Kathleen G. Muth, Joseph G. Arth, John C. Reed, Jr.; A minimum age for high-grade metamorphism and granite intrusion in the Piedmont of the Potomac River gorge near Washington, D.C.. Geology ; 7 (7): 349–350. doi: https://doi.org/10.1130/0091-7613(1979)7<349:AMAFHM>2.0.CO;2
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Near Great Falls on the Potomac River, metamorphosed Piedmont rocks of the Glenarm Group were intruded by small bodies of granite and pegmatite after the peak of high-grade metamorphism in the area. Muscovites from two of the feldspar-quartz-muscovite pegmatites were dated by the Rb-Sr method. They show ages of 469 ± 20 m.y. and 469 ± 12 m.y. The ages may represent the time of intrusion or cooling of the rocks below about 500 °C. The pegmatites crosscut the foliation of the Glenarm metasediments, and thus their age places a younger limit on the time of deposition of the series and of high-grade metamorphism in the area.