Lake Ramparts. 



231 



Figure 2. — Outline of lake 

 with parallel shores, show- 

 ing arrangement of master 

 cracks. 



closed by new ice (G). Cracks of this type run from 



side to side of long narrow 

 lakes (Fig. 2), or from cape 

 to cape of lakes with irregular 

 shores (Fig. 3). 



The enlargement of a sheet 

 of lake ice by this process 

 thrusts its edges against the 

 shores. The force of the 

 thrust is measured by the strength of 

 the ice. Its results are various, de- 

 pending chiefly on the character and 

 outline of the shores. Where a shore 

 is steep, the ice-sheet may break 

 against it (Fig. 4, f/). Where the re- 

 sistance to shoreward movement is 

 strong in both directions, the ice-sheet 

 may buckle up or down (/ or /), 

 creating an ice-ridge, or a lane of open 

 water. These features, known in Sweden as rdkar, 



Figure 3. — Outline 

 of irregular lake, 

 showing arrange- 

 ment of master 

 cracks. 



mm. 



Figure 4. — Ideal section of ice sheet on a lake, illustrating 

 effects of thrust caused by changes of temperature. 



usually follow broad cracks of the type marked G in 

 Figure i. 



On gently shelving shores the edge of the sheet is 

 merely slidden landward, bending or slightly cracking as 

 it rises along the slope {K), and carrying with it what- 

 ever may be frozen to it or in it. Some of the sand, 

 mud, pebbles, etc., constituting the bottom is thus moved 

 tov/ard the land ; and boulders which project high enough 

 to be firmly grasped by the ice are dragged in the same 

 direction, plowing the bottom as they go. In the ensuing 

 summer, when winds stir the surface of the lake, the 



