Perky Rocks EarthCache
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Hornfels Ridge
Visiting this EarthCache you will be learning about hornfels metamorphic rock. What is hornfels you ask? Hornfels is a fine-grained rock that can range from light to dark in color. The rock that was here originally was diabase, clay, shale, sandstone or limestone. This particular formation of rock you will be looking at will be the heat altered version of the original rock. At this location you will be visiting what is called a hornfels ridge. What makes this so interesting? You'll soon see when you learn how this ridge came to be.
First I am going to go over some terms that will be used to help in your understanding.
~ Metamorphic rock - Rocks that are formed from existing rocks that had been exposed to the pressure and extreme temperatures associated with a body of magma or techtonic plate collisions.
~ Contact metamorphism - The process of the existing rock properties being changed due to contact with heat or pressure.
~ Microplate - Island sized piece of the earth's crust.
Hornfels is created when there is contact metamorphism. Many, many moons ago there was an event where microplates collided causing tremendous heat and pressure that deformed rocks in North America's east coast. Magma forced it's way into the older rock beneath the earth's surface because of these plate collisions and in turn it caused the native rock that was already there to change (contact metamorphism). Over time the new rock cooled and solidified into what you see today. At some point in time the softer rock around the hornfels had been exposed to the elements causing it to erode away. This leaves behind the hornfels which has a harder composition due to the change in it's properties millions of years ago.
Fun Facts:
- Did you know that contact metamorphism often results in the formation of garnet and emery? Pretty cool huh?
- Contact metamorphism is also responsible for creating marble!
- Hornfels is a German word which means "hornstone". This comes from the association with glacial "horn peaks" in the Alps.
- Some of the rocks you see in Southeastern Pennsylvania may not have even been formed here. They may have been brought here from somewhere else on one of the microplates!
Your Mission
Your tasks for this EarthCache are as follows if you decide to accept this mission. Please e-mail me the answers through my profile. Do not post your answers here. Let others have the same fun getting their own answers. :-)
Finds without the required e-mail will be deleted.
- Touch the rock. What does it feel like and why do you think it's like that?
- Look closely at the rock. What does the grain of the rock look like? Is there a pattern to it?
- Looking at the whole rock formation what is the angle in degrees (make a good guess) that it slopes down to the waterway? (proof of your visit)
- What is your elevation? (proof of your visit)
- Take a picture of yourself if you would like. (Optional). Please don't show the angle of the rock formation though.
Parking can be found at any of the three listed parking areas. I would suggest parking at Foy Park and enjoying the stoll on the Perkiomen trail. The view from the bridge is really pretty. DO NOT park on Haim Rd. That is not an acceptable place to park.
Congrats to Oceanic815 for the FTF!!!
Additional Hints
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