52 LAKES OF NORTH AMERICA. 



lakes are frozen over, the expansion of the ice pushes up stones and 

 gravel along shelving shores and forms other topographic features. 

 Another process tending in part in the same direction comes into play 

 in the spring when the ice on a lake becomes broken and is moved 

 by the wind. The action under these conditions is the same that takes- 

 place on a much larger scale on the shores* of Labrador and other 

 northern lands, where an ice pack is driven on a shelving beach by 

 the force of the wind. Stones and l)oul(lers are carried up low lake 

 shores, in the manner here noted, and added to the ridge formed by 

 the winter expansion of the ice. Occurrences of this character have 

 been observed by J. B. TyrroU on the shore of Lake Winnipegasie.^ In 

 some instances these ice-built ridges are so marked and appear so much 

 like artificial walls that they are commonly referred to the work of man. 

 In some observed examples in the northern portion of the LTnited States 

 and in Canada, ice-l:)uilt ridges occur 40 to 50 feet from the water's edge, 

 are 20 feet high and l)road enough to furnish convenient roadways. 



The formation of ice-built walls about the margins of small northern 

 lakes by ice expansion was first explained by C. A. White.^ The process 

 has also been clearly stated by Gilbert,^ in his treatise on the topography 

 of lake shores, from which the following is quoted : — 



" The ice on the surface of a lake expands while forming so as to 

 crowd its edge against the shore. A further lowering of temperature 

 produces contraction, and this ordinarily results in the opening of ver- 

 tical fissures. These admit the water from below and by the freezing of 

 that water are filled, so that when expansion follows a subsequent rise 

 of temperature the ice cannot assume its original position. It conse- 

 quently increases its total area and exerts a second thrust upon the shore. 

 When the shore is abrupt the ice itself yields, either b}^ crushing at the 

 margin or by the formation of anticlinals (upward folds) elsewhere ; but 

 if the shore is gently shelving, the margin of the ice is forced up the 

 declivity and carries with it any boulders or other loose material about 

 which it may have frozen. A second lowering of temperature does not 

 withdraw tlie protruded ice margin, ])ut initiates other cracks and leads 

 to a repetition of the shoreward thrust. The process is repeated from 

 time to time during the winter, but ceases with the melting of the ice in 

 the spring. The ice formed the ensuing winter extends only to the water 



1 Geol. and Xat. Hist. Surv. of Canada. Ann. Rep., 1890-91, p. 04 B. 



2 American Naturalist, vol. 2, 1869, pp. 14(i-149. 



3 Fifth Ann. Rep., l^ S. (Jeol. Surv., p. 109. 



