110 = W. Upham—Minnesota Valley in the Ice Age. 
northeastward to Hudson Bay, the Minnesota valley and that 
of the Mississippi below, carrying only a small fraction of their 
former volume of water, have become considerably filled by 
the alluvial gravel, sand, clay and silt, which have been brought 
in by tributaries, being spread for the most part somewhat 
evenly along these valleys by their floods. 
The changes produced by this post-glacial sedimentation 
have been pointed out and ably discussed by Gen. G. K. War- 
ren, who thus added much to our knowledge of the geological 
history of the Minnesota and Mississippi Rivers. Lakes Trav- 
erse and Big Stone and Lac qui Parle occupy hollows in the 
outlet of Lake Agassiz due to inequalities of these recent de- 
. . 
has brought more sediment than its branch, which is thus 
dammed for a distance of thirty miles, to Little Rapids, with a 
depth of 20 to 25 feet at low water. The current of this part 
of the Minnesota through the dry season is very sluggish or 
imperceptible, and its surface often becomes considerably cov- 
ered with the green scum of cryptogamous vegetation charac- 
teristic of pools and lakes. The channel here is from fifteen 
to twenty-five rods wide, with no lake-like expansions; but 
lakes from one to four or five miles long, and from a quarter 
to a half mile wide, lie near the river and parallel with it at 
each side, upon the bottomland. Lake Pepin, having a depth 
of about sixty feet, according to General Warren, lies in the 
continuation of this valley which was deeply channeled by the . 
outflow from Lake Agassiz, because it has become unequally 
filled below the foot of this lake by the deposition of alluvium 
from the Chippewa River. Two of the tributaries of the Mis- 
sissippi from the east were similar outlets of floods supplied 
by glacial melting after they had become free from their modi- 
fied drift by flowing through a lake. Lake Superior, held by 
an ice barrier on the northeast at a level about 500 feet above 
- its present height, overflowed at the head of the Bois Brulé 
River, by Upper St. Croix Lake and the St. Croix River. The 
Mississippi valley at the mouth of this river, as in the case of — 
the Minnesota River, has become more filled by post-glacial de- 
posits than its tributary, which is thus held as back-water 
twenty miles, to the head of Lake Saint Croix, which is 25 feet 
deep. Lake Michigan, till the receding ice-sheet was melted 
from its present outlet at the north, similarly discharged south- 
ward by the Illinois River, which, like the foregoing, is ob- 
structed at its mouth by the alluvium of the Mississippi. At 
_ low water the greater part of its length is dammed, and has a 
very slight and often imperceptible current through the two 
hundred miles from La Salle by Lake Peoria to its mouth. 
Major Long remarked: “This part of the river may with 
posits. At the mouth of the Minnesota River, the Mississipp! 
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