|Geologic age||Lower Devonian|
Igneous > Plutonic > Granitic > Granitelight-gray, even and medium-grained, muscovite-bearing granite
|Comments||Part of Merrimack Belt (Silurian, Devonian and Pennsylvanian Rocks). Secondary unit description per MA007 as reported in USGS Geologic Names lexicon (ref. MA024): Chelmsford Granite intrudes Berwick Formation, Ayer Granite, and Dracut Diorite northwest of Clinton-Newbury fault. Occurs as two elongate, northeast-striking bodies occupying an area of 60 sq km. Described as light colored and gneissic. Zartman and Naylor (1984) could not obtain a definitive age but believed Chelmsford was in same age range as Ayer because the age of the Chelmsford lay within margin of error for their age determinations. Devonian age shown on MA State geologic map is based on their Pb-Pb age of 389 +/-5 Ma (Wones and Goldsmith, 1991).|
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., and Wones, D.R., 1983, Bedrock Geologic Map of Massachusetts: U.S. Geological Survey, Reston, VA, scale 1: 250,000.
Wones, D.R. and Goldsmith, Richard, 1991, Intrusive rocks of eastern Massachusetts, Chapter I, IN Hatch, N.L., Jr., ed., The bedrock geology of Massachusetts: U.S. Geological Survey Professional Paper, 1366-E-J, p. I1-I61.
USGS Geologic Names lexicon found at: http://ngmdb.usgs.gov/Geolex/
|Counties||Essex - Middlesex|