212 THE GLACIAL LAKE AGASSIZ. 



had become established in the course which it now has to its debouchure 

 into Lake Agassiz at the present most southern bend of the river. Its large 

 delta there brought into the lake was already in progress of deposition dur- 

 ing the accumulation of the Milnor beach-ridges and partly supplied the 

 gravel and sand of which they are formed. 



But the River Warren quickly cut down its channel to the base of 

 the earlier Sheyenne deposit of gravel and sand before described, lying 

 above the present valley of the Bois des Sioux, until it reached the harder 

 till, and there Avas stayed during the numerous stages of lacustrine extension 

 and glacial retreat which are represented by the single Herman beach of 

 this southern portion of the lake. The growth of the great Sheyenne delta 

 continued, and the Buffiilo delta was probably mostly completed, during 

 the withdrawal of the ice-sheet to the Fergus Falls moraine and its pause 

 or readvance by which that moraine was made. Through the same stage, 

 excepting its very short early portion, represented by the Milnor beach, and 

 for a long time afterward. Lake Agassiz held its Herman level, changing 

 only very slightly in this southern area by slow erosion of the outlet, but 

 experiencing northward a gradual uplifting of its basin, whereby its Her- 

 man beach, single at the south, becomes double and multiple in proceeding 

 to the north. 



The next stage in the departure of the ice withdrew portions of its 

 border to the ninth or Leaf Hills moraine (p. 163), which is closely associ- 

 ated with the Fergus Falls moraine, the two being merged together through 

 much of their course. Lake Agassiz, therefore, gained only a small exten- 

 sion of its length and area (PI. XIX). The most notable change was the 

 formation of a northwestern bay of the lake, reaching in a reentrant angle 

 of the ice-sheet to Lariinore and McCanna, which received the Elk Valley 

 delta, deposited by a large glacial river flowing from the depression on 

 the ice surface where the descending slopes of its Minnesota and Dakota 

 lobes met. 



After these contiguous and partly combined moraines were formed, 

 the increasing warmth of the climate again jjushed back the ice border 

 a long distance, until its retreat was temporarily interrupted at the line 

 marked by the tenth or Itasca moraine (p. 173). Advancing northward, 



