History of lake Agassis. — Up ham. 193 
sota and 150 to 200 miles in North Dakota and southern Mani- 
toba, with a maximum of probably not less than 300 miles in the 
Red River valley, where lake Agassiz would doubtless cause a 
more rapid melting of the ice-margin. Through this time the 
river Warren eroded a channel about fifty feet deep, approxi- 
mately from 1,100 to 1,050 feet above the sea, or perhaps it 
eroded only the lower half of that depth, in the moderately undu- 
lating sheet of till which reached across the present valley of 
lakes Traverse and Rig Stone. The shortness of the time prob- 
ably occupied in the formation of the beaches of lake Agassiz 
may well astonish us in what it implies concerning the rapidity of 
the recession of the ice-sheet, and the brevity, geologically speak- 
ing, of the stages of pause or re-advance when its moraines were 
accumulated. 
The retreat of the ice seems to have uncovered the southwest 
border of lake Agassiz earlier than its shores farther north and 
on its east side, as is shown by the Milnor beach, a less distinct 
shore deposit than the Herman beach and 20 to 25 feet above it, 
which was observed near Milnor, North Dakota, and along a dis- 
tance of about ten miles thence northwest to the Sheyenne, but 
was not recognized farther north nor in Minnesota. The forma- 
tion of the Sheyenne delta had begun at this time of the Milnor 
beach, and continued through the time of the Herman beach, with 
which latter the Ruffalo, Sand Hill, Pembina, and Assiniboine 
deltas were also contemporaneous. The departure of the ice from 
the Red River valley seems to have been too rapid to permit the 
accumulation of definite shore deposits" on the borders of lake 
Agassiz, excepting the scanty Milnor beach derived from the 
Sheyenne delta, until its outlet was cut down to the level of the 
Herman beach, which probably represents a time of much slower 
erosion of the outlet, due to diminished glacial melting and smaller 
volume of the outflowing stream. 
Compared with the level of the present time, the highest Her- 
man beach has a gradual ascent from south to north which aver- 
ages nearly a foot per mile, amounting to about 175 feel in the 
224 miles from the mouth of the lake at its southern end to the 
international boundary. The mouth of the lake was then about 
1,055 feet, and its surface on the international boundary about 
1,230 feet, above the present sea level. It is further found that 
15 
