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This is a regional study of one of the great unconformities of the geologic record. Cross sections show the wedging-out of the formations above and below the basal Pennsylvanian unconformity as the center of the United States is approached. This is further shown by isopachous maps of the deposits of Pottsville age and of the rocks of Upper Mississippian age. An areal map of the pre-Pennsylvanian rocks shows that as the Pennsylvanian seas covered the country, the pre-Pennsylvanian rocks had the general form of a great arch extending from New Mexico northeast into Minnesota and the pre-Cambrian shield of Canada. Along this axis, pre-Cambrian rocks were exposed and, farther from the axis, both east and west, progressively younger systems were exposed to the advancing Pennsyl anian seas, so that, at the margins of the country, rocks of Upper Mississippian age were in contact with rocks of lower Pottsville age.
The principal results of the study are the evidence about the presence and nature of this continental arch in the west-central states; the thinning of the rocks of Pottsville age as the axis is approached, due chiefly to progressive overlap in this direction; the thinning of the Upper Mississippian rocks in the same direction, due chiefly to progressively deeper erosion as the axis is approached; the character and position of the folds present in pre-Pennsylvanian time; the probable connection of the Pottsville from Pennsylvanian to West Texas; and the source of a substantial amount of the Pottsville sediments through the erosion of the pre-Pennsylvanian rocks from the crest of this continental arch.
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