


















148 THE LAKES OF IOWA, — PAST AND PRESENT. 
thick, and all the bowlders contained in the whole mass 
which has been swept out to form the valleys have gradually : 
rolled down upon their slopes, and many of them into tle 
streams. For this reason we usually find them more mume- 
up the for e ial beneath its w anpali, to be carried away 
in the form of muddy water at the times of its overflow, 
leaving the bowlders and gravel strewn upon its bed; w 
they may not be seen at all upon the prairie surfaces al 
them. ) 
- This latter fact being misunderstood has led to the a l 
position, that, being abanns upon those surfaces, they M 
been gathered up by human hands and carried to the ‘shor’ | 
to build the “walls” of; while the truth is, the embank 
ments, as well as the presence of the materials of sti 
they are composed, are due to natural causes alone, we | 
their origin is wholly referrible to the periodic action of 1, f 
aided in some degree by the force of the waves. 
The water in the lakelets is usually very low in late 
tumn, and when winter comes it is sometimes frozen nal | 
to the bottom in their deepest parts, so that occasionally | 
all the fish are killed by this means. The ice, of 
freezes fast to the bowlders as well as to whatever els! 
be within its reach, and the expansive power of from one 
five miles of freezing water is exerted upon them in @ 
tion from the centre towards the shores,—a powe! 
more than sufficient to move the largest bowlders upon 
gentle slopes, | 
- The embankments are from two to six feet high, a 
two to twenty feet across the top, and always separato 
piece of ground from the lake; because where the 0 
shore is a little abrupt, and higher than the high-water 
no embankment is formed, but the bowlders are ! 
thrust against the shore with such force as to j . 
and often thickly studded with them. 
i 
