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Book Chapter

Rivers, glaciers, landscape evolution, and active tectonics of the central Appalachians, Pennsylvania and Maryland

By
Frank J. Pazzaglia
Frank J. Pazzaglia
Earth & Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA
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Duane D. Braun
Duane D. Braun
Geography & Geosciences, Bloomsburg University, Bloomsburg, Pennsylvania 17815, USA
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Milan Pavich
Milan Pavich
Eastern Earth Surface Processes Team, U.S. Geological Survey, Reston, Virginia 20192, USA
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Paul Bierman
Paul Bierman
Department of Geosciences, University of Vermont, Burlington, Vermont 05405, USA
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Noel Potter, Jr.
Noel Potter, Jr.
Department of Geology, Dickinson College, Carlisle, Pennsylvania 17013, USA
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Dorothy Merritts
Dorothy Merritts
Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA
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Robert Walter
Robert Walter
Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA
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Dru Germanoski
Dru Germanoski
Department of Geosciences, Lafayette College, Easton, Pennsylvania 18042, USA
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Published:
October 06, 2006

Abstract

Welcome to the Appalachian landscape! Our field trip begins with a journey across Fall Zone (Fig. 1), named for the falls and rapids on streams flowing from the consolidated rocks of the Appalachians onto the unconsolidated sediments of the Coastal Plain. The eastern U.S. urban centers are aligned along the Fall Zone, the upstream limit of navigation. Typically, the rocks west of the Fall Zone are part of the Piedmont province. This province exposes the metamorphic core of the Appalachian Mountains exhumed by both tectonics and erosion. At least four major phases of deformation are preserved in Piedmont rocks, three Paleozoic convergent events that closed Iapetus, followed by Mesozoic extension that opened the Atlantic Ocean. A record of Cretaceous to Quaternary exhumation of the Appalachians is preserved as Coastal Plain sediments. Late Triassic and Jurassic erosion is preserved in the syn-extensional fault basins, such as the Newark basin, or is buried beneath Coastal Plain sediments (Fig. 1). The trip proceeds northwest across the Fall Zone and Piedmont and into the Newark basin. Late Triassic and Jurassic fluvial red sandstone, lacustrine gray shale, and black basalt were deposited in this basin. The Newark basin is separated from the Blue Ridge by a down to the east normal fault that locally has contemporary microseismicity. The Blue Ridge represents a great thrust sheet that was emplaced from the southeast during the Alleghenian orogeny (Permian). The summits of the Blue Ridge are commonly broad and accordant. Davis (1889) projected that accordance westward to the summits of the Ridge and Valley to define his highest and oldest peneplain—the Schooley peneplain. North and west of the Blue Ridge is the Great Valley Section of the Ridge and Valley Province (Fig. 1). Where we cross the Great Valley at Harrisburg, it is called the Cumberland and Lebanon valleys. This section is underlain by lower Paleozoic carbonate, shale, and slate folded and faulted during the lower Paleozoic Taconic orogeny. The prominent ridge on the west flank of the Great Valley is Blue or Kittatinny Ridge. It is the first ridge of the Ridge and Valley Province; the folded and faulted sedimentary rocks of the Appalachian foreland basin, deformed during the Alleghenian orogeny. Drainage during most of the Paleozoic was to the northwest, bringing detritus into the Appalachian foreland basin. The drainage reversed with the opening of the Atlantic Ocean and southeast-flowing streams established courses transverse to the strike of resistant rocks, like the Silurian Tuscarora Sandstone holding up Blue Mountain. West and north of the Ridge and Valley is the Allegheny Plateau, that part of the Appalachian foreland that was only gently deformed during Alleghenian shortening. Our trip will traverse that part of the plateau called the Pocono Plateau which is underlain by Devonian to Penn-sylvanian sandstone. At the conclusion of our trip, we will reverse our transverse of the Appalachians by traveling from the Pocono Plateau to the Ridge and Valley, to the Great Valley, to the Newark Basin, to the Piedmont, and then to one of the great Fall Zone cities—Philadelphia—via the Lehigh and Schuylkill rivers.

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