Geology Club Field Trip to Old Rag Mountain, Shenandoah National Park

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Old Rag Summit from Ridge Trail
Old Rag Summit
View toward the southeast from Ridge Trail toward the summit of Old Rag. Old Rag Mountain is underlain by the 1115 Ma Old Rag granite. A variety of textures of granite are exposed at this location. The original type locality for the Old Rag granite (Furcon, 1939) was behine the Old Rag Post Office, which no longer exists.
The precambrian Old Rag granite is Grenvillian in origin. Rocks of Grenville age in the Blue Ridge have been subdivided based upon the grade of metamorphism. The granulitic Pedlar Massif (which contains Old Rag) represents a higher metamorphic grade than the adjecent amphibolitic Lovingston Massif. Matt Hall
View from Ridge Trail
Toward the Crest of the Blue Ridge
Pedro Jugo
Diabase Dike
Near the summit of
Old Rag Mountain
Mafic dikes, either diabasic or metabasaltic (Catoctin Formation) are present cutting the Old Rag granite in places. Over 20 diabase dikes, concentrated near the summit, produce spectacular chimneys (photo at left) as they weather more rapidly than the host granite.
Opferkessels are solution pans which are rarely found in granitic rocks. It has been proposed formation of the opferkessels on Old Rag was initiated by interaction of mafic phases in the rock with organic and other acids produced by conifers near the summit.

Among the Boulders and Opferkessels
Thin Sections

Select thin sections of several of the textures exhibited by rocks on Old Rag Mountain. Textures are extremely diverse, ranging from impingement (a non-equilibrium texture characterized by curved grain boundaries that meet in triple- and quadruple-grain contacts with unequal angles, and range in crystal size and shape) to highly deformed (both mylonitic and cataclastic).
Blue quartz present in the groundmass and as pods (up to 4 feet in length), is common in many of the Grenville-age rocks of the central Virginia Blue Ridge. The origin of the blue color has been attributed to radioactivity, Ti in the form of ilmenite or rutile inclusions, or possibly Rayleigh scattering due to the presence of small inclusions. More information
Pod of Blue Quartz
Pod of Blue Quartz
In a moss-covered
light-green charnockite