IOWA ACADEMY OP SCIENCES. 
23 
The similarity of the higher exposure and the one near the 
river indicates the probable common origin of both. If this be 
so it indicates that the trough of the Missouri was excavated 
much as now, and yet that the water surmounted the Cretaceous 
bluffs with sufficient depth to float small icebergs. This con- 
ception will also explain why the boulder clay is so thin over 
the uplands; also, why it should be distributed so far down the 
Missouri, while it is so thin near the moraine. 
Such a thickness of the drift as is found in southwestern 
Iowa and northern Missouri could seem, perhaps, to have been 
derived in a similar manner from the Des Moines ice lobe. 
PREGLACIAL ELEVATION OF IOWA. 
BY H. FOSTER BAIN. 
[Published by permission of the State Geologist.] 
The long period intervening between the deposit of the Cre- 
taceous rocks of Iowa and the advent of the glaciers has left in 
this state no record in deposits. Its history must be gathered 
entirely from the land forms then created, and from inferences 
drawn from orographic changes known to have taken place in 
other regions. 
Over the greater portion of Iowa the land surface of post- 
Cretaceous time is now covered by a thick mantle of drift. It 
is only in the northeastern corner of the state that it is exposed, 
and it is in this region mainly that the history of the period has 
been read. It is, however, possible to find in other portions of 
the state much which confirms the results obtained from a study 
of the driftless region. 
Throughout Iowa the records of well borings show the pres- 
ence of numerous buried drainage channels, some of which can 
be traced with a measurable degree of accuracy for a considera- 
ble distance. In the course of recent detailed work in connec- 
tion with the Geological Survey a number of these have been 
noted. One of the best examples is what may perhaps be called 
the Washington Channel, as it has been studied most in that 
county. 
