26 
IOWA ACADEMY OF SCIENCES. 
the inter- stream divides alone remaining to outline the former 
plain. 
At the opening of Cretaceous times the greater portion of 
Iowa was a land surface. The open sea lay to the west and the 
drainage was in that direction. As the sea line crept farther 
inland successive portions of the country were submerged, and 
at the same time the unsubmerged portion was exposed to 
erosive agencies. At the close of this period about one-half of 
the state was probably covered by the newly deposited beds and 
the remainder was reduced to a monotonous plain barely above 
sea level. It is known that at that time a very large portion of 
the United States had been similarly* reduced to a penneplane. 
The close of the Cretaceous was everywhere marked by 
orographic changes. It was a time of elevation and of 
re-arrangement. The changes which took place in the Rocky 
mountains at that time had a most important influence upon 
Iowa, and indeed the whole upper Mississippi valley. The ele- 
vation produced a corresponding, though much smaller elevation 
over the great plains. Uphamf estimates this in northwestern 
Minnesota and westward at from 5,000 to 10,000 feet, decreasing 
towards the east. This had the effect, as pointed out by West- 
gate |, of turning the direction of drainage over this region 
from the west or southwest to the southeast, and of setting in 
motion the influences which first blocked out our present drain- 
age system. 
During the whole of Tertiary times Iowa was probably a. 
land surface. 
Whether the buried channels here described had their origin in 
the general Tertiary erosion period, or whether they were formed 
during the second period of base-leveling differentiated by 
Upham§ at the close of the Tertiary and during early Pleistocene 
times, cannot now be definitely stated. The width and depth of 
these channels, so far as they are known, bear out the conclusion 
reached by Chamberlin and Salisbury and by McGee in studies 
of the driftless region that the period of erosion v^^as a long 
one, and may possibly be taken as confirmatory of the belief 
that the two periods of base- leveling are not here so distinct as 
elsewhere. 
Woodworth: Am. Geol , XIV, 309-235. Minneapolis, 1894. 
t Am. Geol., XIV, 338. Minneapolis. 1894. 
Am. Geol., X[, 357. Minneapolis, 1893. 
§Op. cit., p. 238. 
