98 POLAR PROBLEMS 



of hummocks) or of more or less large "pressure areas" (hummocked, 

 piled-up, and telescoped areas). ^ With a change of direction of the 

 motion of parts of the Arctic Pack, pressure ridges are destroyed 

 where they are and are replaced by water areas, and other pressure 

 ridges and pressure areas are formed elsewhere, and so on. 



In winter the number, development, and size of areas of open water 

 are considerably less, and therefore the parts of the Arctic Pack, not 

 being so free in their motion as they are in summer, form only cracks 

 and leads, which now shut, now open, or else freeze together with 

 new ice, the rapid growth of which progressively handicaps the 

 motion of these parts. There then takes place the formation of level 

 areas of one-year-old ice amidst the chaotic surface of the Arctic 

 Pack that had been created by the shifting and moving of its parts 

 in the autumn. 



The formation of pressure areas in the autumn takes place only 

 among these areas of new ice, while the formation of pressure ridges 

 takes place in winter also, although it is not so widespread as it is in 

 autumn. At the same time, during the whole winter and up to May, 

 the thickening of the ice proceeds by the natural accretion of freezing. 



In this manner, late in summer and early in autumn, the area of 

 the Arctic Pack has diminished, but, at the same time, its power, 

 weakened during the period of its thaw, increases in a mechanical 

 way — through hummock formation, telescoping, piling up. During 

 the winter the power of the Arctic Pack increases mainly through 

 accretional freezing; so does its area. 



With the beginning of the melting of the snow in June there 

 begins the decay of the winter solidity and compactness of the Arctic 

 Pack and the beginning of its melting and therefore of its decrease in 

 strength and later, after its breaking up, of its decrease in area. The 

 most powerful factor in this process is the thawing of the interstitial 

 snow and ice by whose freezing in the preceding autumn, when they 

 were in a melted form, the blocks of ice were compactly cemented 

 together; this thawing honeycombs and ultimately destroys the 

 hummocks, hummocked fields, and other piled-up ice formations. 



In the process of time pools are formed in the hollows and depres- 

 sions of the ice surface and around the hummocks. This nearly 

 fresh water, freezing in the cracks, widens them; the hummocks 

 themselves no longer present the solid compacted ice masses they 

 did before, and all protuberances on the surface of the ice are rounded 

 (Fig. 1 6). The hummocks and pressure ridges are finally ready to 

 fall into ruin when favorable conditions develop, i. e. if the wind 

 and tide are of sufficient force to cause a first displacement among 

 the parts of the Arctic Pack. This in turn leads to the destruction 

 of the hummocks and to the formation of channels and leads, which, 



6 The parts of the ice most subjected to this kind of pressure are the areas of the weakest, thin ice. 



