SURFACE FEATUBES. 75 



which, it contains and which was at first hardly visible, 

 becomes in time even more conspicuous than the sand itself, 

 because a part of the latter has been wasted while the gravel 

 all remains. Thus it has been, upon an extended scale, with 

 the drift upon which all these lakes rest. It is composed of 

 soil, clay, sand, gravel, and boulders ; the former being usually 

 so much in excess upon the surface around the lakes as to 

 cover the boulders so completely that sometimes none are to 

 be seen there. They doubtless exist in greater or less numbers 

 everywhere in the drift, although they may not appear on the 

 surface, for wherever a stream has cut its valley down through 

 it, all the boulders contained in the whole mass removed, 

 become and remain exposed, while the fine material has been 

 swept away as sediment suspended in the flood-waters of the 

 streams. The boulders thus left collect upon the valley-sides 

 and many of them roll down into the stream collecting in its 

 bed. For this reason, we find them more numerous there and 

 upon other eroded surfaces than anywhere else. 



Again, the ceaseless dashing of a lakelet's waves stirs up 

 the finer material of the drift which rests beneath its waters. 

 This passes off as sediment at the time of their overflow, 

 leaving the gravel and boulders strewn abundantly upon its 

 bed, even if none of either is to be seen upon the prairie 

 surface around. This latter fact being misunderstood, has led 

 to the belief that their absence upon the adjacent prairies was 

 caused by their having been collected by human hands and 

 brought to the lakes to build the " walls " (?) of; while the truth 

 is, the embankments, as well as the presence of the separated 

 material of which they are composed, are due to natural 

 causes alone, the origin of the embankments being wholly 

 referrable to the periodic action of ice, aided to some extent 

 by the force of the waves. ' 



The lakes are almost without exception very shallow and 

 the water in them is usually low in late autumn, so that when 

 winter comes, it is frozen to the bottom over so wide a margin 

 from the shore, as to leave in some of them very little unfrozen 

 water in the middle. In some of the shallower ones, indeed, 



