Lunga Reservoir Formation (Pavlides, 1989; 1990). Metadiamictite, characterized by a nonstratified, micaceous quartzofeldspathic matrix resembling a granitoid, containing rounded to subrounded silt- to sand-sized quartz and plagioclase grains, fine-grained and porphyroblastic muscovite, green porphyroblastic biotite, garnet and magnetite. Granule-, pebble-, and cobble-sized lumps of milky quartz are ubiquitous; mica schist and gneiss clasts are common; calc-silicate clasts are rare. Pebble-, cobble-, and boulder-sized fragments of mafic and ultramafic rock are sparsely distributed in the southern part of the outcrop belt, but are locally abundant in the north. Locally along the contact with the Chopawamsic Formation, the Lunga Resevoir contains exotic fragments similar to lithologies found in the Chopawamsic. The Lunga Resevoir is intruded by Occoquan Granite.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.
Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.