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 Shepherd:|: 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. 

 fMissoun Geol. Sur. , Vol. I, p. 167, 1891. 

 IMissouri Geol. Sur., Vol. XII, p. 127, 1898. 



