480 THE GEOLOGY OF MINNESOTA. 



[Chains of lakes. 



that was taken by the ice-sheet. The swells and undulations of the till 

 have their greatest extent in this direction, and the lakes fill the hollows 

 that are formed by its unequal accumulation. Among the hills of the 

 terminal moraines, however, the longer axes of the lakes are apt to be 

 transverse to the course in which the ice came, but parallel with its border. 

 In each case, such lakes are due to variable glacial erosion and deposition; 

 and the basins in which they lie are not more remarkable features of the 

 contour than are its swells, hills, and areas of highland. The deepest lakes 

 contained in depressions of the till in this state are from fifty to one hun- 

 dred and fifty feet in depth, reaching as far below the average level of the 

 drift-sheet as its most elevated portions rise higher; but a great majority of 

 these lakes, especially upon regions of only slightly undulating surface with- 

 out prominent elevations, are shallow, ranging from five to twenty-five 

 feet in depth. They mainly have very gently ascending shores, but some- 

 times on one or more sides are partially bounded by steep banks five to 

 twenty or thirty feet high, formed by the wear of waves which have eaten 

 away projecting portions of their margin of till, leaving its boulders, but 

 strowing its finer detritus over the lake-bed. 



In regions of modified drift, consisting of stratified gravel and sand 

 that were supplied from the dissolving ice-sheet, the lakes, from ten to 

 fifty feet or more in depth, and often bordered by level or undulating 

 tracts of modified drift, from twenty-five to one hundred feet or more 

 above them, lie in depressions which at the time of the fluvial deposition of 

 this drift were probably still occupied by unmelted masses of ice, prevent- 

 ing sedimentation where they lay and consequently leaving hollows en- 

 closed by steep and high banks, whose top is the margin of plateaus or 

 plains of gravel and sand. No examples of lake basins thus surrounded by 

 modified drift were found in Watonwan and Martin counties, neither of 

 which have any noteworthy deposits of this class, nor any such rough 

 morainic areas as to influence the distribution and trend of their lakes. 



Most of the lakes of Minnesota, and of all glaciated regions, present 

 only such forms and arrangement as are readily explained thus by the 

 modes of excavation and accumulation, and the diverse deposits of the ice- 

 sheets. The first described and most common type of lakes found upon 

 the surface of the drift, trending in parallelism with the course in which 



