SURFACE FEATTTKES. 77 



around the lake, and these are the walls that have excited 

 so much wonder. 



Fig. 2. 



38Ssak_. 



LOW WKniR'Jj^ 



,-.=sss -- ----i-'i.'. c „_- s r , - ;sl-;t/ag UH£. 



The above diagram shows the relative positions of the embankments, the high 

 and low water levels, and the lowest level of freezing. 



The embankments vary in height from two to ten feet, and 

 from five to twenty or thirty feet across the top, their size 

 and outline varying according to the materials which com- 

 pose them. If boulders were numerous upon the bottom, the 

 adjacent embankment is largely composed of them; if sand 

 prevailed, a broad, gentle, rounded embankment resulted, 

 just such as might be expected from that material; and if 

 mud, filled with the fibrous roots of water plants and sedges, 

 were brought out by the ice, a steep, narrow embankment 

 was formed, because such material will stand more erect in a 

 ridge or embankment than sand and boulders will. Such 

 embankments of fibrous mud are often found separating a 

 peat-marsh from the lake which was once a part of it. These 

 are erroneously called Beaver dams by those who forget that 

 beavers never attempt to dam still waters ; but dam running 

 streams only, that they may have ponds of still water for the 

 use of their colonies. 



It has been observed that the embankments are often 

 largest on the sides opposite the prevailing winds. This may 

 be accounted for, at least in part, by the fact that the ice 

 being burthened with the materials to which it had frozen 

 fast, would be floated to those shores when the spring-floods 

 had raised the water in the lakes ; and in part also, by the fact 

 that the dashing of the waves would be most constant and 

 forcible against those shores. The objection that the material 

 thus taken up and floated by the ice would be as likely to drop 

 anywhere else as at the shore, loses force when it is remem- 

 bered that wherever else they may drop they are subject to be 



