Blue Ridge


 
Blue Ridge
 
The Blue Ridge province extends from southernmost Georgia to Pennsylvania.  In Virginia the physiographic region includes both the Blue Ridge mountains (Skyline Drive and the Blue Ridge Parkway), and the strip of land to the east running through Galax, Charlottesville, Culpepper, and Warrenton. This is an instance where the physiography and the geology do not exactly correspond. The geologic province is defined primarily by the rocks underlying it (coarse grained igneous and metamorphic Grenville basement rocks) rather than its topography (the eastern part of the geologic province blends in topographically with the Piedmont in many places, and appears distinct from the Blue Ridge mountains). In northern Virginia the Blue Ridge province crossed by Interstate 66 extends from about 5 miles east of Front Royal to Bull Run mountain just west of Manassas, Va. Here it is about 20 miles wide.
     
Structurally the Blue Ridge province is a large, eroded anticline overturned to the west (Cross Section). The core of the anticline is composed of igneous and metamorphic rocks collectively known as the Grenville, although there are also late Proterozoic intrusives and sediments present as well. They are the oldest rocks in the state at 1.1 billion years old (and a protolith [earlier rock now modified to something else] dating back to 1.8 billion). 
     
The east and west flanks of the anticline are much younger volcanics (Crossnore event) and clastic sediments. The clastic sediments fill rift grabens on the Northwest and Southeast flanks of the anticline (Lynchburg, Ocoee, Grandfather Mtn., Mt. Rogers Groups). Stratigraphic thicknesses range from about 3000 meters to 7000 meters. The final filling of the graben and creation of a divergent continental margin is preserved in the metamorphosed lava flows (Catoctin Formation) and sedimentary rocks (Chilhowee Group and Evington Formation) about 570-600 million years old (Blue Ridge Cross Section). The Crossnore igneous suite includes a volcanic pile (Mt Rogers in the south) and Granite batholiths (Robertson River) intruding into Grenville plutons. 

Contributed by Lynn Fichter 

Tuesday, July 29, 2014
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