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.