THE COAL MEASURES OF ARKANSAS 3^3 



latitude it occupied during the St. Louis, or approximately to 

 the latitude of the present mouth of the Missouri River. This 

 permitted erosion of the land surface north of this point ; and 

 immediately south of it the disposition of coarse sediments, 

 covering the other marine beds of the Mississippian. Marine 

 deposits of the later Kaskaskia, were again laid down over the 

 coastal deposits cf the same region. With a further recession 

 of the sea some of the territory occupied by Kaskaskia rocks 

 was also made land and was subjected to erosion. According 

 to the recent suggestions of Marbut 1 the shore line at this time, 

 when it had traveled farthest to the southward, coincided very 

 nearly with the present crest of the Ozark uplift, that extends 

 along a line drawn from Iron Mountain to the southwest corner 

 of the state of Missouri. 



It is now a well-established fact that everywhere to the north 

 of the present Ozark crest profound erosion took place between 

 the time when the St. Louis limestone was deposited and that 

 when the Coal Measures were formed. Moreover, the effect of 

 this land degradation must have left the country very much in 

 the condition of a peneplane, for the old land surface is a rather 

 even one, with no marked contrasts of relief. It beveled the 

 previously slightly tilted strata, for the Coal Measures rest on the 

 rocks of all ages from the Silurian to the top of the Lower 

 Carboniferous. This stratigraphic hiatus, represented by the 

 plane of unconformity at the base of the northern Coal Measures, 

 manifestly indicates an episode in the physical history of the 

 region, the importance of which has been heretofore little appre- 

 ciated. 



In the southerly retreat of the sea during the close of 

 Lower Carboniferous time, there was, of course, a point beyond 

 which the shore line did not advance. Beyond this point, sea- 

 wardly, deposition went on uninterruptedly, the succession of 

 beds was continuous, and the layers were conformable through- 

 out the Carboniferous system. Such conditions appear to have 

 prevailed in the region of southern Missouri and northern 



1 Missouri Geol. Surv., Vol. X, p. 82, 1896. 



