76 



PHYSICAL GEOGRAPHY. 



the water is occasionally all frozen and the fish killed by 

 that means. This was the case a few years ago with Walled 

 lake in Wright comity, bnt it has since been partly re-stocked 

 by the fry that reaches it from the rivers by way of the ontlet 

 at the time of overflow. 



The ice, of conrse, freezes fast to everything npon the 

 bottom, whether bonlders, sand, gravel, or mnd, and the 

 expansive power of the water in the act of freezing is exerted 

 npon them, acting from the center of the lake in all directions 

 towards its circnmference. Those who are familiar with the 

 expansive power of ice in the act of forming, will readily 

 see that under snch circumstances it would be more than 

 sufficient to move the largest boulder up the gentle slope of 

 the bed of the lake. It is true that the motion resulting from 

 one winter's freezing would hardly be perceptible, but the act 

 repeated from year to year, and from century to century, 

 would ultimately move everything upon the bottom beyond 

 the reach of the ice. The tracks of boulders thus moved 

 have been observed, being as unmistakable in their character 

 as those are which the river mussel leaves behind it in the 

 sand. 



Thus it will be seen that whatever was originally upon the 

 bottom of the lake within the reach of the ice, whether 

 boulders, sand, gravel, or mud, has been constantly carried 

 toward the shore, where we find them collected in perfectly 

 natural disorder, and forming a ridge just where the expan- 

 sive power of the ice ceased. Below the line of freezing the 

 same kind of material would of course remain unmoved 

 upon the bottom, because there is nothing to disturb it. 



The real embankments are only found separating a piece of 

 low land from the lake, because the material thrust out by 

 the ice against the steeper banks, merely accumulates there 

 upon the slope which rises above any such accumulation, 

 and also, of course, above the high water line of the lake. 

 In such cases the shore is often thickly studded with the 

 boulders thrust against it, but the material meeting no such 

 obstruction on a marshy side accumulates in circular ridges 



