering only moderate 

 resistance to further spreading and which does not close 

 again on removal of the force it is apparent that a single 



