20 



Indiana University Studies 



associated shales appear in the section. Farther east in the 

 vicinity of Cincinnati, only the highest parts of the ridges 

 are capped by the shales containing a thin coal, and the sand- 

 stones of the basal Pottsville. These strata reach lower levels 

 to the west, the dip of the Pennsylvanian-Mississippian con- 

 tact being considerably greater than the general westerly 

 slope of the land surface. Consequently most of the western 

 portion of the region is capped by the Mansfield formation. 



The lower portion of the Mansfield consists locally of sandy 

 shale, which usually contains a thin seam of coal ; but typically 

 the Mansfield is a coarse, often gritty, massive, decidedly 

 cross-bedded sandstone with occasional streaks of sandy shale. 

 The occurrence of the shaley phase containing the thin coal 

 seam is not haphazard, but takes place where the erosion of 

 the Mississippian has been greater, previous to the deposi- 

 tion of the Mansfield, than in the adjacent regions. That 

 is, the shales and thin coal seams appear to have been de- 

 posited in local basins. Above the shales, the sandstone phase 

 usually comes in and becomes quite characteristic of the 

 Mansfield formation. Many cases are known where the sand- 

 stone rests disconformably on the green shale of the upper 

 Chester. The Coal Measures in Indiana contain little or no 

 olive shale. The massive sandstone on the ridge near Ridge- 

 port is typical of the Mansfield formation as it occurs in the 

 "American Bottoms" region. It is about 50 feet thick east 

 of the village, and considerably thicker west. Its resistance 

 to weathering and erosion is well illustrated by the higher 

 elevation of the ridge west of Ridgeport where the Mans- 

 field is excellently developed. It is vv^ell known that massive 

 sandstones where well cemented must give rise to bluffs along 

 streams, and that sandstones when they occupy the inter- 

 stream areas, whether they are well cemented or not, cause 

 sharp ridges of rather uneven crests. These features are 

 characteristic of Mansfield sandstone topography. 



The above discussion includes all of the Paleozoic rocks 

 found in the region of the ''American Bottoms". The remain- 

 ing deposits belong to the Pleistocene and Recent periods. 

 Since these latter deposits occur at various levels, and are 

 concerned directly in the development of the physiography 

 of the region, rather than indirectly as conditioning factors, 

 they will be discussed under other headings below. 



