32 The American Geologist. January, 1903. 
rock fra^"mcnts, such as arc inclosed in this section and in 
most other sections of loess adjoining' the rock bluffs of the 
valley, especially where it receives a small tributary. In this 
connection we may notice that the Concannon tunnel in its 
outer half, dug" during the winter of 1900-1901, had perfectly 
maintained its vertically cut sides and slightly arched top a 
year and a half before our visit, behaving in the manner so 
characteristic of the loess, which would not be possible in any 
Postglacial deposit of alluvivun. 
The second objection to Chamberlin's view relates to his 
assumption that the JMissouri river during some part of the 
Postglacial period had a fioodplain at Lansing about 25 feet 
above that of the present time. Changes then taking place as 
to the course of the river's channel are supposed to have per- 
mitted the stream in the ravine to deposit the silt above the 
skeleton. On the contrary, if I rightly read the geologic his- 
tory of the Missouri and Mississippi rivers, the fioodplain at 
Lansing and southward was lower during the Wisconsin stage 
of glaciation than now, and has in general been somewhat 
built up, instead of being cut down, during all Postglacial 
time. \'\ ith the uplift of this region which preceded the form- 
ation of the Wisconsin and Minnesota moraines, the valleys ac- 
quired their present slopes, and the valley loess was rapidly 
eroded. Its deposit along the Missouri, once having the bight 
of probably 150 feet above the Lansing skeleton, was removed, 
giving the valley its present width and even a greater depth 
than now, before the moraine-fonning stage of the waning Ice 
age. During this process of reexcavation, the remnant of loess 
at and above the Concannon tunnel was spared; and here and 
there, as close north of Council Bluffs, a terrace would be left 
midway between the crest of the valley loess bluffs and the 
present bottomland. Sufificient time intervened betv/een the 
lowan and Wisconsin stages to allow the great valleys to be 
sculptured in the easiW eroded loess to nearly their present 
form with a somewhat greater depth. Through this interval 
the ice accumulation and melting were almost evenly balanced ; 
but there next ensued a time of general and prolonged re- 
cession of the ice border, interrupted by many halts shown by 
parallel and parti v interlocking moraine belts. From the abun- 
dant melting of this Wisconsin stage, floods again poured 
