EXTENSION FOLLOWING THE GLACIAL RETREAT. 209 



agnin lol)ate, extending from the west shore of Lake Ag-assiz southward 

 and then westward and northward, between the hike area and the Sheyenne 

 River, to the prominent and typical moraines that are found south of 

 Stump and Devils lakes, on the Big Butte, about Broken Bone Lake and 

 northward, and on Turtle Mountain. In their remarkable development 

 these moraines are similar to tlie massive Leaf Hills, with which they seem 

 to have been contemporaneous. The laving action of Lake Agassiz caused 

 the thick portion of the ice-sheet filling the Red River Valley to melt back 

 somewhat faster than its thinner portions on the higher land areas at each 

 side. 



The highest of the Herman beaches of Lake Agassiz extends in Min- 

 nesota, as traced in this survey, to the north side of Maple Lake, 20 miles 

 east-southeast of Crookston, and probably it continues thence into the for- 

 est region on the east, where it is impracticable to follow its course, to the 

 vicinity of Red Lake; and on the west it reaches through North Dakota 

 and at least 14 miles into Manitoba, terminating on the northern part of the 

 Pembina escarpment somewhere between Thornhill and its northern end, 

 that is, between 14 and 40 miles nortl\of the international boundary. Be- 

 fore the formation of this beach was completed the ice-sheet had retired 

 from the lake area as far north as the beach extends. During pauses of 

 this glacial recession the Dovre, Fergus Falls, Leaf Hills, and Itasca mo- 

 raines were formed, showing a northward retreat of the ice border from the 

 DoNi-e moraine across a distance of about 150 miles in central Minnesota 

 and 150 to 200 miles in North Dakota and southern Manitoba, with a max- 

 imum of probably not less than 300 miles in the Red River Valley, where 

 Lake Agassiz produced a more rapid melting of the ice margin. Through 

 this time the River Warren, outflowing from this glacial lake, fed by abun- 

 dant ice-melting and rains, eroded a channel about 50 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 undulating sheet of till 

 which reached across the present valley of Lakes Traverse and Big Stone. 

 The shortness of the time thus indicated as probably occupied in the for- 

 mation of a single one of the beaches of Lake Agassiz, reaffirmed as it is 

 by the small amount of the littoral erosion and resulting beach deposits, 

 MON XXV 14 



