120 
IOWA ACADEMY OF SCIENCES. 
extends is not known, since the horizon soon is covered 
too deeply by the overlying strata. 
On the highest parts of the Ozark dome in Missouri, the 
coal measures are still found resting upon the uneven 
channeled surface of the Lower Carboniferous. South of 
the southern boundary of Missouri there is no evidence 
that any break in sedimentation occurs between the coal 
measures and Lower Carboniferous formations. 
How far east of the Mississippi river the unconformable 
relations exist is not known. However, to the points 
where the basal line of coal measures dips beneath the 
eastward sloping strata, the unconformity is everywhere 
observable. 
The plane of unconformity at the base of the coal meas- 
ures represents clearly an old land surface that was sub- 
jected to erosion for a period long enough for the tilted 
strata to be completely beveled off from the Kaskaskia 
limestone down to the Cambrian sandstones. During the 
interval between the deposition of the last of the Lower 
Carboniferous formations of the region and the coal meas- 
ures of the upper Mississippi valley enormous denudation 
had taken place. Heretofore the extent of this erosion has 
been little appreciated. 
The evidence already at hand indicates plainly that the 
surface on which the coal measures of the upper Missis- 
sippi valley were laid down was quite diversified. There 
were hills and vales, differing in elevation by several hun- 
dreds of feet. Some of these have been especially noted 
by Bain* and other members of the Iowa Geological Survey. 
There were broad drainage basins and deep narrow gorgesf. 
In some localities even traces of extensive dendritic stream 
systems are discernible. Some of the most notable of 
these are those recently described by ShepherdJ in south- 
west Missouri. 
If we wish to get a general conception of what this old 
surface relief actually was, we gather something of its real 
character by comparing it with the relief now existing. 
*Iowa Geol. Sur. , Vol. I, p. 174, 1893. 
fMissouri Geol. Sur., Vol. 1 , p. 167, 1891. 
^Missouri Geol. Sur. , Vol. XII, p. 127, 1898. 
