392 7?. F. Griggs — Divided Lakes in Western Minnesota. 



entirely explicable on this basis. Though such cases are less 

 striking than the larger ramparts such as that in Lake Detroit, 

 they are very much more numerous. Almost every large lake 

 in the region has several sloughs cut off by ice ramparts. 



The effect of the formation of ice barriers across the bays 

 of a lake is very marked on the later history of the lake. The 

 primary effect is to dredge out the shallows to clean out the 



Fig. 4. 



Fig. 4. Small lake with three ramparts. 



advancing marsh vegetation and to pile up steep banks, thus 

 enlarging the lake and counteracting the most potent agencies 

 tending to fill it up. But when the barriers are thrown across 

 so as to divide the lake into small parts, secondary effects 

 appear which reverse the primary. As the reaches of water 

 over which the wind sweeps are reduced, the consequent erosive 

 action of waves on the shores is lessened and transportation 

 by long shore currents is checked. This greatly accelerates 

 the filling up of the lake by favoring the growth of vegetation 

 in its shallow water. This is so marked that it is difficult to 

 find such a bay which is not already far along toward extinc- 

 tion. Most of them are so choked with vegetation that though 

 their character is plainly evident on inspection it cannot be 

 made to appear in a photograph such as figure 1. It is obvious 

 that in regions where its action is well developed ice-shove is 

 to be reckoned as an indirect aggradational factor of first impor- 

 tance. 



Literature cited. 



Bulkley, E. R., Ice Ramparts. Trans. Wis. Acad. Soi., Arts and Letters, 

 xii, pp. 141-157, 1900. 



Gilbert, G. K., Lake Bonneville. U. S. G. S. Monogr. 1, p. 57, 1890. 

 , Lake Ramparts. Bull. Sierra Club, vi, pp. 225-234, 1908. 



Columbus, Ohio, Feb., 1909. 



