PENNSYLVANIAN. 431. 



is largely or wholly eroded, and still farther east the folding was closer, with concomitant 

 faulting, usually overthrusting, as in the anthracite and eastern Alabama regions. Tlie folds 

 dating from the great post-Paleozoic movement conform in system to those in the older rocks 

 to the east and the principal faults are similarly nortliwestward overtlirusts. The high eleva- 

 tion and consequent erosion of large areas of the closer folded basins, in the northern portion 

 of the trough, resulted in the isolation of the anthracite and Broad Top fields in Pennsylvania 

 and the Potomac and upper Cheat basins of Maryland and West Virginia. Similar detachments 

 are seen in the southern Appalachian region. 



DIVISIONS OF THE PENNSYLVANIAN IX THE APPALACHIAN TROUGH. 



Pottsville group. — The Pennsylvanian of the Appalachian trough is, as a whole, divided 

 into subdivisions which in practical application have the value of groups, although in the 

 northwestern areas they are mapped as formations. Of these the Pottsville, or lower group, 

 occupies the_ early narrow estuary, but the later divisions extend over the greater part of the 

 Pennsylvanian overlap. The Pottsville, like the succeeding formations, is composed of sand- 

 stones, shales, and clays (including fire clays, coals, and Umestones), but it contains a larger 

 proportion of sandstones and arenaceous shales than the later formations. This is a natural 

 result of the conditions of encroachment and rapid filling under which it was laid down. The 

 Pottsville is thickest in the southern exposures, where, near the eastern outcrops, it probably 

 exceeds 7,500 feet. In the northwestern bituminous area, partly as the result of less volumi- 

 nous sediment, partly by reason of the absence of all but the latest members of the group, it 

 measures locally less than 200 feet. The older divisions of the PottsviUe — that is, those witliin 

 the earher Umits of the Pennsylvanian sea — occur only (1) in the soutliern Appalachian area, 

 (2) along the eastern border of the coal field in the central Appalacliian region, ami (3) in the 

 southern and middle anthracite fields of Pennsylvania. 



Mariae conditions, attested by the presence of marine ^loUusca, were present at a number 

 of stages in Pottsville time, particularly in the central and soutliern Appalachian regions, 

 though limestone is extremely rare except in the upper PottsA-ille of the central Appalachian 

 region, and in the Mercer shale member, in the upper part of the Pottsville in the northern 

 bituminous areas. The Pottsville contains all the workable coals south of the Kentucky- 

 Tennessee State line. Subdivisions of the Pottsville and their characters wiU be cited on a 

 later page in connection with the discussion of the map units. 



AllegJieny formation. — The Allegheny, next succeeding the PottsviUe, is a thin formation 

 characterized by a larger proportion of coal, shale, limestone, and iron ore. Like those of the 

 Pottsville, its sediments are generally grayish or whitish, though they are more apt to be 

 somewhat ferruginous. Red beds are absent except- possibly in northeastern Kentucky. Its 

 Hmestones, several in number, 'are distinctly marine, though relatively tliin, and may be 

 traced continuously over large areas. In the bituminous districts, where it is sometimes known 

 as the "Lower Productive Coal Measures" ("XIII"), the Allegheny ranges generally between 

 250 and 350 feet in thickness near the northern outcrop, though it thins southwestward to 160 

 feet in northeastern Kentucky. It is also greatly thinned in the bottom of the broad sjrncline 

 of the main coal field. The Allegheny appears to have been completely eroded from the region 

 south of the Kentucky- Tennessee line, but it is possible that the sandstones tipping the summits 

 of several of the highest mountains in the coal field of northern Teimessee may represent its 

 basal terranes. 



Conemaugh formation. — The Conemaugh, which succeeds the Allegheny, is generally 

 marked at its base by sandstone or conglomerate. It is especially characterized by sandstones, 

 shales, and limestones, intermingled, particularly in the western area, with red and green* 

 shales, clays, and sandstones. It contains less coal than any of the other Pennsylvanian 

 formations of the Appalachian trough, and in most of the earlier reports of the Pennsylvania 

 and Virginia State surveys it was consequently known as the ' ' Lower B arren Measures " (" XI V" ) . 

 A thin limestone known as the Ames (or "Crinoidal") limestone member, which occurs near 

 the middle of the Conemaugh formation, is locally present over large portions of the field, and 



