Valley and Ridge


 
Valley and Ridge
 
This province lies east of the Allegheny Plateau and west of the Blue Ridge; its western boundary is known as the Allegheny Front and marks an abrupt change from the flat lying rocks of the plateau to the folded/faulted rocks in the valley and ridge. The Allegheny Front boundary can be seen in the Cumberland Image as a northeast to southwest diagonal trend separating the parallel blue and green valleys and ridges in the east from the dominantly flat lying green part in the west. 

On the ground these ridges can be seen looking west from Skyline Drive or the Blue Ridge Parkway, for example this view west from the Little Stony Man overlook. The row after row of parallel mountains and valleys (beginning with the Shenandoah Valley) gives the province its name. In the picture Page Valley is in the foreground, backed by Massanutten Mt., followed by Shenandoah Mt, and eventually the Allegheny Front in the distance. The underlying structures formed during the Alleghenian Orogeny.
     
The rocks here are sedimentary, thrust faulted and folded into anticlines and synclines, lower and middle Paleozoic in age (Cambrian through lower Mississippian; 16 Page History Index), and have a trellis drainage pattern. A trellis drainage is when rivers are forced to run parallel to long ridges (see Cumberland Image). The ridges exist because they are underlain by hard rock that erodes less easily than the softer rock in the valleys (for example in this Cross Section, the Massanutten mountain is held up by a resistant sandstone, while Page and Shenandoah valleys to the east and west are underlain by more easily weathered Shale and Limestone.
 
The lowest part of the stratigraphic section is mostly Cambrian and lower Ordovician Carbonate (Limestone and Dolomite) that were deposited in tidal flat and coastal environments. Today these are best exposed in the Shenandoah and Page valleys. Most of the rest of the sedimentary rocks are sandstones and Shales deposited in deep marine basins during times when the Piedmont region was a large mountain.
 
Contributed by Lynn Fichter 
Thursday, July 24, 2014
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