The Accotink Schist is here named as the lower unit of the Eastern Fairfax sequence, the structurally lowest metamorphic rocks in northern VA. It consists of beds of deformed pelitic quartz-muscovite-biotite-chlorite-plagioclase schist with thin basal intervals of fine-grained metasiltstone and interbeds of metasandstone and characterized by turbidite sequences. Gradationally underlies the Lake Barcroft Metasandstone; exotic blocks of Accotink Schist are contained in the Sykesville Formation, which is believed to have been emplaced by subaqueous sliding on top of the Lake Barcroft. Intruded by the Early Cambrian Occoquan Granite. Thickness is unknown because the base is not exposed.

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

The Accotink Schist is the lower unit of the here named Annandale Group, originally the informal Eastern Fairfax sequence of Drake and Lyttle (1981). The Accotink and the upper unit of the Annandale Group, the Lake Barcroft Metasandstone, lie between two melanges, the lower newly recognized Indian Run Formation and the upper Sykesville Formation. Fragments of Accotink and Lake Barcroft are restricted to a certain area of melange originally defined as Sykesville, but now considered to be a separate unit, the Indian Run. Age is Late Proterozoic or Cambrian.

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

The age of the Annandale Group, including the Accotink Schist and Lake Barcroft Metasandstone, is now believed to be Late Proterozoic and (or) Early Cambrian because the Annandale Group has had a much more complicated geologic history than the younger Popes Head Formation, which unconformably overlies it, and the precursory melanges, the Sykesville and Indian Run Formations. All the lithotectonic units in the area are intruded by the Occoquan Granite, which may be as young as Late Cambrian, based on an Sr-Rb age of 494+/-14 Ma.

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