80 IOWA ACADEMY OF SCIENCES. 



The water in the lakelets is usually very low in the late autumn and 

 when winter cornea they are frozen nearly to the bottom in their deepest 

 parts, so that occasionally all the fish are killed by this means. The ice, 

 of course, freezes fast to the boulders as well as to whatever else may be 

 within its reach, and the expansive power of from one to five miles of 

 freezing water is exerted on them in a direction from a center towards the 

 shore — a force much more than sufficient to move the largest boulders on 

 these gentle slopes. 



The embankments are from 2 to 6 feet high and from 2 to 20 feet across 

 from the top, and always separate a low piece of ground from the lake; 

 because where the original shore was a little abrupt and higher than the 

 high-water level, no embankment was formed, but the boulders were 

 merely thrust against the shore with such force as to render it steep and 

 often thickly studded with them. 



Meeting no such obstruction on a marshy side, the material thrust out 

 accumulates just where the expansive force of the ice is spent. This process, 

 repeated year after year and age after age, has cleared the bottoms of the 

 lakelets of their boulders and other material and piled them up in circular 

 ridges upon their shores; and these are the walls that have excited so much 

 wonder. It has been observed that the embankments are heaviest on the 

 sides opposite the prevailing winds. This may be accounted for, at least 

 in part, by the fact that the ice being burdened with the material to which 

 it has frozen fast, would thus be floated against those shores when the 

 spring floods had raised the water of the lakes, and in part, also, by the 

 fact that the dashing of the waves would be almost constant against these 

 shores. 



It will thus be seen that whatever was originally upon the bottom, 

 whether boulders, sand, gravel or mud, has been carried to the shores and 

 we find the embankment composed of all these materials, arranged in 

 perfectly natural disorder. If bDulders were numerous we find the 

 embankment largely composed of them. If sand prevailed, a broadly 

 rounded embankment was formed, just such as we would expect from such 

 material; and where a peat marsh extends out into the land, an embank- 

 ment of turf is thrown up at the water's edge, which, being supported by 

 living rootlets, is frequently high and very narrow. 



I found nothing in my observations to contradict these con- 

 clusions of White. The accompanying photo, by Professor 

 Calvin, taken at Clear lake, illustrates nicely White's explana- 

 tion. 



It is probable that formerly the lake was drained into the 

 Boyer river. A very slight increase in the elevation of the 

 east shore or depression of the shore at the southwest would 

 again turn its waters to this course. White has indicated that 

 ice and ice pressure are shifting material constantly, and build- 

 ing up walls where shores are low and the slopes gradual. It 

 is easy to believe that at these two rather distant points on 

 this lake, the building up would not be perfectly uniform, and 



