422 G. K. Warren— Valley of the Minnesota and Mississippi. 
restored ; then deposition continues immediately in front of the 
last deposit. Such deposition, therefore, does not extend lat- 
erally from the course of the current into any contiguous dead 
water, as depositions from water holding a clayey or vegetable 
matter would. 
Thus in the valley of the Mississippi, the lakes alongside the 
river’s course are deeper than the river, which has continued 
to raise its bed by deposits of sand after the lakes were cut off 
rom its current. 
From the Falls of St. Anthony down to the St. Croix River 
the Mississippi Valley receives no considerable tributary. The 
St. Croix comes from a region of trap rock now furnishing little 
or no large and heavy sedimentary matter. The result has been 
that the Mississippi deposit of sand and gravel has been thrown 
across its mouth, holding back its water and forming the St. 
Croix Lake. At low water, while the depth of the Mississippi 
at the junction is two and one-half feet, the depth in Lake St. 
Croix is twenty-five feet. It is not probable that twenty-five 
feet depth represents the amount of filling of the ancient valley 
at this point, because the lake itself must have been somewhat 
shoaled with fine deposits of clay and vegetable matter. 
he next considerable tributary to the Mississippi Valley is 
the Chippewa River. This, entering at right angles wit 
steep river slope and a probable high-water volume of at least 
40,000 cubic feet per second, comes from a region inexhaustibly 
supplied with siliceous sand and gravel containing a considera- 
ble of the heavy magnetic sand, whose oxidation often cements 
the other sand deposits.* It brings quantities of these materials 
which, spread out below, give a very steep slope to the Missis- 
sippi River, and very bad shoals for navigation. pete 
Lying just above this deposit is Lake Pepin, which it com- 
pletely accounts for. The reason this lake has not been filled 
pee the Mississippi above is that the supply of sand from the 
Chippewa is so great as to raise the level more rapidly than the 
filling above can keep pace with. The Chippewa from the left 
bank pushes its sand-bar out, so as to confine the outlet of the 
lake to the opposite shore. There is an observable relation 
between the condition of the lake and the deposits of the Chip- 
pewa. The deepening of the waters by the deposit of Chippewa 
sands is felt at low water sometimes as far up as the mouth of the 
St. Croix, when floods in the Chippewa make these deposits large, 
and on the other hand, in times of droughts the waters of the lake 
cut the outlet deeper, and lower its level, so that the shoal water 
is moved down the river two to three miles below the St. Croix. 
_ * “The analysis i hi ippewa River (the Yellow 
Banks) gives Seetiees sa bomeger NS an ak is chiefly white 
sand, with only two per cent of organic matter, less than four per cent of soluble 
saline matter, consisting chiefly of oxide of iron and alumina with only a trace 
3 earth.”—Owen’s Report, p. 56. 
