16 THE GLACIAL LAKE AGASSIZ. 



part of Lake Traverse, and first exteuds southwest to the head of this 

 lake, thence southeast to Mankato, and next north and northeast to the 

 Mississippi at Fort Snelhng, its length being about 250 miles. Its width 

 varies from 1 to 4 miles, and its depth is from 100 to 225 feet. The 

 couutrv through which it lies, as far as Carver, about 25 miles above its 

 junction with the Mis.sissippi, is a nearly level expanse of till, only 

 moderately undulating, with no prominent hills or notable depressions, 

 excepting- this deep channel and those formed by its tributary streams. 

 Below Carver it intersects a belt of terminal moraine, composed of hilly 

 till. Its entire course is through a region of unmodified drift, which has 

 no exposures of solid rock upon its surface. 



Bluff's in slopes from 20 to 40 degrees, and rising 100 to 200 feet to 

 the general level of the country, fonn the sides of this trough-like valley. 

 They have been produced b}' the washing away of their base, leaving the 

 upper portion to fall down and thus take its steep slopes. The river in 

 deepening its channel has been constantly changing its course, so that its 

 current has been tiu-ned alternately against the opjjosite sides of its valley, 

 at some time undermining every portion of them. In a few places this 

 process is still going forward, but mainly the course of the Minnesota 

 River is in the bottom-land. Comparatively little excavation has been done 

 by the j^resent river. As we approach its source it dwindles to a small 

 stream flowing through long lakes, and we finally pass to Lake Traverse, 

 which empties northward ; yet along- the upper Minnesota and at the di^^de 

 between this and the Red River this vallev or channel and its inclosing 

 bluffs are as remarkable as along the lower part of the ^Minnesota River. 

 It is thus clearly shown to have been the channel of outflow from a lake 

 formerly extending northward from Lake Traverse. 



The ]\Iinnesota Vallev in many places cuts throug-h the sheet of ch'ift 

 and reaches the underlving rocks, which have frequent exposm-es tdong- its 

 entire course below Big Stone Lake. This excavation shows that the 

 thickness of the general drift slieet upon tliis part of Minnesota averages 

 about 150 feet. The contour of the (dd rocks thus brought into view is 

 much more rineven than that of the drift. In the hundred miles from Big 

 Stone Lake to Fort Ridgely the strata, are Archean gneisses and granites, 



