Welcome to Fort Jefferson
Even before you enter the fort you can tell what a massive undertaking it was to build it. The amount of materials needed, transportation, the man hours, the cost – it is incredible to think about what went in to building this fort.
One piece of the fort that you may not think about, is the floor you walk on as you enter the fort. What type of rock is it and why was it used?
The stone you are walking on is greywacke (pronounced gray-wacky). It looks like slate, but is actually a type of sandstone. This type of rock contains angular grains in a fine, clay matrix. It typically appears gray, brown, yellow or black in color. This type of sandstone contains fewer grains made of quartz and more made of feldspars, volcanic rock fragments, as well as silt and clay than most sandstone making it a “dirty sandstone.”
Let's talk about why it was used. Greywacke is found in many places, notably in the United States, United Kingdom and New Zealand. The greywacke used for Fort Jefferson was quarried in Pennsylvania and New York. Greywacke was used because it is an incredibly durable material.
When you’re building a fort in the middle of the Gulf of Mexico and you’re going to have heavy cannons positioned all the way around the fort, you want a rock that can withstand the weight of those cannons. Since greywacke is so hardy, it was a great choice to use as the floor of Fort Jefferson. Today, greywacke is used as aggregate in road building, as armor rock for sea walls, and for flooring because of its durability and abundance.
What makes greywacke so durable? It is durable because of how it came to be and what it is made out of. Again, it is a type of sandstone. Sandstone is a sedimentary rock. Meaning that when sand-size grains eroded from the Acadian Mountains 380 million years ago, they settled in the Catskill Delta and over time built up. As more layers are added, the pressure to the earlier layers increases and they are compacted. Over millions of years this adds up and eventually greywacke is formed. It is not just the formation process that makes it durable, but also the contents of greywacke. Greywacke is made up of a lot of different minerals (Augite, Calcite, Clay, Clay Minerals, Feldspar, Quartz and many more) and compounds (Aluminium Oxide, Iron(III) Oxide, Magnesium Carbonate, Phosphorus Pentoxide, Titanium Dioxide and many more).
In order to log this Earthcache...
Please read the following questions and send me your answers.
- Run your fingers along the stone floor. What does it feel like? Is it rough, smooth, gritty, flat? How has it weathered?
- How was the stone used? Is the floor one large piece or a few large pieces, or is the floor several smaller pieces placed together?
- As you walk through the entrance into the interior of the fort there is an object to your left (southwest) on the ground. What is it? What do you think it was used for?
- Make your way to either the northeast (waypoint 4A) OR southwest (waypoint 4B) along the fort wall and enter one of the casements. Is the stone cracked or does it appear in good condition? Can you tell where the cannons were positioned in the casement? How does the stone appear around those areas?
- Was greywacke a good stone choice for the floor of Fort Jefferson? Explain.
- Post a picture of you at the fort. Or perhaps your favorite view from one of the casements.
Sources
Dry Tortugas National Park Geologic Resources Inventory Report, Revised April 2014
https://en.wikipedia.org/wiki/Greywacke
https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/greywacke.html
https://www.nps.gov/goga/learn/education/graywacke-sandstone-faq.htm
https://teara.govt.nz/en/rock-and-mineral-names/page-1
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