The external massifs along the Appalachian orogen include Precambrian basement rocks with attached cover. To the northwest (cratonward), in the Appalachian foreland fold and thrust belt, Palaeozoic sedimentary rocks, but no basement rocks, are exposed; that belt was the subject of the classic debate about thin-skinned (deformed cover rocks detached from undeformed basement) and thick-skinned (basement deformed with attached cover) structural styles. Presently available data indicate detached cover rocks and thin-skinned style in the fold and thrust belt: large-scale thrusting occurred late in the orogenic history. In the external basement massifs, late Precambrian graben-fill sedimentary and volcanic rocks :indicate early basement faults; and within the craton, steep basement faults bound graben blocks of Cambrian age. Distribution of known basement faults suggests that basement rocks beneath the fold and thrust belt may also be faulted. Local episodic synsedimentary structural movement through much of the Palaeozoic is documented by stratigraphy in the fold and thrust belt.
Axes of early synsedimentary structures are approximately coincident with axes of late folds and thrust fault ramps, but stratigraphic data show that magnitude of the early structures was much less than that of the late structures. These relations suggest the interpretation that early low-magnitude structures formed in cover rocks over basement faults and that the early structures, or the basement faults, significantly influenced the geometry of later detachment structures during large-scale horizontal translation.