Here There Be Dragons EarthCache
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Crazy folded rocks or a petrified serpent? You decide.
SAFETY FIRST: This stop is a roadcut of rock along a busy
highway. While there is plenty of room to pull over and see the
rocks, be careful of the potential dangers from rocks falling from
above and of vehicles in the road.
This site is a roadcut on ON-35, located between the towns of
Dorset & Dwight, Ontario.
Tools You'll Need:
1) your GPSr;
2) a way to estimate a fairly long distance; you can do this with
your GPSr if you're techie-clever (see explanation below), or you
can use a really long tape measure, or use the length of your foot
pace & count the number of paces; and
3) a camera (optional).
To log this EarthCache:
1) Send an email to the EarthCache owner with an estimate of
the length of the dragon from head to tail. Provide your answer in
feet or meters. Probably the easiest way to do this is to carefully
mark a POI in your GPSr at one end of the dragon, tell your GPSr to
GOTO that POI, then walk to the other end of the dragon and have
your GPSr tell you how far away you are from the POI.
2) Look closely at the layers of amphibolite in the "neck"
of the dragon and count how many thin layers are present; include
this answer in your email. This can give a rough idea of the number
of times the layer has been folded and refolded on itself. See
below for more information.
3) Optional task: Geologists always document their field
work with good quality photographs. If you are able, take a
photograph of the dragon's head, and include something for scale.
That is, include a person or object in the photograph of known size
so that the size of the rocks in the picture can also be
understood. Objects commonly used for scale include things such as
a coin, a hammer, a compass, a camera lens, a person, etc.; feel
free to use whatever you like that suits the purpose. Taking good
documentary photographs is part of the training of a field
geologist.
Geology of the Area:
The rocks throughout much of Ontario are part of a region known as
the Grenville Province. The rocks of the Grenville Province record
evidence of a huge, ancient mountain belt that formed on what was
then the margin of North America, about 1.2 to 1.0 billion years
ago. The Grenville Mountains would have been very high, similar to
the modern day Himalayas, and were formed similarly by plate
tectonic processes - by continents and pieces of continents
colliding together.
The Grenville Mountains are long eroded away, and the rocks exposed
in the Grenville Province are the deep roots of those mountains.
These rocks are high-grade metamorphic rocks, meaning they formed
deep in the Earth by very high heat & pressure. Many faults are
commonly found in mountain ranges, and also present throughout the
Grenville are intense zones of shearing, where large masses of
rocks were sliding past one another. One can think of these shear
zones as being like faults - only at this depth in the Earth, the
rocks along the "fault" don't break, they slowly flow by
recrystallization of the solid minerals. These shear zones are also
not small - they can be hundreds of meters wide and hundreds of
kilometers long. They also form boundary zones between major
packages of rocks. In this case, this shear zone marks the boundary
between the Central Metasedimentary Belt to the SE, and the Central
Gneiss Belt to the NW.
One of the structures sometimes found in high-grade shear zone
rocks is called fold transposition. Imagine a thin layer of
rock within a shear zone, where the rocks above the layer are
moving one way and the rocks below are moving the opposite. Our
thin layer, at times, could become folded, and start to bend into
an "S" or "Z" shape. Now imagine that shearing continues, and that
folded layer itself begins to get refolded. and again. and again.
and again.... What started out as a simple layer of rock gets
folded over an over upon itself as it gets rolled up inside the
middle of the shear zone like a really long spaghetti noodle
wrapped around a fork many times. That is fold transposition.
The "dragon" roadcut as it is known to geologists is one of the
most phenomenal examples of fold transposition known. It consists
of a thin layer of black rock called amphibolite, layered within
with a pink rock called a granitic gneiss. The black layer has been
folded over on itself many, many times, and the end result gives a
very serpent like appearance.
Additional Hints
(No hints available.)