1922] EDWARDS, GEOLOGICAL WORK AT RAINER NATIONAL PARK 



69 



Among these, special mention might be made of the various forms 

 left upon the ice by differential melting and especially those forms 

 resulting from the protection afforded by blocks of rock and small 

 coverings of rock dust which shield the ice beneath and prevent melting. 

 Such blocks of rock are frequently left standing, as shown in figure 

 38, upon a pedestal of ice, which finally grows so tall that its base is 



Fig. 38. — A perched boulder on the Nisqually Glacier. 



no longer within the shadow. Then the melting of the ice, causes the 

 stone to be dropped, usually on the south side of the pedestal, as this 

 is the direction from which the most direct rays of the sun strike. 



In a few places the dust accumulations were very thick and instead 

 of increasing the rate of melting, have protected the ice beneath and 

 formed projections upon the surface of the glacier. The core of these 



