Alternating beds of coarser-grained micaceous metasiltstone and metasandstone and finer-grained metapelite and micaceous metasiltstone with interbedded felsic and mafic metatuff layers are here named the Old Mill Branch Member of the Popes Head Formation. The unit is isoclinally folded and phyllitic cleavage is subparallel to bedding. Discordantly overlies all other rocks of the Wissahickon terrane in Fairfax County, VA. Gradationally underlies the Station Hills Phyllite Member of the Popes Head. Maximum thickness is about 730 m. Age is Late Proterozoic and (or) Early Cambrian based on the age of the Occoquan Granite, which has been determined radiometrically as Early Cambrian.

Source: GNU records (USGS DDS-6; Reston GNULEX).

The age of the Popes Head Formation and its Old Mill Branch Metasiltstone and Station Hills Phyllite Members is now believed to be Cambrian because the Popes Head unconformably overlies the other lithotectonic units in the area, and is intruded by the Occoquan Granite, which may be as young as Late Cambrian, based on a Sr-Rb age of 494+/-14 Ma.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Popes Head Formation is divided on the State map into Station Hills Phyllite and Old Mill Branch Metasiltstone [Members]. The Popes Head unconformably overlies the Mather Gorge, Sykesville, and Yorkshire Formations, the Annandale Group, and the Piney Branch Complex. The Old Mill Branch is intruded by the Occoquan granite, which has been dated at 479+/-9 Ma by J.N. Aleinikoff (1993, written commun., U-Pb single-crystal zircon). Map symbol indicates that age of the Old Mill Branch is considered Cambrian and Ordovician.

Source: GNU records (USGS DDS-6; Reston GNULEX).