390 B. F. Griggs — Divided Lakes in Western Minnesota. 



extremely sharp point both resemble such spits. Its structure 

 and position show plainly, however, that water can have had 

 practically no part in its construction. There is nothing of the 

 Y-shaped ridging characteristic of cusps built out directly from 

 the shoreline, nor are there any hooks such as are often built 

 by long shore currents. Moreover this lake has no currents or 

 waves competent to build such a spit. Its longest diameter is 

 less than three miles and there are no currents along its shores 

 except those due to local winds. Such waves as are raised by 

 the prevailing westerly winds are directly opposed to the 



Fig. 2. 





Fig. 2. Front slope of rampart shown in fig. 1. 



building up of such north and south spits and, especially at 

 their extremities, tend to destroy them and scatter the materials 

 of the sharp ridge. 



That water currents can have no part in the formation of 

 these spits is shown very clearly by another small lake some 

 miles west of Detroit. This lake (tig. 4) is completely divided 

 by one barrier while two others project from its shores. It is 

 so small that water work of any magnitude is impossible. 

 Indeed it is so small as to raise the question whether ice-push 

 could become sufficiently great to heap up the ramparts 

 observed. Such action is undoubtedly greatly favored in this 

 lake by the very slight depth which would enable the ice to 

 seize abundant' materials for its work. It is known that ice- 

 shove is entirely absent from deep lakes, but to just what degree 

 extreme shallowness would favor the contraction and expansion 

 needs to be determined by careful field studies. 



