Evidence for the Progressive Closure
Of the Proto-Atlantic Ocean in the Valley and Ridge
Province of Northern Virginia and Eastern West Virginia
Lynn S. Fichter: James Madison University
Richard J. Diecchio: George Mason University
INTRODUCTION
PHYSIOGRAPHIC SETTING
This field trip runs entirely within the Valley and Ridge province of the Appalachians. The Valley and Ridge is underlain by Paleozoic strata that have been deformed during the Alleghenian orogeny into a fold and thrust belt. Topography and relief within the Valley & Ridge is controlled primarily by the differential erosion of these folded and faulted strata. Generally speaking, in this part of the Appalachians, sandstones are the most resistant strata, and are therefore the ridge formers. Limestones and shales are generally much less resistant, and are the valley formers.
The Valley and Ridge is bounded on the west by the Allegheny Plateau. The Plateau is underlain by basically the same strata as the valley and ridge, but does contain younger parts of the stratigraphic column. Structurally, the Plateau is much less severely deformed than the Valley and Ridge. The Plateau is underlain by flat-lying to gently-dipping strata, in contrast to the much steeper dips in the Valley and Ridge. The structural and physiographic boundary between these two provinces is referred to as the Allegheny Front, visible in the western part of the field area (German Valley overlook, Stop 4).
The Valley and Ridge is bounded on the east by the Blue Ridge, which is visible from some places along the field trip route. The Blue Ridge contains Precambrian plutonic and metamorphic rocks that are the oldest in the area, and which form the basement upon which the Valley and Ridge stratigraphic succession was deposited. The Blue Ridge is discussed in more detail in the field guide by Conley and Gathright in this volume.
The Piedmont Province is located east of the Blue Ridge, and will not be accessible at all on this field trip. References are made to the Piedmont, however, because many of the sedimentary, volcanic, plutonic and metamorphic rocks of the Piedmont are of the same ages as some of the strata in the Valley and Ridge. These piedmont rocks contains information about geologic events happening in the Proto-Atlantic east of the Appalachian margin while the strata of the Valley and Ridge were being deposited and deformed.