SOUTHWARD COXVERGEXCE XORTHWARD THRUST 329 



SOUTHWARD COXTERGEXCE 



Attention has already been called to the parallelism of the close Appa- 

 lachian folds and of the major syncline of the Appalachian coal field. 

 This parallelism is so phenomenal as to deserve strong emphasis. Xot 

 only do the major folds rise and fall in nnison, but they swerve from the 

 general trend in the same way and along the same cross-lines. The paral- 

 lelism is not perfect, however, and the axis of the Appalachian coal basin 

 converges in Kentucky and Tennessee with the stronger eastern folds. 

 This was the result of the excess advance of the Cumberland thrust 

 block in front of the North Carolina salient. West of the coal basin 

 synclinorium the other major axes also converge southward; these in- 

 clude the Nashville-Cincinnati anticline, the Michigan-Illinois syncline, 

 and the Ozark- Wisconsin anticline. 



NORTHWARD THRUST FROM LLANORIA 



This convergence of axes appears to be the result of two factors : ( 1 ) 

 the excess northwestward advance before the North Carolina salient, and 

 (2) a similar but slight northeastward advance of the Ozark mass. The 

 latter advance seems to have been the result of pressure delivered by the 

 landmass of Llanoria northward through the Ouachita uplift and the 

 Arkansas synclinorium against the borders of the Ozarks. Evidence of 

 this northward shoving is seen in the overturning of the folds toward 

 the north in Arkansas and eastern Oklahoma and in a similar though 

 slight overturning with faulting around the north of the Ozark dome. 

 The latter zone of steeper northward dips extends eastward into Ken- 

 tucky, where it is complicated by an east-west zone of faulting. 



The northward compression evidenced in the Ouachita and^ Ozark 

 region might be equally plain in the Appalachians were it not masked 

 by the greater thrust from the southeast. The general pressure and 

 northward motion of the crust, so plain in the Ouachitas, is part of the 

 general system underlying similar structures throughout eastern North 

 America. It is even traceable dimly under the Cretaceous cover by 

 means of drill records. It is a curious fact that a section running- east 

 of north from the Ouachita Mountains through the Ozark uplift to north- 

 ern Illinois is almost a duplicate of a section running at right angles to 

 it from northwest Georgia through the Nashville dome to northern Illi- 

 nois. The chief structural difference is that the Ozark dome is slightly 

 higher than the Nashville dome, so that the Precamlirian is exposed. 



