This is an Earthcache – as such, there is no physical cache. Instead after examining the rock at the posted coordinates, you will answer 6 questions and send me the answers. This earthcache is placed in in Black Hill Regional Park with permission from the Maryland-National Capital Park and Planning Commission. The park grounds are open daily from dawn to dusk.

Here at the posted coordinates, you will find a curious rock. Located near the entrance of the park this stone stands guard. There are parking spots along the center of the road nearby, so pull over and check out this lone sentinel.

Upon inspection you can easily tell that this rock is layered, as if you were looking at the side of a stack of paper. You may come to the conclusion that this is a sedimentary rock, and maybe that was once true. This rock did start out as silt and clay. Then after years of being buried it changed into shale, a sedimentary rock.

Thanks to the Rock Cycle this rock didn’t stop changing there. You see, rocks are in a constant state of transformation. This rock went through another change effected by pressure, heat, and water that resulted in a more compact and more highly crystalline condition. So this rock went from silt, to shale, to slate, and then to Phyllite.

It became a Metamorphic rock. These rocks start out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, and some combination of these factors.

This rock went through what is called Barrovian metamorphism, which is the most commonly encountered metamorphism in geology. It occurs in intense tectonic conditions associated with Major Mountain Building and Volcanic Arcs. Look at the diagram below. Looking from left to right represents an increase in temperature, and from top to bottom corresponds with an increase in pressure. The silt/clay settling on bottom of an ocean or lake bed eventually gets buried, and under pressure and time forms a siltstone. As more geological forces push this layer deeper below the surface of the earth, both temperature and pressure increase on the rock.

In a Barrovian Sequence, a shale parent rock undergoing a full sequence of Barrovian metamorphism produces the following rocks, in order: slate, phyllite, schist, and gneiss.

A 1968 map from the Maryland Geological Survey has Black Hill Regional Park being located within the Ijamsville Phyllite formation. This formation includes many rocks that were originally sedimentary, but then over time, were altered by pressure and heat. Some of the specific rock types identified in that 1968 survey include blue, green, or purple phyllite, as well as phyletic slate. These rocks are often interbedded metasiltstone and metagraywacke.

LOGGING REQUIREMENTS:

To log this Earthcache: Read the geology lesson above. Answer all five questions posted below and send them via e-mail or messenger contacts on my Geocaching profile.

QUESTION 1. What is the most commonly encountered metamorphism in geology?

QUESTION 2. If this phyllite would have continued to metamorphise, what would have been it's next change in the Barrovian Sequence?
A) Slate
B) Gneiss
C) Schist

QUESTION 3. Would you say that the rock demonstrates conformity (do the layers appear to be laid in the same consistent direction)?

QUESTION 4. Does the layers of this rock appear to be stable, or does it appear crumbly and fragile?

QUESTION 5. Of the specific rock types identified in that 1968 survey, observe the color of this phyllite. Would you identify it as being blue, green, or purple?

QUESTION 6. Does the rock across the street (see top photo) also appear to be phyllite?

OPTIONAL PHOTO: Posting a photo that readily indicates that you (and anyone else logging the find) are at the location.

Awesnap has earned GSA's highest level:

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