A Very Brief Look at Local Geology
Formation of Blue Hill Mountain
Blue Hill is an example of a pluton. Plutons are large masses of intrusive
igneous rock which form when a vast plume of magma rises up into the crust and
intrudes into preexisting rock. The magma plume cools and solidifies while still
in the crust. Plutons may be anywhere from tens of meters to over 100 km
across. Geologists refer to the original rock that surrounds a pluton or other
intrusive igneous rock formations as country rock.
About 380 to 420 million years ago, the pluton which became Blue Hill
Mountain formed. As the pool of magma welled up within the crust, it slowly
cooled forming the granite mass we call Blue Hill Mountain. At the time of its
formation, it was about 2 km beneath the surface of the Earth. What happened to
all the rock that lay on top of it? It eroded away along with the softer rock that
surrounded each side of the pluton. The mountains of Acadia National Park,
other coastal mountains and Mt. Katahdin are also plutons.
In other areas magma seeped through vertical cracks creating intrusive
igneous rock formations known as dikes. Black basalt dikes running across the
pink granite can be observed at Schoodic Peninsula. The basalt erodes faster than
the surrounding granite leaving gaps that the explorer must sometimes jump
across. Sills form when magma seeps into horizontal cracks in rocks. Both dikes
and sills may be seen along many stretches of roads where rock has been blasted
exposing a rock face. The country rock is older than the dikes and sills that cut
through it.
Ice Ages
Our planet has repeatedly experienced periods of glaciation. During these
periods ice sheets extended outward from the poles but did not cover the entire
Earth. The most recent ice age peaked about 18,000 yrs ago. At that time ice
sheets extended from the polar region south to Cape Cod, MA and Long Island,
NY. A sheet of ice approximately one mile thick covered Blue Hill. The ice
sheets disappeared from this region 10-12,000 years ago.
Evidence of Glaciation
As the ice sheets advanced they scraped away soil and loose rock and
deposited them elsewhere. Where these were deposited by ice they are unsorted,
a jumble of gravel, stones and rocks of various sizes. Where they were deposited
by meltwater from the ice, the material was sorted. The valuable gravel deposits
mined throughout Maine are gifts left by the melting ice sheets. The sands that
form Cape Cod, MA, Long Island, NY and Martha's Vineyard were deposited by
retreating ice sheets.
Glacial erratics are boulders that were picked up by the advancing ice
sheets and deposited, sometimes miles from their place of origin. They are visible
evidence of the ice sheets that scoured this area that can be seen easily when
hiking or driving- look at the huge boulders scattered in woods, across blueberry
fields etc.
Glacial striations are parallel scratches gouged into exposed bedrock as the
glaciers moved over the rock.
Advancing ice sheets also reshaped mountains. The sediment-laden ice
scoured and smoothed the side of the mountain from which it approached. As it
moved over the top of the mountain it left the other side steep and rough as the
glacier tore away rocks.
Other Evidence of Past Periods of Glaciation
Eskers are deposits of sand and gravel sorted by the meltwater flowing
under a glacier. They tend to be oriented from north to south.
Areas that were glaciated tend to have soil containing unsorted sand,
gravel and rocks that were picked up and transported by the glaciers. This soil is
referred to as glacial till.
Glaciers reshaped the land in other ways as well. The weight of a layer of
ice a mile thick actually depressed the land. As the ice sheets grew, sea level
dropped. 14,000 years ago the average sea level was about 110 meters (360 feet)
below its present level. During the peak of the last ice age, the shoreline was
miles from its current position. In places the shoreline was up to 60 miles out
from the present-day coastline (Glaciers of the Northeastern US). Much of what
is now the Gulf of Maine was inland during the last ice age.
When the ice sheets finally melted, the land slowly rebounded a little like
a marshmallow that has been pushed down on. This rebounding effect is very
slow however and takes place over a period of thousands of years. As the glacial
ice melted, sea level rose. Fossil and soil evidence shows that after the last ice
age the shoreline was as far inland as Millinocket. We can find fossils of marine
shells and bluish-gray marine clay (very fine sediment that is a characteristic of
the ocean floor) that identify regions of Maine that were once underwater.
As the land slowly rose, sea level dropped.
Glaciers of the Northeastern US. http://www.priweb.org/ed/TFGuide/NE/glaciers/glaciers.pdf. The
Paleontological Research Institution. 2003.
Define each of the following terms.
PlutonCountry RockDikeSillGlacial ErraticGlacial StriationEskersGlacial Till-
1. Describe how Blue Hill Mountain formed.
2. When did the most recent ice age peak?
3. Describe how advancing and retreating sheets of ice changed the shape of the land.
4. Compare sea level today and sea level 18,000 years ago.
5. Compare sea level today and sea level 10,000 years ago.
6. Describe three pieces of evidence that Maine was covered in glaciers.