SOUTHERN AND CENTRAL APPALACHIANS 



123 



schist. Granitic stocks and pegmatite dikes cut the domes and meta- 

 sedimentary mantle. 



Foliation in the Baltimore gneiss parallels the contact with the mantle 

 and arches over the domes in asymmetrical form with the steep flanks 

 to the southeast. Lineation appears like raindrops running off an umbrella 

 (Cloos, 1953). 



The Baltimore gneiss has been considered Precambrian in age and 

 possibly as old as any rock in the Piedmont. The Glenarm sediments are 

 thought to have been deposited in late Precambrian or early Paleozoic 

 time unconformably upon the gneiss. It is clear, however, that the same 

 degree of metamorphism pervades the overlying Glenarm rocks as the 

 Baltimore gneiss, and since the foliation of one parallels the other, it 

 has been assumed that metamorphism and doming of the mantle has 

 obliterated the original basement structures and produced a new con- 

 cordant foliation. 



If the unconformity existed, two periods of tectonism are implied, one prior 

 to Glenarm sedimentation and another following it. If the unconformity did 

 not exist, a single period of deformation, metamorphism, and injection can 

 explain observed relationships. All previous investigators of the domes favor 

 existence of the unconformity, but conclusive proof is lacking (Tilton et al., 

 1958). 



The age of the post-Glenarm tectonism is generally considered 

 Taconian or Acadian. Evidence bearing on this conclusion will be pre- 

 sented later. The age of the Precambrian tectonism will also be taken 

 up later. 



The third division of the Piedmont shown on Fig. 8.26 consists mostly 

 of the Glenarm series with generally horizontal fold axes. Foliation is 

 vertical on the east border and is inclined to the southeast on the west 

 border. The fold axial planes dip to the southeast also. Metamorphism 

 lessens toward the west, and mica schists become phyllites; amphibolites 

 become epidote- and chlorite-rich greenschists. 



The rocks of this zone have not yet been correlated with the fossil- 

 bearing early Paleozoic strata west of the Martic line. It is possible, 

 according to Cloos, that the Cambro-Ordovician limestones of the 

 westerly zone (6) are facies of the once sandy rocks of the Glenarm. 



Zone four encompasses the Sugarloaf structure which, as shown by 

 the closure of the bedding, is an anticlinal dome. The western limb is 

 overturned. Cleavage confirms the domal structure. The rocks arc in 

 the chlorite and greenschist facies of zone three. The local phyllites are 

 correlated with the Cambrian Harpers phyllite, and the quartzites which 

 are below the Harpers are most likely the Lower Cambrian Weverton 

 quartzite (Scotford, 1951). 



The Martic line is called division five. It was first recognized as an 

 overthrust in which the then presumed older YVissahickon schist was 

 thrust westward over the presumed younger Paleozoic strata, and all 

 rocks southeast of the "fault" were regarded as Precambrian and north- 

 west of it as Paleozoic. Careful work has shown that the line is not a 

 discrete plane of major displacement, but that in most places complicated 

 conditions pertain (Cloos and Heitanen, 1941). It was also presumed 

 that the Martic "thrust" is a boundary between highly metamorphosed 

 schists and little metamorphosed Paleozoic strata. Cloos and Heitanen 

 have demonstrated that metamorphism is not restricted to the YVis- 

 sahickon schist but that all rocks including the Cambrian Antietam schist, 

 Vintage dolomite, and Ordovician Conestoga limestone show the same 

 intensity of metamorphism. At one place the sequence is repeated five 

 times, where the Conestoga is capped by the YVissahickon schist, which 

 in several ways is similar to the Antietam. At another place the Antietam 

 schist almost meets a spur of YVissahickon. 



Along the Martic line the fold axes are horizontal or plunge predominantly 

 to the southwest. All folds are overturned southward. Flow cleavage is an 

 axial plane cleavage and dips to the north. Bedding is intensely crumpled and 

 at manv localities is entirely obscured by later cleavage. Since all members of 

 the sequence are thin and underlie large areas, it can safely be assumed that 

 bedding is roughly parallel to the boundary planes and thus largely conform- 

 able in all members of the sequence (Cloos and Heitanen. 194H. 



Zone six consists mainly of Cambro-Ordovician limestones which are 

 strongly cleaved and overturned to the west. The zone is covered in large 

 part with the Triassic deposits (division seven). 



Zone eight is the Blue Ridge belt previously described, and cleavage 

 and lineation extend northwestward to the position labeled "tectonite 



