Gile Mountain Formation (MADgma;0)

State	Massachusetts
Name	Gile Mountain Formation
Geologic age	Lower Devonian
Original map label	Dgma
Comments	Part of Connecticut Valley Belt (Silurian and Devonian Rocks) Secondary unit description per MA006 as reported in USGS Geologic Names lexicon (ref. MA024): Rocks mapped as Conway Schist by Emerson (1898, 1917) and subsequently subdivided by Segerstrom (1956) and Willard (1956) were mapped across the MA-VT State line as Waits River and Gile Mountain Formations by Doll and others (1961) on Centennial Geologic Map of Vermont. Although controversy still exists over relative ages, detailed reconnaissance mapping by authors and S.F. Clark, Jr., L.M. Hall, and J.W. Pferd shows that Waits River and Gile Mountain Formations are readily distinguished in the field. For these reasons, and to maintain continuity across the State line, authors chose to follow VT nomenclature here and on MA State bedrock geologic map (Zen and others, 1983). Primary difference between Waits River and Gile Mountain is presence in Gile Mountain of beds of noncalcareous, commonly micaceous quartzite. Both formations contain conspicuous beds of punky brown-weathering impure marble or calcareous granulite, mostly in Waits River and less abundant in Gile Mountain. Predominant lithology of both formations is typically contorted gray, graphitic, locally very sulfidic, moderately aluminous mica schist containing quartz veins. Gradational but definitely significant boundary can be mapped between both formations. Amphibolite in both formations may correlate with Standing Pond Volcanics occurring at or near Waits River-Gile Mountain contact in VT. Report goes into great detail regarding informal subdivision of each formation (Hatch and others, 1988).
Primary rock type	amphibolite
Secondary rock type	amphibole schist
Other rock types
Lithologic constituents	Major Metamorphic > Schist > Amphibole-schisthornblende schist Metamorphic > Amphibolite
Map references	Unpublished Digital Geologic Map of Massachusetts received from Rudi Hon at Boston College in 1998.
Unit references	Zen, E-An (ed.), Goldsmith, R. (comp.), Ratcliffe, N.M. (comp.), Robinson, P. (comp.), Stanley, R.S. (comp.), Hatch, N.L., Jr., Shride, A.F., Weed, E.G.A., Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000. Hatch, N.L., Jr., Robinson, Peter and Stanley, R.S., 1988, Stratigraphy of the Connecticut Valley belt, Chapter B, IN Hatch, N.L., Jr., ed., Bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-A-D, p. B1-B34. [http://pubs.er.usgs.gov/publication/pp1366AD] USGS Geologic Names Lexicon (GEOLEX) [http://ngmdb.usgs.gov/Geolex/]
Geographic coverage	Franklin