278 THE GLACIAL LAKE AGASSIZ. 



the lake and of its sliore deposits and planes of ei'osion in each of the more 

 than thirty stages which these shore-lines exhibit. The high-water surface 

 of the summers, however, had probably a nearly uniform elevation during 

 many years in each stage, producing therefore a beach or eroded line of 

 nearly constant height. On the other hand, the reduced lake level of the 

 winters, when the superficial melting of the ice-sheet ceased and the lake 

 doubtless became mostly frozen over, was likewise at nearly the same ele- 

 vation from year to year; but the beach ridges formed by the strong wave 

 action of the autumn, winter, and spring storms, with the effects of the 

 drifting lake ice during the breaking up in spring, would be mostly washed 

 away by the ensuing high water of the summer, when the glacial melting 

 attained its maximum. As the result of these annual oscillations of the lake 

 sm-face, gravel and sand from the material eroded during the storms of 

 winter, both from bordering cliffs and from the shallow lake bed close along 

 the shore, have been chiefly preserved in beach deposits at the higher plane 

 of the fluctuation reached in summer. 



Periodic oscillations occupying several years between successive max- 

 ima of the lake level were also probably caused by cycles of increase and 

 diminution in temperature and rainfall, with consequent in-egularity in 

 the yearly amount of the glacial melting. The cycles of rise and fall 

 of the great Laurentian lakes have a somewhat uniform average length of 

 ten to twelve years, as stated in Chapter XI, the maximum heights of these 

 lakes being 5 to 6 feet above their lowest recorded stages. But, on account 

 of the great variation of the tribute received by Lake Agassiz from the 

 departing ice-sheet in the alternating warm and cold portions of each year, 

 probably its annual fluctuations of level equaled or exceeded the changes 

 of longer periods in the Laurentian lakes, which receive a somewhat steady 

 supply through all the seasons, but are raised by excess of rainfall during 

 a few years together and then lowered by a series of drier years. 



THE UPPER OR HBRMAK BEACHES AND DELTAS IN MINNESOTA. 



Our description of the highest shore-lines of Lake Agassiz may well 

 begin at the mouth of this lake, the present site of the northern end of 

 Lake Traverse. Thence the uppermost or Herman beach was traced 



