560 JOSIAH BRIDGE 



this are patches of non-fossiliferous sandstone and fire clays, which 

 on lithologic grounds are classed as basal Pennsylvanian. The 

 area covered by these Pennsylvanian deposits is not definitely 

 known, but recent work has shown that its extent is much greater 

 than has heretofore been supposed. It probably caps the divide 

 almost continuously from Rolla to Cuba and for a considerable 

 distance northeast of Cuba. Around the edges of this plateau 

 where the Pennsylvanian is wanting, but where the Jefferson City 

 formation is still in place, there are found small areas which are 

 covered with Mississippian bowlders. They rarely cover more 

 than a few acres and may or may not be associated with similar 

 deposits of Pennsylvanian age. They have an average vertical 

 distribution of about thirty-five feet and a maximum of eighty feet. 

 The total relief of the Rolla area is about five hundred feet, but 

 the bowlder patches are confined to the upper two hundred feet. 

 They occur at lower levels in the northern part of the area, owing 

 to the slight dip of the underlying formations. 



The bowlders almost invariably occur on the hillsides or in 

 the heads of small ravines. A few patches have been found upon 

 the hilltops, but in every case the crest of that particular hill was 

 below the general summit-level. In the places where one of 

 the bowlder areas is associated with a small outlier of Pennsyl- 

 vanian it may completely encircle it, but in most cases forms a 

 fringe on one side only, which would seem to indicate that these 

 bowlder areas were localized in some manner before the deposition 

 of the Pennsylvanian. 



The exact nature of the Mississippian-Pennsylvanian uncon- 

 formity is still doubtful. Outliers of Mississippian rocks occur in 

 some parts of the Ozark region, but up to the present none have 

 been found in this area. Small isolated patches of stratified Missis- 

 sippian may be buried beneath the Pennsylvanian, but at the present 

 time no such occurrences have been reported. At one locality 

 fragments of the Mississippian have been found in the basal con- 

 glomerate of the Pennsylvanian, 1 but such association is not com- 

 mon. It may be that these patches of bowlders are the remnants 

 of small outliers which have been reduced to this stage since the 



1 Wallace Lee, op. cit., p. 44. 



