Maine's Red Beach Granite EarthCache
Maine's Red Beach Granite
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This earthcache will bring you to a roadside rest area that has
been special to me because I find the rock here to be unique and
tell a special story.
Granite is an igneous rock that forms by slow cooling and
solidification from molten rock (magma). The magma is produced by
melting at depth and intrudes into the overlying rocks without
breaking the surface. As it solidifies underground, the various
mineral grains grow from the melt to produce the interlocking
mosaic of grains we see in the rock today. Since granite must form
underground, the fact that we see it at the surface today means
that through geologic time a significant amount of overlying rock
has been removed by erosion. The coordinates bring you to a
roadside Rest Area and Boat Launch located in Robbinston, about 12
miles south of Calais. The granite gets its name from the striking
natural brick-red color mineral in the rock, called alkali
feldspar. There are two other predominant minerals in the granite
as well, quartz which is light gray and translucent and another
variation of feldspar (plagioclase) which is milky white. Upon
close inspection of the rocks and outcrops at this location, you
will notice that the granite is made up of individual grains of
those minerals that are very fine. If you look closely you will
find tiny black minerals here that are needle-shaped hornblende.
Granite with this particular grain size of the minerals is a result
of the magma solidifying very fast. You may also notice tiny holes
in the rock (called miarolitic cavities) where gas bubbles escaped
as it was being formed. Similar to way carbonation is released by
taking the cap off a soda bottle, gas bubbles can only be released
under relatively low pressures. You will also notice thin red dikes
cut through the granite. These dikes are even finer-grained than
the granite, with grains too small to see without magnification.
Their microscopic grain size is similar to that of volcanic rocks
and requires very rapid cooling. Such dikes are thought to
represent the remnant portion of magma remaining after the granite
had mostly solidified. This last bit of magma was injected into a
thin crack and cooled instantaneously to a very fine-grained rock.
On the surface of the outcrops you will see another story that
involves glacial movement during the Ice Age, about 14,000 years
ago. Some of the rocks here have grooves and scratches on their top
surfaces. This is a result of stones that were frozen in the base
of the continental ice sheet and then dragged across the bedrock,
causing the surface marks. The alignment of these marks is parallel
to the direction the glacier was moving. You will need to identify
the direction that you think the glacier moved. The reason this Red
Beach Granite in important is that it holds a special place in the
geology of eastern Maine because of its relationships to
surrounding rocks. It intrudes into deformed sedimentary and
volcanic rocks of the Eastport Formation, so it must be younger
than the Eastport Formation. The Red Beach Granite is overlain, in
turn, by sandstone and conglomerate of the Perry Formation, which
contains cobbles that were eroded from the Red Beach Granite. This
relationship demonstrates that the granite must have intruded,
cooled, been uplifted, and eroded before the Perry Formation was
deposited. The sequence of events including eruption of volcanic
rocks, deformation of the earth's crust, intrusion of Red Beach
Granite is a part of the formative process of the Appalachians
called the Acadian orogeny and finally the erosion. What you are
looking at today is a result of these processes that took place
under your feet. At the coordinates listed above you will be in the
general area of seaside outcrop with glacial grooves and an
exceptional view of the beach.
To log this Earthcache: Post a photo of you and your GPSr and
then send an email to me through my profile with the following
information: What is the direction of the glacial grooves? Also at
Waypoint (N 45 04.959 W 67 06.494) you will find a section of the
beach, explain how the rock came to rest and what is different
about it. Please begin your email with the name of the earthcache
and make sure your log includes the number of people in your
group.
There is another location on Route 1 about 3 1/2 miles north of
Site 1, U.S. St. Croix Island Historical Site in Red Beach, Maine.
The town is named Red Beach because of there are several miles of
underlying red bedrock called Red Beach Granite. Take a walk along
the shore beside the boat launch where you will find a bedrock
outcrop and many blocks and stones of Red Beach Granite. Here the
grain is much larger and is a result of the magma cooling slower.
Some of these blocks have clean surfaces where the texture of the
granite can be seen. Identify the three minerals mentioned earlier.
Some of these blocks were left from quarrying operations in the
late 1800's and early 1900's when Maine granite was used for
buildings and monuments up and down the eastern seaboard. Among the
buildings which incorporate Red Beach Granite is the American
Museum of Natural History in New York. For further information
please go to
http://www.maine.gov/doc/nrimc/mgs/explore/bedrock/sites/oct03.htm
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