SEDIMENTARY ROCKS OF SOUTH MOUNTAIN 219 



Thus far his conclusion is good, but inasmuch as the sandstone 

 series along the mountain front is, according to his interpretation, 

 older than the orthofelsite series, and dips away from the limestone 

 of the valley, he concludes further that 



A master fault must therefore run along the northwest foot of the mountains, 

 along the low drift-filled valley of Yellow Breeches Creek [Mount Holly area], 

 in which nowhere can any rock be seen in place, but only a series of brown hem- 

 atite iron ore deposits, some of them of great size and once extensively mined 

 in open quarry work. The northwest face of the mountain mass is therefore 

 in fact the eroded basset edge of the quartzite series dipping away from the fault. 

 The thickness of the quartzite and conglomerate series may be imagined from 

 cross-section No. 10, laid 2^ miles north of Greenwood [east of Chambersburg], 

 along which for five miles quartzite beds on a prevailing southeast dip are either 

 seen or indicated suggesting a total thickness of 14,000 feet [a partial measure 

 of the throw of the "master fault"]. 



Walcott accounts for the mountain offsets chiefly by thrust- 

 faulting, as expressed in the following extract:' 



My impression is that these offsets [in the mountain front], and also the com- 

 plicated structure of the mountain, arise partly from folding, but more largely 

 from the westward thrusts of masses of strata along the lines of fault of a low 

 hade. This westward thrusting on the fault plane, complicated by previous 

 folding of the strata, leaves masses of the subjacent, pre-Paleozoic rocks resting, 

 in various places, on different members of the Lower Cambrian series, and also 

 appears to interbed the quartzites and schists of the Cambrian in the schists, 

 eruptive, etc., of the Algonkian. 



Keith also holds the view that strike faults have a prominent 

 part in the structure of the mountains, especially along the western 

 border. He shows their presence in the Catoctin belt,^ and carries 

 them into the area southeast of East Branch of Little Antietam Creek. 



CONCLUSION 



It has been demonstrated in this paper that the conclusions of the 

 writers quoted above are not applicable to the area here discussed; 

 that thrust- faulting cannot account for the offsets in the mountain 

 front near Fayetteville and opposite Waynesboro, but that they 

 are due to folds plunging steeply to the south, with the strata in 

 unbroken sequence"; that along the straight portions of the moun- 

 tain front the youngest beds of the sandstone series are next to the 



I Loc. cit., p. 27. 2 Loc. cit., pp. 358-62. 



