page_title: Blue Ridge Mountains
section_title: Geology
hierarchical_section_title: Blue Ridge Mountains / Geology
caption_attribution_description: English: Oblique air photo of South Mountain in Maryland, looking north. Interstate 70 bisects the image. Taken and uploaded by James Stuby.
context_page_description: The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range. The mountain range is located in the eastern United States, and extends 550 miles southwest from southern Pennsylvania through Maryland, West Virginia, Virginia, North Carolina, South Carolina, Tennessee, and Georgia. This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap. To the west of the Blue Ridge, between it and the bulk of the Appalachians, lies the Great Appalachian Valley, bordered on the west by the Ridge and Valley province of the Appalachian range.
  The Blue Ridge Mountains are noted for having a bluish color when seen from a distance. Trees put the "blue" in Blue Ridge, from the isoprene released into the atmosphere, thereby contributing to the characteristic haze on the mountains and their distinctive color.
context_section_description: Most of the rocks that form the Blue Ridge Mountains are ancient granitic charnockites, metamorphosed volcanic formations, and sedimentary limestone. Recent studies completed by Richard Tollo, a professor and geologist at George Washington University, provide greater insight into the petrologic and geochronologic history of the Blue Ridge basement suites. Modern studies have found that the basement geology of the Blue Ridge is made of compositionally unique gneisses and granitoids, including orthopyroxene-bearing charnockites. Analysis of zircon minerals in the granite completed by John Aleinikoff at the U.S. Geological Survey has provided more detailed emplacement ages.
  Many of the features found in the Blue Ridge and documented by Tollo and others have confirmed that the rocks exhibit many similar features in other North American Grenville-age terranes. The lack of a calc-alkaline affinity and zircon ages less than 1.2 billion years old suggest that the Blue Ridge is distinct from the Adirondacks, Green Mountains, and possibly the New York-New Jersey Highlands.  The petrologic and geochronologic data suggest that the Blue Ridge basement is a composite orogenic crust that was emplaced during several episodes from a crustal magma source.  Field relationships further illustrate that rocks emplaced prior to 1.078–1.064 billion years ago preserve deformational features. Those emplaced post-1.064 billion years ago generally have a massive texture and missed the main episode of Mesoproterozoic compression.
  The Blue Ridge Mountains began forming during the Silurian Period over 400 million years ago. Approximately 320 million years ago, North America and Europe collided, pushing up the Blue Ridge.  At the time of their emergence, the Blue Ridge were among the highest mountains in the world and reached heights comparable to the much younger Alps. Today, due to weathering and erosion over hundreds of millions of years, the highest peak in the range, Mount Mitchell in North Carolina, is only 6,684 feet high – still the highest peak east of the Mississippi River in the US.
caption: The northernmost extension of the Blue Ridge Mountains, in northern Maryland
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