East of Cameron’s Line EarthCache
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A fun and educational Earth Cache in the Mountain Laurel Open Space
in the Town of Fairfield, CT.
A huge thanks to Geosliders for their help with this one!
Parking access is on Mountain Laurel Rd.
Read the following and answer the four questions.
Approximately 600 - 700 million years ago the North American
continent developed a rift (a tear in the earth’s crust that
continuously erupts magma and pushes the two sides apart) and began
splitting apart, each side moving in the opposite direction of the
other. As this occurred the area between the two land masses filled
with sediments and volcanic deposits (from the magma at the rift
zone). Over time sea level rose and much the area was covered by
this ancient shallow sea known as the Iapetus Ocean.
Then the rifting ended. The ancient sea began to close and our
continent collided with the island arc that had developed during
the rifting process. This event is called the Taconic Orogeny (or
mountain building event). During this event the sedimentary
deposits that made up the shorelines were scraped off and thrust
onto the continent as the rest of the mass was forced deep into the
earth. Following the Taconic Orogeny was the Acadian Orogeny which
occurred roughly 400 million years ago. This tectonic event also
altered the landscape through active folding and faulting of the
entire area.
The Geology of western CT can be highly complex due to the
significant deformation that occurred during the Taconic Orogeny. A
massive thrust fault (one layer being shoved up, onto and over
another layer) in western CT known as Cameron's Line is basically a
large suture zone between rocks that have different origins. To be
overly general, the rocks on one side of the line have been
significantly moved by tectonic forces. To the west of Cameron's
Line we find metamorphosed sedimentary material initially formed as
shallow continental slope deposits. The metamorphic rocks to the
east of Cameron's Line are mainly derived from deepwater basin
sediments. Each depositional zone environment creates a different
type of rock – which means a different end result after
metamorphism.
Why is this significant? Under normal circumstances as sediments
are deposited the youngest ones are on top and as you dig deeper,
the sediments get older. In some areas of CT one can find older
metamorphosed deep water sediments on top of younger slope
deposits. In other words, something BIG happened to move them
there. The area of the Earthcache is located east of Cameron’s
Line.
The rocks in the outcrop at the IP consist of beautiful mica
schist. Mica….what? Lets first look at how Schist forms. We start
with deposits of clay minerals off the continental slope into a
deep water setting. Over time these sediments are subjected to
temperature and pressure changes and form shale. During various
geologic events these sediments can be further subjected to various
degrees of temperature and pressures and “metamorphosed” into
something else. As metamorphism occurs and increases in intensity,
the shale will change into various types of rocks. A shale will
change to Slate, then Phyllite, then Schist (intermediate grade)
and finally to Gneiss. In this outcrop you will find many
interesting geologic features. Use the following definitions to
help answer the questions at the bottom:
Schist: a coarse grained, intermediate grade metamorphic
rock. Coarse grained means the individual mineral grains are
visible with the naked eye. Schist is typically named for the
dominant mineral from which they have formed (in this case mica
schist).
Mica: a commonly found silicate mineral that forms in
flakes, sheets and books and is found in sedimentary, igneous and
metamorphic rocks. It is most commonly observed as shiny light
colored flakes. The flakes can also be stacked on top of the other
and “mica books” can be observed.
Garnet: a metamorphic index mineral (meaning it forms at a
specific temp/pressure point) commonly found in intermediate to
high grade metamorphic rocks. Garnet is typically found as a dark
reddish brown mineral and is used for jewelry as well as industrial
abrasives. Other index minerals include chlorite (green) and
biotite (black).
Pegmatite: a pegmatite is an igneous intrusive rock with a
grain size of 20mm or greater, that forms deep in the earth and
cools slowly. When a mineral bearing rock cools slowly the minerals
have more time to form properly and the crystals are able to grow
larger. Pegmatites are commonly composed of quartz, feldspar and
mica.
So, after all that, how do you earn your smiley?
Walk around the outcrop and use what you have read as a reference.
EMAIL answers and post a photo where required for the following
questions:
1. Do you see any evidence of index minerals? If yes what is it
and what are the coords?
2. Take a photo of your GPS and a pegmatic intrusion and email the
composition (quartz, feldspar, etc)
3. Take a photo of mica and email it’s form (sheet, book or
flake)
4. Since we are EAST of Cameron’s Line where did these sediments
most likely form prior to undergoing metamorphism? – EMAIL the
answer.
There are no signs to get your answers for this one –
hence the 3 difficulty. This EC is intended to make you read, think
and interpret some interesting geology. I'm not looking to delete
smiley’s for wrong answers – I just want to be able to see that you
have read the material and attempted to find the corresponding
answer.
Most of all – HAVE FUN!
Additional Hints
(No hints available.)