Abstract

The rocks of the Glenarm series in southeastern Pennsylvania, of probable Paleozoic age, show strongly overturned and locally recumbent folds. Detailed mapping of structural features indicates that not only the bedding but also the cleavage and axial planes of folds have undergone deformation which increases in intensity from northwest to southeast along with increasing grade of metamorphism. These observations, together with the outcrop pattern, point to large-scale refolding of earlier folds; recumbent structures result from the warping of axial planes that originally had steeper dips. This structure accords with the pattern of folding of unquestioned Paleozoic rocks in the adjoining Philadelphia area without recourse to the hypothetical “Martic overthrust.” The writer suggests that the late-stage deformation is related mechanically to the change in strike of the Appalachian belt; the belt strikes generally north-south in New England and Virginia but is nearly east-west in Pennsylvania.

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