Buckley — Ice Ilamparts. 153 



temperature begins to rise compressive stresses are set up, which, 

 as in the case of the tensile stresses, are greatest at the upper 

 surface of the ice sheet, where the rise in temperature is great- 

 est. They are absent at the lower surface where the tempera- 

 ture is constant. These stresses find immediate relief only nor- 

 mal to the surface of the ice. The horizontal stresses occa- 

 sioned bv the expansive force of the molecules accumulate until 

 they are povv^erful enough to overcome either the rigidity of the 

 ice and the tensile strength of the lower layers of ice, or the rig- 

 idity of the bank and the tensile strength of the lower layers. 



The enormous force with which water expands when passing 

 from a liquid to a solid state is well known, but must not be con- 

 fused with the force of expansion of ice due to a rising tempera- 

 ture. If the force in the case of freezing water and rising tem- 

 peratures can be said to be proportional to expansion, then the 

 force exerted by water upon freezing will be 666 times the ex- 

 pansive force of ice when raised 1° F. In other words it would 

 require a change of QQ%^ F. in the temperature of the ice, to 

 make the expansion caused therebv eaual to that of the same vol- 

 ume of water in passing from a liquid to a solid state. 



If the rigidity of the banks is greater than that of the ice cap 

 relief will take place by the bulging or arching up of the latter. 

 If the banks are weaker than the ice cap, ramparts v/ill form 

 along the shore. The initial relief usually takes place suddenly, 

 as shown by observations on Lake Mendota. Ice ridge-s have 

 been known to form instantaneously on this lake, with a report 

 and shock resembling a miniature earthquake. After the ridge 

 or rampart has once formed relief takes place 'pari passu with 

 the compressive stresses. When the relief is once initiated it re- 

 quires but little force to keep the mass moving. 



Owing to the greater compressive stresses in the upper than in 

 the lower layers of the ice sheet, when expansion takes place, 

 tensile stresses are set up in the latter which must overcome the 

 strength of these layers before relief can take place in any man- 

 ner. When the compressive stresses are powerful enough to 

 overcome the strength of these lower layers, as well as the rigid- 

 ity of the ice sheet or bank, relief takes place in the lower layers, 

 through the formation of cracks which begin at the under sur- 

 face and extend upwards. (See Plate XIV.) These cracks 



