438 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



occurred at the close of the Pennsylvanian, but I can not see that they 

 were any more important than those in late Devonian or those in late 

 Silurian time. I certainly fail to see any sufficient reason for saying that 

 the Appalachian Mountains were "born" at that time or that the valley 

 rocks were folded then and not before or after. The evidence of repeated 

 pre-Pennsylvanian warping and folding in southeastern North America 

 is so convincing that it seems a waste of opportunity to further discuss it. 

 And that the late Mesozoic and subsequent movements which warped and 

 faulted the Triassic rocks in Virginia, Pennsylvania, and New Jersey may 

 at the same time have expended their energies chiefly in close folding the 

 southern and middle parts of the Appalachian Valley is, to say the least, 

 not an unreasonable hypothesis. But certainly the present result was not 

 all accomplished in any single period. On the contrary land shrinkage 

 movements seem to have prominently exerted themselves in the Ap- 

 palachian region at frequent intervals throughout geologic time. 



Twenty years ago Walcott*^ pointed out that great downwarps or 

 troughs were developed near the eastern and western borders of the North 

 American continent prior to the beginning of the Cambrian; and more 

 recently Ulrich and Schuchert** have shown that the more important 

 overthrust faults in the Appalachian Valley began as subparallel folds 

 that had attained sufficient magnitude already in Eopaleozoic ages to 

 divide this great tract into distinct marine troughs. That some of these 

 earlier barrier folds locally attained considerable altitude and suffered 

 great erosion is shown, as on the Eome barrier in the vicinity of Birming- 

 ham, Alabama, by the removal of 1,000 to 4,000 feet of Knox dolomite 

 before the eroded surface was covered and preserved by an early Ordovic- 

 ian deposit. Whether the Eome barrier was reduced to baseievel at this 

 time near Birmingham has not been determined. However, it seems 

 doubtful from the fact that some 15 miles northeast, at Foster Mountain, 

 Stones Eiver and Mohawkian limestones, nearly 700 feet in thickness, 

 were laid down in a narrow embayment scooped out of the Knox dolomite. 



The remains of the Foster Mountain embajnnent are preserved in a 

 manner so satisfactory as to leave no question regarding the topography of 

 the immediate region at the time of its occupancy by a small arm of the 

 Ordovician sea. Thick banks of coarse chert conglomerate (Attalla con- 

 glomerate) lined the shores, but in the space of a few hundred feet these 

 pass into calcareous shale and this a short distance farther out into pure 

 limestone. Occasionally a thin bed of small-pebbled conglomerate is 



*8 C. D. Walcott : Bull. U. S. Geological Survey, No. 81, 1891, pp. 363-369. 

 ** B. O. Ulrich and Charles Schuchert : Paleozoic seas and barriers in eastern North 

 America. New York State Museum, Bull. 52, 1902, pp. 633-663, 



