2i6 GEORGE W. STOSE 



arch, the limestones around the pitching end are crumpled into 

 numerous close folds, as may be seen on the geologic map, Fig. 2. 



Just east of this Big Flat arch the Antietam sandstone, contain- 

 ing Scolithus and fragments of Olenellus, is crushed to a coarse 

 breccia and cemented by iron. This crushed zone is the closely 

 compressed, faulted syncline between the Big Flat and Montalto 

 arches, and apparently the quartzite of Little Mountain is brought 

 to the surface along this line of fracture by a minor anticline. 



Origin of the structures. — It has been shown that the general 

 structure of this part of South Mountain is a fiat-topped arch, with 

 the sedimentary beds on the western flank disposed as a steep mono- 

 cline, dipping away from the older volcanics in the heart of the 

 range, and beneath the limestones of the valley. This simple struc- 

 ture is complicated at several points along its strike by secondary 

 folds, giving rise at their pitching ends to offsets in the mountain front. 



The steep, often overturned, dips of the western limbs of the 

 anticlines both in the mountain and valley rocks indicate that the 

 crustal movement was to the west, and that the compressive force 

 at the surface came from the east. This conclusion is also borne 

 out by the schistosity dipping 35° to the southeast. The ultimate 

 force, however, may have acted as a deeper-seated stress from the 

 northwest. According to the theory of isostatic adjustment, the 

 loading of the sea bottom during all of Paleozoic time would produce 

 a deep-seated flowage toward the land. Isostasy need not, how- 

 ever, be depended on to account for such movement, for the thou- 

 sands of feet of sediments in themselves represent an equivalent 

 sinking of the sea bottom, whether the result of loading or of some 

 independent cause. This landward motion would be transmitted 

 in a certain degree to the overlying sediments, and they would be 

 moved, by a force acting from below and from the northwest, against 

 the consolidated land mass as a buttress, which would produce a resist- 

 ant force in the opposite direction acting at the surface. The but- 

 tress in this case would be the Archean rocks of the Piedmont to 

 the southeast, now partially covered by Triassic sediments. Thus 

 we should have exhibited at the surface structures produced by the 

 resistant stresses from the southeast, i. e. folds overturned to the 

 northwest and schistosity dipping to the southeast. 



