Calculation of crustal shortening ratios in the northern Appalachians has been undertaken on the basis of continuous exposures provided by the anthracite fields of Pennsylvania. The ratios increase from northeast to southwest with a maximum approximately in the Harrisburg section. The increase is accompanied by convergence of axes from the northeast toward the Susquehanna River, and consequently the Appalachian trends bulge northwestward. An arc is formed with Baltimore approximately at the center of curvature. In the same measure as the folds crowd northwestward thrusts appear in the crystalline axis and involve the Glenarm series and the lower Paleozoic rocks. The gneiss basement does not participate with equal intensity in the folding, and an axial divergence between basement and cover becomes visible. Most of the “Appalachian folding” seems to have taken place above the gneiss complex and the basement is only superficially involved. The Catoctin volcanic complex and the Glenarm series participate in the Appalachian trend. This seems to indicate a major structural break above the Baltimore gneiss complex.

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