226 GEOLOGY 



level. This constant process of concentrating bowlders at the 

 shore-line gives rise to the "walled" lakes (Fig. 182), which arc 

 not uncommon in the northern part of the United States. The 

 "wall" does not commonly extend entirely around a lake. In 

 making the walls, the ice driven up by the waves, especially in the 

 spring when the ice is breaking up, co-operates. 



If a lake is bordered by a low marsh, the ice and frozen earth 

 of the latter are really continuous with the ice of the lake, and the 

 push of the latter sometimes arches up the former into distinct 

 anticlines, the frozen part only being involved in the deformation 

 (Fig. 183). A succession of colder and less cold periods may give 

 rise to a succession of such anticlines. 1 If the shore is steep, the 

 crowding of the ice against a low cliff of yielding material, such 

 as clay, disturbs all above the shore-line (Fig. 184). Where the 

 cliff is sufficiently resistant, it withstands the push of the ice, and 

 the ico itself is warped and broken. 



Ice on rivers. Rivers also freeze over in cold climates, and 

 when the ice breaks up in the spring, the stones and bowlders to 

 which it was frozen in the banks are sometimes floated miles down 

 the river. At Montreal stone buildings 30 to 50 feet square, pro- 

 jecting so as to have river ice form about them, have been moved 

 by the ice of the St. Lawrence. 



When the river ice breaks up, masses of it are carried down- 

 stream, and may accumulate in vast fields or "jams" behind 

 obstructions in the river. Where a jam forms above a bridge, the 

 bridge may be swept away. The jams also occasion disastrous 

 floods above their sites, and when they break, the waters accumu- 

 lated above may sweep down the valleys with destructive violence. 



Northward-flowing rivers in the northern continents are es- 

 pecially subject to such floods. The snows of their upper basins 

 often melt while the lower parts of the streams are still frozen 

 over. The free discharge of the upper waters is thus prevented 

 for a time, and freshets follow later. 



Ice on the sea. In high latitudes, ice is formed aloni^ the 

 shore. Unlike fresh water, sea-water condenses until it free 



1 Buckley, Wis. Aoa.l. of Sci., Vol. XIII. I'l. I. !'.)(). A study ..f iofl 

 ramparts formed about the.shores of Lake McmlotM. \Vi- . in l.sirs '.!). 



