Coastal Maine Geopark

Geoheritage Overview 

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The story of our planet really begins around 4.56 billion years ago when our planet was formed as one
of the 8 planets in our solar system. However, the chronicle of coastal Maine’s history appearing in the
rocks and landscape, only records the last ~600 million years (Ma), when the ancient supercontinent
Rodinia began breaking apart into two small continents: Laurentia and Gondwanan. The ocean between
the two was called the Iapetus Ocean, which is where some of the oldest rocks found along Maine’s
coast were first deposited as fine sediments. Over time these sediments were lithified into rocks and
later folded, altered, and accreted upwards as a series of ancient continents collided together.
Between 500 – 290 Ma when these ancient continents forcefully collided with the eastern margin of
Laurentia, a series of orogenies (mountain building events) took place and terranes were accreted
onto Laurentia, thickening the land mass and forming much of the Appalachian Mountains.
(Braun and Braun, 2016).

Around 420 – 390 Ma, the Acadia Orogeny took place as the ancient microcontinents, Gander and
Avalon, collided into Laurentia and closed the Iapetus Ocean. As the oceanic lithosphere
subducted, Gander and Avalon accreted onto the edge of Laurentia. As a result of these events, a long
chain of super volcanoes was formed along the eastern shoreline, one of which was Mount Desert
Island. (Braun and Braun, 2016).

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Within each volcano, magma gathered deep inside the magma chamber, rising to surface, and
occasionally extruded out the top. Along the coastline, it was like a series of kettle pots full of hot,
molten magma, which cooled and crystallized over millions of years with bursts of intense activity and
magma intrusions into the older sedimentary rocks. A series of younger granite intrusions occurred
when the Meguma terrane (proto-Nova Scotia) collided with the Laurussia supercontinent (Laurentia-
Baltica-Avalonia) around 360 – 340 Ma, forming the youngest granites along Maine’s coastline. (Braun
and Braun, 2016; and Loiselle, 2002).

The Rheic Ocean – the ocean in between Laurussia and Gondwana – eventually closed during the
Alleghenian Orogeny around 270 – 255 Ma when the two continents collided, forming the
supercontinent Pangea. This was the final stage of the formation of the Appalachian
mountain building event and the present day coastline of Maine was uplifted on the northwestern
flank of the Appalachian mountain range that would have been a massive mountain range the size of
the modern-day Himalayas. Over the next 200 Ma, the landscape was extensively eroded by water,
wind, and ice. Rivers carved into fractures and plains of weakness, forming valleys, slowly eroding
away sediments. When Pangea eventually broke apart, the Atlantic Ocean was formed separating the
North American and Eurasian plate that continues to separate around 20cm every year along the Mid-
Atlantic ridge. (Braun and Braun, 2016; and Loiselle, 2002).

While the last ~180Ma has been tectonically quiet for the rocks of coastal Maine, over the last ~2Ma
there have been several glaciation events that have carved out the landscape and altered coastlines. The last continental glacier, called the Laurentide Ice Sheet, which extended all the way out to Georges Bank
around 25 thousand years ago (Ka), covered the landscape in ice about a mile high. The ice sheet retreated and advanced several times over thousands of years, with the last glacial maximum ~18Ka and the coastal
Maine region ice free by ~12Ka. These glaciers carved out U-Shaped valleys, scratched striations on bedrock, eroded and carried sediments over thousands of miles, and deposited those sediments as moraines, eskers, erratics and clay throughout Maine.

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The glaciers eroded all prior sediment and top soil, leaving behind barren and exposed bedrock when
the ice retreated. For the last ~15-12Ka, soils have slowly developed within this glaciated landscape
and certain plants have thrived. However, when the ice retreated, glacial sediments were also
deposited that has greatly influenced the geomorphology and ecology of the region. Maine’s iconic
wild low-bush blueberry (Vaccinium angustifolium) thrives on top of paleodeltas. The Pineo Ridge
glacio-marine paleodelta, today high above current sea level, is known for the famous “blueberry
barrens” because of the sandy-loam drainage that provides idea growing conditions for the plant.
Blueberries have been important food source over the past 10 Ka and continues to be an important
part of the cultural identify and economy of Downeast Maine. (Thompson and Borns, 2007).

The coastline of Maine has been constantly changing in response to the retreat of the Laurentide Ice
Sheet. After the ice sheet retreated, around 12.5 Ka sea level reached a lowstand ~60m
below present day and then rapidly rose to 20m below present ~12 Ka where it remained during a
slowstand” until ~8Ka before rising to near present-day levels (Kelley et al., 2010).

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Archaeological evidence shows that the Wabanaki were able to walk out to several islands that were
connected to the mainland when they first settled in Maine around 13 – 10 Ka. Several archaeological
sites along coastal Maine show inhabited areas that would have been exposed during these lowstand
periods that are today submerged under water. 

Sea-level rise and changes in ecology as a result of changing climates over the last 8 Ka have been
recorded in coastal shell middens, which are mounds of bone and shell fragments discarded by the
Wabanaki. Layers of history recorded in these human-made formations tells the story of how sword
fish once swam in the Atlantic Ocean and the types of tools people used to hunt and fish. Yet, these
valuable archives of geoheritage are rapidly eroding away as sea levels continue to rise today (Kelley
and Newsom, 2019).

When Europeans began exploring the coast of Maine, they built settlements at the heads of bays for
ease of access for launching and hauling in ships. These coastal towns are now are the forefront of
coastal erosion, as sea levels rise and increase of flooding erodes away roads and infrastructure.

While the Geopark as a whole is connected the same geological story featuring the opening and
closing of ancient ocean basins, volcanism, mountain building events, glaciation, and human
settlement, every site within the Geopark tells a unique chapter of this story.