270 Wisconsin Academy of Sciences, Arts and Letters. 
tending to fix the age of the deposits save such limits as their 
capacity for endurance may suggest. There are two features 
however, which serve to fix their age within rather narrow 
limits. The inferior limits are fixed by the fact that in sev¬ 
eral' places the deposits are seen to pass under Loess showing 
that they are older than at least a part of that formation. Its 
superior limit is fixed by the fact of its occurrence on the outer 
scarps of the foot hills facing the river. When the Missis¬ 
sippi forsook its old channel north of the bluffs and adopted 
the captured valley of a small tributary on the south there be¬ 
gan a process of readjustment to adopt the small valley to its 
new service. This readjustment consisted in an energetic 
basal erosion which removed the lower slope of the bluffs giv¬ 
ing rise to steep frontal scarps. 
Text Fig. 2. The entire line gives a sectional outline of the hill as 
it now exists. The broken lines c. d. show the approximate position of 
the rock surface and will give some idea of the amount of filling on the 
lower slopes of the bluffs. 
The outline in dots and dashes shows the outlines of the bluff as it 
must have been to have furnished material for the deposit a. I have 
indicated Lm. the position of the Lower Magnesian cap. 
The distance from a. to e. was something like 300'. 
Hear Trempealeau Bay this scarp is particularly well de¬ 
veloped, rising in one place almost precipitously from the 
waters edge. The circ opening out on this front is a hanging 
valley whose lip is about 100 feet above present river level. 
A few rods east of the outlet of this circ, on the steep slope, 
is one of the characteristic deposits, an isolated sub-circular 
patch perhaps 100 feet in diameter. It is evident that no 
appreciable recession of the scarp has occurred since the for¬ 
mation of the deposit, since it itself must have been among the 
first things to be removed. 
