GENERAL MEETING. 95 
yielded to erosion that their area is topographically represented 
by a broad, shallow trough, of which the altitude is only from 
600 to 1,000 feet, and of which the sides rise and culminate in the 
Niagara escarpment on: the east and in the Mississippi-Missouri 
water-shed on the west. There is, however, a subordinate general 
topographic feature which is independent of geologic structure. A 
wide, gentle, indefinitely outlined depression extends directly across 
the great eastward projection (the “ Cromwell’s Nose’’) of Iowa and 
diagonally across the Upper Silurian, Devonian, and Carboniferous 
rocks alike, in the line of the general course of the Mississippi, from 
near the’ mouth of the Turkey to the mouth of the Iowa. Itis 
manifestly of great antiquity. 
Thus, in its general topography, eastern Iowa is characterized, 
primarily, by an elevated escarpment near its eastern border, by a 
broad depression intersecting its western portion diagonally, and 
by a general southwesterly slope extending over most of its area; 
and secondarily, by an indefinite ancient valley cutting off its eastern 
projection. And its general drainage system is almost absolutely 
independent of this general topography ; for not only do the prin- 
cipal streams flow at right angles to the prevailing slope and cut 
through the elevated escarpment when it lies in their way, but, with 
the single exception of the Cedar, they preserve their courses directly 
across the ancient valley. 
In their relation to minor topographic features the rivers of 
eastern Iowa conform to two diametrically opposite laws: 1. for 
two-thirds or three-fourths of their combined length they flow in 
the axes of the ill-defined, shallow valleys which characterize the 
drift-plain ; and, 2, for the remaining portion of their courses they 
flow in narrow gorges which they have excavated for themselves in 
the axes of the elongated ridges that constitute the leading features 
in the local topography of the region. Moreover, they have in 
many instances, at the same time gone out of their direct courses, 
and deserted valleys already prepared for them, to attain the anoma- 
lous positions assumed under the second law of association. And let 
it be noted that in every such case the gorges have demonstrably 
been carved by the streams themselves through the quaternary and 
older formations alike; that the pre-existent valleys which they 
avoided have not been appreciably eroded since the quaternary ; 
and that there has been no localized orographic movement in the 
region since long antecedent to the quaternary. 
