Let's Build Something Together!
In order to keep this short and to the point, I am omitting the main events that led up to the formation of the rocks that would become the core of Rhode Island. For an excellent summary, check out the Out of Africa Earthcache established by Nerves in Jamestown.
The eastern margin of North America consists of a series of terranes (large bodies of rock distinctly different from neighboring bodies of rock) that have been added onto the core of the North American continent (known as Laurentia) for the past 470 million years. The primary bedrock that underlies Rhode Island is known as the Avalon Terrane. Geologists believe that this rock originated as a volcanic island chain or microcontinent that formed approximately 600 million years ago (MYA). Eventually, the Avalon Terrane was added onto the eastern edge of Laurentia.
Breaking Up is Hard to Do
After the Avalon Terrane was added to the eastern edge of Laurentia, rift basins began to open up, threatening to break these newly added pieces of bedrock from the continent. The rifting wasn't meant to be, however. The forces that created the rifting subsided, leaving a 1,000 square mile basin within the Avalon Terrane. This basin is known to geologists as the Narragansett Basin, and that is where the story for this Earthcache begins.
The Narragansett Basin is labeled as NB.
Welcome to the Jungle
Approximately 356 MYA, Earth was in the early stages of the Carboniferous Period. The majority of Earth's landmass was located in the southern hemisphere as the supercontinent Gondwana. The Narragansett Basin was located along the southern coast of a smaller continent known Euramerica, located near the equator.
Earth during the early Carboniferous Period.
In the millions of years after the failed rifting, the Narragansett Basin filled with sediment. This low-lying area became a vast complex of wetlands and marshes. Being close to the equator, the Narragansett Basin was a warm, lush tropical forest filled with life. The dominant vegetation included mosses, giant horsetail-like plants, and club moss. Amphibians were the major vertebrate land animals, and giant dragonfly-like insects buzzed through the skies.
How early Carboniferous forests may have appeared.
As the plants of this period died, they would fall and eventually be covered by the swamp sediments. The water-saturated sediment prevented the plants stems from decomposing quickly. This cycle of plant growth, death, and covering with sediment continued for millions of years, producing thick layers of minimally decomposed plant material, known as peat.
The Big Crunch
The tropical paradise wasn't going to last forever. By the late Carboniferous, all of the continents had collided, forming the super-continent Pangaea. Land that would become Rhode Island was now located deep in the interior of Pangaea, as Africa was colliding with the eastern edge of North America.
Earth during the later stages of the Carboniferous.
The massive pressure and heat from this collision turned the swamp sediment into rock over millions of years. In many places, the swamp sediments were so rich in peat that coal beds were formed. In fact, the Narragansett Basin is the largest coal deposit outside the big coal states of Pennsylvania and West Virginia.
Today
Rhode Island has a rich record of those tropical days of the Carboniferous. Besides numerous historic coal mines, one can easily find fossil evidence of the plant-life during this geologic period if you know where to look. Numerous sites within the Narragansett Basin have yielded plant fossils. A common fossil to find is that of a Calamite. Calamites are an extinct genus of plants similar to plants of the horesetail family. Unlike horsetails, Calamites were quite large, with some species growing as tall as 30 meters!
What a Calamite may have looked like. An example of a horsetail.
Your Assignment
You will need:
Tape measure
Magnifying lens (optional)
Welcome to Rock Island, a series of small bedrock outcrops connected by some salt marsh within the lower reaches of the Providence River. Your first stop is at the listed coordinates. Here you will find numerous Calamite fossils within the bedrock.
An example of a Calamite fossil.
Examine the multitude of fossils here. In addition, look closely at the rock surrounding the fossil. A magnifying glass may help, but is not necessary. Please note that the picture above is not from Rock Island. Don't waste time trying to find it!
Based upon the appearance/texture of the surrounding rock, what type of sediment do you think covered the plants after they had died?
After making these observations, you will return to the first island. The included waypoint will bring you to a remarkable Calamite specimen right beneath your feet.
Find this fossil specimen. Using a tape measure, measure the length and width of the Calamite fossil.
To log this Earthcache, please email your answers to the two bold questions above prior to logging.
Please check the tides before going to this Earthcache. High tides can make it very difficult to walk to the breakwater that leads to Rock Island. In addition, the marsh that connects the two islands may be submerged at high tide. Please be careful, as the rocks can be quite slippery, and keep a close eye on your children! There is abundant poison ivy on the islands, although you should not encounter any on the walk to either site.
After examining the fossils, you may want to poke around the island for a while in hopes of finding your own Calamite fossils. I found an awesome piece on my first visit, and I'm sure there are many more. Please post pictures of any fossils you may find!
Congrats to Nerves and RecipeForDisaster for FTF!