Seawall Beach
(Above) Located in Southwest Harbor on Mount Desert Island, Seawall Beach is a famous stop as a part of Acadia National Park. Scattered boulders cover the beach like a natural wall between the ocean and the woods.
(Left) Bedrock Geology Map: The gray circle highlights the Seawall Beach area and its bedrock composition. According the the Maine Bedrock Map, the Seawall area contains mostly volcanic breccia and intrusive granite. Coordinates: lat. 44. 240682 long. -68.300338 (Edited after Gilman & Chapman, 1998)
Cranberry Volcanics: These gray rocks are part of the Cranberry Island Volcanic series. Since they are brecciated, large and light-colored pyrocrysts can be found in the rock.
Granite: A fine-grained granite with a distinctive sugary texture and a pink hue. It extends from Seawall to Bass Harbor and contains prominent veins of quartz and feldspar.
Rocks types
Granite [Devonian]:
Where to find it: Seawall to Bass Harbor
It is a medium to fine grained phaneritic granite, with pegmatitic pockets. It has a sugary, pink color. There are visible veins of coarser quartz and feldspar. The pretty, blue mineral is amazonite, a type of potassium feldspar.
Volcanic Breccia [Late Silurian]
Formation: Cranberry Island Series
Texture: Clasts imbedded in an aphanitic groundmass
Minerals: Feldspar, quartz, hornblende
Other: Xenotliths were larger than 2mm, and were usually a light gray color.
Basalt
Formation: Mafic Dike
Texture: Aphanitic
Minerals: Olivine, pyroxene, amphibole, plagioclase feldspar
Other: Width of dike is roughly 6 ft and trends NE-SW
Minerals
Amazonite [Tectosilicate]
Chemical Formula: KAlSi3O8
Crystal System: Triclinic
Cleavage/Fracture: Fracture
Luster: Nonmetallic
Color: Blue, Blue-Green
Streak: None
Hardness: 6-7
Other: A blue variety of microcline feldspar
Quartz [Tectosilicate]
Chemical Formula:SiO2
Crystal System: Hexagonal
Cleavage/Fracture: Fracture
Luster: Nonmetallic
Color: Colorless, clear to pearly white
Streak: None
Hardness: 7
Other: Forms 6 sided crystal
Biotite [Phyllosilicate]
Chemical Formula: K(Mg,Fe)3(AlSi3O10)(OH)2
Crystal System: Monoclinic
Cleavage/Fracture: 1 direction of cleavage
Luster: Nonmetallic, Pearly
Color: Black
Streak: Black
Hardness: 2.5 (Can be scratched by nail)
Other: A mafic type of mica. Chunks of biotite were sparse. It can easily split into thin layers.
References
Caldwell, D. W. (1998) Roadside Geology of Maine, Missoula, Mountain Press Publishing Company, 317 p
Gilman, R. A., Carleton, A. (1988) “Bedrock Geology of Mount Desert Island,” Maine Geological Survey: Department of Conservation, Sate Geologist: Anderson, W. A.