188 C. R. KEYES — A DEPOSITION AL MEASURE OP UNCONFORMITY 



On the highest parts of the Ozark dome in Missouri the Coal Measures 

 are still found restinoj on 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 now known. However, along the line where the basal plane of 

 Coal Measures dips beneath the eastward sloping strata the unconformity 

 is everywhere observable. 



The plane of unconformity at the base of the Coal Measures represents 

 clearly an old land surface that was subjected 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 Measures of the Upper Mississippi 

 valle}'' enormous denudation took place. Heretofore the extent of this 

 erosion has been little appreciated. 



TOPOGRAPHY OF THE OLD LAND SURFACE 



The evidence already at hand indicates plainl}' that the surface on 

 which the Coal Measures of the upper Mississippi valley were laid down 

 was quite diversified. There were hills and vales, differing in elevation 

 by several hundreds 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 gorges.f In some locali- 

 ties even traces of extensive dendritic stream systems are discernible. 

 Some of the most notable of these are described by Shepard X 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. The topographic contrasts are certainly 

 nearly as marked in the old Carboniferous as they are today over the 

 same area. 



SIGNIFICANCE OF THE UNCONFORMITY 



The phenomenon under special consideration has been generally re- 

 garded as local in its nature — the same as many unconformities occurring 



* Iowa Geol. Survey, vol. i, 1893, p. 174. 

 t Mis.souri Geol. Survey, vol. i, 1891, p. 107. 

 J Missouri Geol. Survey, vol. xii, 1898, p. 127. 



