122 



WORK OF SNOW AND ICE 



sand, gravel, and bowlders at the bottom. If the land at the shore 

 is very low, the ice in expanding may shove up over it, carrying the 

 debris frozen in its bottom, and it may push loose gravel, sand, etc., 

 in front of its edge. Where bowlders are frozen to the bottom of 

 the ice, the shoreward thrust as the ice expands shifts them toward 

 the shore, and they may be shoved up a little above the normal 

 water-level. The concentration of bowlders at the shore-line, year 



Fig. 125. Shore of Wall Lake, Iowa. (Photo, by Calvin.) 



by year, gives rise to the "walled" lakes (Fig. 125), which are 

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

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



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 may arch up the former into distinct ridges 

 (anticlines), the frozen part only being involved in the folds (Fig. 

 126). A succession of colder and less cold periods may give rise 

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

 ing of the ice against a low cliff of yielding material, such as clay, 

 disturbs all above the shore-line (Fig. 127). Where the cliff is 

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

 itself is warped and broken. 



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 may be floated miles down the river. At 



1 Buckley, Wis. Acad. of Sci., Vol. XIII, Pt. I, 1900. A study of ice ramparts 

 formed about the shores of Lake Mendota, Wis., in 1898-99. 



