cover of Bath 100K report

Bedrock Geology of the Bath 1:100,000 Map Sheet, Coastal Maine


Arthur M. Hussey II1,2
Henry N. Berry IV1

1 Maine Geological Survey
Augusta, Maine 04333

2 Bowdoin College
Brunswick, Maine 04011

Maine Geological Survey
Bulletin 42

Online Edition, 2006

This bulletin is a companion to Maine Geological Survey Geologic Map 02-152,
Bedrock Geology of the Bath 1:100,000 Quadrangle, Maine (4.9 Mb PDF file)

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Bedrock that underlies the area of the Bath 1:100,000 map sheet includes metamorphosed sedimentary and volcanic rocks of Late Cambrian to Silurian age, gabbroic and granitoid intrusive rocks ranging from Late Silurian to Permian age, and basaltic or diabasic dikes of Mesozoic age. The metamorphic rocks have been assigned to seven tectono-stratigraphic sequences: Central Maine, Falmouth-Brunswick, Casco Bay, East Harpswell, Fredericton, Benner Hill, and Megunticook. These sequences are separated by inferred early tectonic thrust faults and late tectonic or post tectonic strike-slip faults of the Norumbega fault zone, so have uncertain relationships to each other. For a few structurally isolated units, the sequence assignment is not clear. Several stages of folding characterize each sequence and are believed to be mostly the result of Late Silurian to Early Devonian phases of the Acadian orogeny. Prograde metamorphism of the volcanic and sedimentary rocks ranges from staurolite grade to sillimanite + K-feldspar grade in a Buchan-type low-pressure metamorphic facies series. Multiple stages of Buchan series metamorphism are clearly indicated by large muscovite pseudomorphs at Small Point. At the highest grade (sillimanite + K-feldspar), pelitic rocks are extensively migmatized. 40Ar/39Ar cooling ages on hornblende from sequences east of the Flying Point fault suggest that major regional high-grade metamorphism occurred during the Silurian-Devonian Acadian orogeny. Younger hornblende cooling ages from the Falmouth-Brunswick sequence in the Brunswick area suggest a more limited region of high-grade metamorphism of Late Paleozoic age possibly related to the intrusion of Permian pegmatite and granite lenses. No unequivocal evidence of large-scale Alleghenian folding has been documented in the map area.

Table of Contents


Central Maine sequence

  • Torrey Hill Formation (SOth)
  • Richmond Corner Formation (SOrc)
  • Hutchins Corner Formation (SOhc)

Falmouth-Brunswick sequence

  • Nehumkeag Pond Formation (Onp, Onpr)
  • Mount Ararat Formation (Oma)

Casco Bay Group

  • Cushing Formation (Ocp, Oca, Ocm, Ocw)
    • Peaks Island Member (Ocp)
    • Merepoint Member (Ocm)
    • Wilson Cove Member (Ocw)
  • Cape Elizabeth Formation (Oce, Ocer, Ocep, Ocea, Ocec)
  • The Cape Elizabeth - Cushing contact
  • Spring Point Formation (Ospb, Ospf)
  • Diamond Island Formation (Odi)
  • Scarboro Formation (Osc)
  • Spurwink Metalimestone (Osk) and Jewell Formation (Oj)

East Harpswell Group

  • Yarmouth Island Formation (SOy, SOya)
  • Bethel Point Formation (SObp)
  • Sebascodegan Formation (SOs, SOsa, SOsr, SOspi)

Fredericton sequence

  • Bucksport Formation (SOb)

Megunticook sequence

  • Penobscot Formation (OCp)

Benner Hill sequence

  • Mosquito Harbor Formation (Omh)
  • Hart Neck Formation (Ohn)
  • Benner Hill Formation (Obh)
  • Rocks exposed on the Georges Islands, St. George

Sequence uncertain

  • Cross River Formation (OCcr, OCcrg)
  • Unnamed amphibolite unit (Ouv)
  • Unnamed sulfidic schist (Ouss)

Correlations and ages of the stratified rocks

  • Central Maine sequence
  • Falmouth - Brunswick sequence
  • Casco Bay Group
  • East Harpswell Group
  • Fredericton sequence
  • Benner Hill sequence
  • Megunticook sequence
  • Sequence uncertain

Intrusive rocks

  • Raccoon pluton (Srgb, Srgbh)
  • Spruce Head pluton (Ssg, Ssgb, Ssgd)
  • Monhegan pluton and other gabbro and diorite bodies (Sgb)
  • Miscellaneous metamorphosed mafic to intermediate plutons (DSmdg)
  • Oak Island Gneiss (DSoi)
  • Edgecomb Gneiss (DSeg)
  • Two-mica granite (DSg) plutons, Woolwich - Phippsburg belt
  • Lincoln Sill (DSls)
  • Quartz diorite at Georgetown (Dqd)
  • Waldoboro pluton (Dwga, Dwgp, Dwgn, Dwgl)
  • Granitic pegmatites
  • Christmas Cove dike

Structural geology

  • Folds
    • Folds in the Falmouth-Brunswick sequence and the Central Maine sequence
    • Merepoint anticline
    • Harpswell Sound syncline and Hen Cove anticline
    • Phippsburg and Cape Small synforms
    • Robinhood Cove synform, Georgetown antiform, and Bay Point synform
    • Boothbay anticline
    • East Boothbay recumbent folds
    • Folds in the Linekin Neck - South Bristol area
    • Pemaquid Harbor anticline
    • Port Clyde refolded anticline
  • Faults and shear zones
    • Thrust faults
    • Norumbega fault system
    • Brittle faults
  • Lineation

Metamorphism and migmatization

Timing of volcanism, sedimentation, deformation, metamorphism, and plutonism

Mineral resources and past mining activity


References cited

Last updated on December 10, 2009