THE WORK OF THE OCEAN. 389 



changes have taken place and are now in progress in many other lakes. 

 Lake ice/ — Since fresh water is densest at 39° Fahr., ice does not 

 commonly form on the surface until the temperature from top to bot- 

 tom is reduced to this point. Cooled below this temperature, the sur- 

 face-water fails to sink, and with sufficient reduction freezes. If the 



Fig. 331. — Ice crowding upon low shore. Clear Lake, la. (Calvin.) 



lake be small, and especially if it be shallow, it is likely to freeze over 

 completely in any region w^here the temperature is notably below the 

 freezing-point for fresh water for any considerable period of time. It is 

 under these circumstances that the ice becomes most effective. 



Suppose a lake in temperate latitudes, where the range of tempera- 

 ture is considerable, to be frozen over when the temperature is 20° Fahr. 

 If now the temperature be suddenly lowered to — 10°, and such change 

 of temperature is not uncommon in the northern part of the United 

 States, the ice contracts notably. In contracting, it either pulls away 

 from the shores or cracks. If the former, the water from which the 

 ice is withdrawn quickly freezes; if the latter, water rises in the cracks 

 and freezes there. In either case, the ice-cover of the lake is again 

 complete. If the temperature now rises to 20° the ice expands. The 

 cover is now too large for the lake, and it niust either crowd up on the 

 shores (Fig. 331) or arch up (wrinkle) elsewhere. It follows the one 

 course or the other, or both, according to the resistance offered by the 

 shore. 



If the water near the shore is very shallow, the ice freezes to the 



* Gilbert, Lake Bonneville, Mono. I, IT. S. Geol. Surv., p. 71, and Topographic 

 Features of Lake Shores, Fifth Ann. Rept. U. S. Geol. Surv., p. 109. 



