230 



Sierra Club Bulletin. 



Figure i. — Diagrammatic section of sheet ice on a lake, illus- 

 trating modes of cracking from changes of temperature. 



In Figure i, x y represents, in section, a sheet of ice 

 resting on the water of a lake. We may think of the 

 temperature of the ice as initially 32°. When the air 

 becomes quickly colder the upper part of the ice is cooled 

 and made to shrink, with the result that many shallow 

 cracks are formed, as diagrammatically shown at ^. If 

 the cold continues, its effect is soon felt through the whole 

 thickness of the ice, but the bottom layer is kept at 32° 

 by the water in contact with it. The crack extends to the 

 bottom, but retains its wedge form (B). Water finds 

 its way into the crack, rises nearly to the top, and then 

 freezes (C). If, now, the weather moderates and the 

 upper part of the ice-sheet is warmed, expansion takes 

 place, but the process is not the simple inverse of the 

 contraction. The deeper of the cracks that were opened 

 cannot be closed again, because they are occupied by the 

 wedges of new ice. The upper part of the sheet there- 

 fore elongates, and in so doing carries with it the lower 

 part of the sheet. The elongation of the lower part 

 causes a new system of cracks (D), which start from the 

 bottom. Unless the temperature has fallen quite to 32°, 

 there is more or less freezing in these cracks, so that 

 they become partly plugged by wedges of new ice (F). 

 Thus, by to-and-fro changes between weather moderately 

 cold and weather very cold, wedges of new ice are in- 

 serted in the lake ice from above and below, and it is 

 made to expand horizontally. 



Usually some of the cracks from above, instead of 

 merely penetrating to the bottom (B), gape widely, so 

 as to open a lane of water (F), and such a lane may be 



