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Underhill Formation - Silvery, gray-green, quartz-sericite-albite-chlorite-biotite schist, containing abundant lenticular segregations of granular white quartz; locally quartz-sercite-albite-chlorite phyllite; porphyroblasts of albite, garnet, and magnetite are common and locally very abundant in gneissic facies in axial anticlines of the Green Mountain anticlinorium . (Northern and Central Vermont). In study area consists mainly of greenish quartz-chlorite-sericite phyllites lying stratigraphically between Pinnacle and Cheshire Formations, where author would place rocks of type locality within Underhill facies of Pinnacle Formation, for they are clearly stratigraphically equivalent to rocks of Pinnacle Formation in Enosburg area, being below an excellent horizon marker, the Whitebrook dolomite and slate. However, Underhill facies of the Pinnacle and phyllites of Underhill Formation are practically indistinguishable in the field, and it is unavoidable, wherever dividing White Brook dolomite and slate are absent, to map all rocks of Underhill facies as one unit. In western part of outcrop belt, Underhill rocks are well defined between White Brook Dolomite or coarse Pinnacle graywacke below and Cheshire Formation above. Rocks in this clearly defined area are here recognized as Fairfield Pond Member. As mapped, the Underhill includes Fairfield Pond Member, Bakersfield Greenstone, Peaked Mountain Greenstone, White Brook Member, Jay Peak Member, and West Sutton Slate Member. Eastern facies of Underhill is named Bonsecours facies (Dennis, 1964).
|Original map label||Cu|
|Comments||Part of Green Mountains, Camels Hump Group Secondary unit description per VT019. The Underhill lies stratigraphically above basement rocks of Mount Holly Complex of Grenville (1 b.y.) age. Age changed to Late Proterozoic and(or) Early Cambrian (Slack and others, 1988) per VT020.|
|Primary rock type||mica schist|
|Secondary rock type||phyllite|
|Other rock types|
Metamorphic > Schist > Mica-schist > Biotite-schistSilvery, gray-green, quartz-sericite-albite-chlorite-biotite schist, containing abundant lenticular segregations of granular white quartz.
Metamorphic > Metasedimentary > Metaclastic > PhylliteLocally quartz-sercite-albite-chlorite phyllite; porphyroblasts of albite, garnet, and magnetite are common and locally very abundant in gneissic facies in axial anticlines of the Green Mountain anticlinorium .
|Map references|Nicholson, S.W., Dicken, C.L., Horton, J.D., Foose, M.P., Mueller, J.A.L., and Hon, Rudi, 2006, Preliminary integrated geologic map databases for the United States: Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, Rhode Island, and Vermont: U.S. Geological Survey Open-File Report 2006-1272.
Doll, C.G., Cady, W.M., Thompson, J.B., Jr., and Billings, M.P., 1961, Centennial Geologic Map of Vermont: Vermont Geological Survey, Miscellaneous Map MISCMAP-01, scale 1:250,000.
Dennis, J.G., 1964, The geology of the Enosburg area, Vermont: Vermont Geological Survey Bulletin, no. 23, 56 p.
Slack, J.F., Atelsek, P.H. and Grosz, A.E., 1988, Geology and geochemical survey of the Bristol Cliffs Wilderness, Addison County, Vermont: U.S. Geological Survey Miscellaneous Field Studies Map, MF-1593-A, 1 sheet, scale 1:24,000.
|Geographic coverage||Addison - Chittenden - Franklin - Lamoille - Orleans - Rutland - Washington - Windsor|
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