84 
Wisconsin Academy of Sciences , Arts and Letters. 
It seems necessary to suppose that this mantle of loess and loess-like silt 
was originally deposited as a horizontal stratum across the entire Missis¬ 
sippi bottoms. At the present time it undulates over the hills. At first 
thought it would seem that the depositing waters might have been deep 
and the silt laid down as an undulatory mantle; but it would seem neces¬ 
sary to extend the same hypothesis to the deposition of the gravels where 
its application is manifestly excluded by the nature of the deposit. I feel 
sure from observation in certain cases that full investigation will show 
that this seeming mantling is the result of the gradual degradation of the 
hills, accompanied by creep of the pliant and plastic material. This phe¬ 
nomenon of creep has a wide expression entirely independent of the area 
uncer consideration; but upon that I cannot dwell. 
During the first glacial episode the altitude and slope of the lower Mis¬ 
sissippi basin was so low as to permit the deposit of this silt on the bluffs 
which are now 200 feet more or less above the present Mississippi bottoms. 
Before the second glacial epoch, according to the division I make, there 
was an elevation sufficient to permit the erosion of the great trench of the 
lower Mississippi by the predecessor of the present river. This erosion 
amounts in round numbers to a. trench about 300 feet in depth and about 
sixty miles in width. Some of the bluffs that are crowned by these silts 
are 200 or 250 feet in height, and Professor Call’s recent investigations 
show 80 to 100 feet of silt in the bottom. It is, therefore, I think, safe to 
say that in round numbers there was an erosion during the interval be¬ 
tween the two epochs of the magnitude named and reaching from Cairo 
south to the gulf with corresponding erosion trenches along the upper 
branches. This great erosion represents the interval between the forma¬ 
tion of the silts of the earlier glacial epoch and the filling in of the valley 
deposits of the later glacial epoch, which now demand our attention. If 
we go back on the glaciated area to the moraines which mark the limit of 
the later glacial incursions, we shall find, starting from the outer side of 
these moraines, valley streams of gravel formed contemporaneously with 
these ice incursions. Tracing these gravel streams along their courses, we 
find that they run down into the channels cut in the inter glacial interval, 
and partially fill them. On the upper Mississippi, on the Chippewa, on 
the Wisconsin and on other tributary rivers, we find gravel trains heading 
on the outer edge of the outer moraine of the later epoch. Passing down 
through the interglacial trenches, these are found represented in the lower 
Mississippi valley, as I think we may safely say from recently gathered 
evidence, as deposits in the Mississippi bottoms, overlaid of course by 
the more recent deposits. The work of the earlier glacial epoch in the 
lower Mississippi I conceive to be the deposit of the loess and loess-like 
silt, that of the interglacial epoch to be the erosion of the great trench in 
which the Mississippi bottoms now lie, and that of the later glacial epoch 
to be the partially filling of this trench. The trenching is the measure of 
the interglacial interval, or at least is a partial measure of it. 
