156 Wisconsin Academy of Sciences, Arts, and Letters. 



from expansion conies in the shape of a bowing up of the ice. 

 The ice sheet breaks either at a place where it is weakest or 

 where for some other reason it is arched before expansion be- 

 gins. Contraction, however, precedes expansion, and it has 

 been observed that where the ice ridges occur, there the widest 

 cracks have previously been opened. The ice shove which ac- 

 companies expansion often elevates the ice to a height of eight 

 or ten feet, in consequence of which the ice sheet on either side 

 is depressed. Through the cracks in the synclinals thus formed 

 water flows out at the surface, forming lakelets which have been 

 observed 150 feet long by 75 feet wide and eight or ten inches 

 deep. When the temperature changes, the water of these lake- 

 lets freezes, resulting in a thickening of the ice at this place. 

 It has been observed that during alternating warm and cold 

 periods the ice ridges do not form at the same place. This is 

 probably due to the thickening of the ice at the place where each 

 successive ridge is formed, as above noted. The increased 

 thickness adds to the strength and rigidity of the ice at such 

 places, and at the next shove the ridge forms where the ice is 

 weaker. This change in the location of the ice crack on Lake 

 Mendota occurred three or four times during the winter of 1899 

 and 1900. 



The analoc:v between the deformation of a sheet of ice cover- 

 ing a lake and that of the crust of the earth is in some respects 

 very striking. The deformation in both instances is by both 

 tensile and compressive stresses, although the more marked 

 deformation is due to horizontal compressive stresses. The re- 

 sults of the expansion and contraction are in both cases very 

 nearly alike, representing a region in which movement takes 

 place by combined fracture and flowage. Faulting, folding, 

 jointing, and the welling up of liquid material from below 

 through cracks are pheaiomena common to both. In the case of 

 the ice sheet the student has the advantage of watching these 

 phenomena and noting their peculiarities. The cause is also 

 definitely known and the observer can measure relatively the 

 forces which are operating. 



In conclusion I will briefly summarize the changes involved 

 in the process of ice rampart formation. 



The 1st condition is one of lowering temperature owing to 

 which the water of the lake freezes. 



