R. F. Griggs— Divided Lakes in Western Minnesota. 391 



Even a superficial examination is sufficient to show that ice 

 is the only agent now at work in the formation of these divid- 

 ing barriers. It is easy to see that the effects of ice action on 

 such submerged ridges as that described in Lake Detroit would 

 accentuate the ridge and pile up the rampart till it emerged 

 from the water and formed a complete barrier across the lake. 

 It is not difficult to see how such a ridge would be formed on 

 any shoal which the ice could reach and grapple, especially if 

 it happened to be located, as is very often the case, along lines 



Fig. 3. 



Fig. 3. Partially completed dividing rampart. Lake Detroit. 



of the major cracks in the ice which tend to run from head- 

 land to headland because of the strains set up by the independ- 

 ent expansion and contraction of the separate bays of the lake. 

 But it is difficult to understand how ramparts could be built 

 out from the shore even along the major cracks where the ice 

 buckles up and forms high ice ridges, unless there were a shal- 

 low place in the beginning. But though these lakes are shallow 

 as lakes go, areas shallow enough to be reached even by the very 

 thick ice of the Minnesota winter are by no means abundant. 

 Though it be demonstrated that ice is responsible for accentuat- 

 ing and extending these ridges after they reach a certain point 

 in their history, it is a question how much of their formation 

 may be assigned to this agency and how much must be attributed 

 to the original irregularities of the lakes, such as lines of moraine 

 crossing them. This question can only be answered by extended 

 field study in the region conducted during the winter, when the 

 ice may be seen at work and its actual effects determined. 



But while it may be difficult to explain the division of large 

 lakes completely by ice action, it is evident that the cutting off 

 of small ponds and swamps from the main body of the lake is 



