Eight hundred and forty-three reverse faults were observed, measured, and analyzed statistically in the section of Susquehanna Gap and Cove Mountain syncline, Pennsylvania. Displacements are from several inches to 12 feet. The grooves and striae on fault surfaces strike perpendicular to fold axes independently of the fault strikes. The predominant dip is southeast and northwest including an angle which increases gradually from 45° on the south limb to 90° in the center of the Cove Mountain syncline. The faults tend to thicken the rock column vertically and to shorten it horizontally in the same manner as folding. Total accumulated thickening and lateral shortening is several hundred feet. The forces necessary to produce such faults are ample to explain brecciation and slickensiding even with very slight displacements and with dip angles of more than 55° but the forces must exceed the weight of the overlying rock column because of friction. Greater dip variation of northwest-dipping fault planes apparently indicates rotation of shear planes during folding. Steep average dip of northwest-dipping planes south of Marysville may be due to rotation after faulting since work done on steep planes would exceed that done on gentle ones.
The authors believe that reverse faulting of the described kind is an important type of Appalachian deformation. Reverse faults are probably much more numerous and may be observed elsewhere.