EarthCache

Here There Be Dragons

A cache by Narnian Rockhound Send Message to Owner Message this owner
Hidden : 10/19/2010
In Ontario, Canada
Difficulty:
1.5 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

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.


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Last Updated: on 7/23/2017 8:13:47 PM Pacific Daylight Time (3:13 AM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum