CHAPTER IX. 

 CHANGES IN THE LEVELS OF THE BEACHES. 



THE NORTHWARD ASCENT OF THE WESTERN SHORE-LINES. 



The successive shore-lines of Lake Agassiz are not jiarallel with each 

 other and with the present levels of the sea and of Lakes Winnipeg and 

 Manitoba, but have a gradual ascent from south to north, which is greatest 

 in the earlier and higher beaches and slowly diminishes through the lower 

 stages of the lake, being at last only slightly different from the level of 

 the present time. On the west side of Lake Agassiz the elevations of its 

 beaches have been determined by my continuous leveling, referred to sea- 

 level by railway survej's, through a distance of more than 300 miles from 

 its mouth at Lake Traverse northward to near Riding Mountain in Mani- 

 toba; and the accompanying table, on page 476, shows ap2:)roximately the 

 stages of the lake during the formation of these shore-lines in their rela- 

 tions to each other and to the present level. These stages of the water 

 surface have been assumed to coincide generally with the foot of the lake- 

 ward slope of the beach ridges, and with the base of the eroded shore 

 escarpments, the crests of the beaches having had a variable height from 

 5 to 15 feet above the lake, coiTesponding with their less or more massive 

 development, while the escarpments rose from the water's edge 10, 20, or 

 rarely 30 feet. 



In this table the estimated stages of the lake are noted for comparison 

 at its mouth, where it outflowed by the River Warren at the north end of 

 Lake Traverse, and on four lines of latitude which are nearly equidistant 

 from each other, passing through Fargo, Grand Forks, Emerson, and Glad- 

 stone, respectively 75, 150, 224, and 308 miles north of Lake Traverse. 

 Thougli tlie fourth of these intervals is somewhat greater than the others, 

 it may still be considered equivalent to them in the observed elevations and 

 northward ascent of the lake shores, because, as will appear further on, the 



