Petrology trip #3: Cockeysville Marble
We are now halfway through our documentation of the University of Maryland petrology field trip. As a reminder, we've already seen the Port Deposit Tonalite and the Setters Schist. Today, we meet the Cockeysville Marble.
The Cockeysville is famed in some quarters because of its role in the construction of the Washington Monument. The upper portion (most) of the monument is made of this rock, although it is a purer (higher CaCO3 content) marble than we see here at this outcrop near the Hunt Valley Shopping Mall. Really, this is more of a marble gneiss.
Rich and Roberta talk with the students about this new rock:
The Cockeysville Marble has a well-developed foliation at this outcrop. Impurities in the limestone protolith (probably clay) have metamorphosed into muscovite mica:
Whether these foliations reflect bedding is an open question in my mind. Here's a look at how the outcrop is weathering out. Lovely, just like a limestone in the way it's dissolving away:
It's reasonably coarse-grained:
Here's two close-ups of the stringers of muscovite mica:
Some structural geology was also apparent. Here for instance, is a fault/shear zone
...And here's a fold. It's an overturned fold; Note how the foliation dips at two different angles, though in the same direction:
Still can't see it? Okay, let me help:
The sharp-eyed among you probably noticed the boudinage on the left side of the fold. Here's a portrait of the most prominent boudin:
The most mica-rich domains acted relatively stiffly under deformation, while the calcite-rich domains flowed more easily.
I also found a surface decorated with slickenfibers (crystal fibers growing aligned in small spaces along the surface of a fault):
... and a close-up, so you can see that the opposite block of rock was moving from the bottom of the photo towards the top. Running your finger over this outcrop from bottom to top would feel relatively smooth, while running your finger over it from top to bottom would feel rough as your fingertip would catch on the little mineral "steps":
The Cockeysville is a lovely marble. But we needed lunch at the Twin Kiss drive-in. The day was advancing, and refueling became a serious issue. And then? On to Mineral Hill...
The Cockeysville is famed in some quarters because of its role in the construction of the Washington Monument. The upper portion (most) of the monument is made of this rock, although it is a purer (higher CaCO3 content) marble than we see here at this outcrop near the Hunt Valley Shopping Mall. Really, this is more of a marble gneiss.
Rich and Roberta talk with the students about this new rock:
The Cockeysville Marble has a well-developed foliation at this outcrop. Impurities in the limestone protolith (probably clay) have metamorphosed into muscovite mica:
Whether these foliations reflect bedding is an open question in my mind. Here's a look at how the outcrop is weathering out. Lovely, just like a limestone in the way it's dissolving away:
It's reasonably coarse-grained:
Here's two close-ups of the stringers of muscovite mica:
Some structural geology was also apparent. Here for instance, is a fault/shear zone
...And here's a fold. It's an overturned fold; Note how the foliation dips at two different angles, though in the same direction:
Still can't see it? Okay, let me help:
The sharp-eyed among you probably noticed the boudinage on the left side of the fold. Here's a portrait of the most prominent boudin:
The most mica-rich domains acted relatively stiffly under deformation, while the calcite-rich domains flowed more easily.
I also found a surface decorated with slickenfibers (crystal fibers growing aligned in small spaces along the surface of a fault):
... and a close-up, so you can see that the opposite block of rock was moving from the bottom of the photo towards the top. Running your finger over this outcrop from bottom to top would feel relatively smooth, while running your finger over it from top to bottom would feel rough as your fingertip would catch on the little mineral "steps":
The Cockeysville is a lovely marble. But we needed lunch at the Twin Kiss drive-in. The day was advancing, and refueling became a serious issue. And then? On to Mineral Hill...
Labels: marble, maryland, metamorphism, structure
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