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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.

 

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Weathering & Rock Features

The prominent fractures on the rocks run at a Northwest to Southeast direction. All the rocks seem fairly rounded on the surface due to high energy waves crashing on them.

Thin Section

Geologic History

Seawall Beach consists of boulders of granite, shatter zone, and various exotic rocks that the glaciers brought in from distant sources [then left in the glacial deposits]. The pebbles are smooth and nicely rounded. Heavy waves breaking on the beach is what formed the high beach ridge during strong ocean storms. During all but the most severe storms, that beach ridge acts as a natural seawall. Some of Seawall beach consists of boulders of the Cranberry Island volcanics. They were deposited during late Silurian or early Devonian time. Rocks are rhyolites and breccias laid down from volcanic ash flows and interbedded with slates and siltstones. Many formed during explosive eruptions.

(Caldwell, 1998)

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