Potholes in the Granite EarthCache
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This Earthcache will take you to a beautiful natural setting surrounded by the city of Richmond. A short hike is necessary, and some of the features described are wheelchair accessible on the north side of the island (look for the new concrete wheelchair ramp taking you out on the rock) 37° 31.758'N 77° 27.345'W
However, the best viewing is on the south side of the island, and is not wheelchair accessible.
What you are standing on is Petersburg Granite, formed in late Paleozoic period of the Earth's creation.
The Paleozoic Era (from the Greek palaios (παλαιός), "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic Eon, spanning from roughly 542 to 251 million years ago (ICS, 2004). It is the longest of the Phanerozoic eras, and is subdivided into six geologic periods; from oldest to youngest they are: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. (wikipedia)
That is some pretty hard rock, so what are all those holes in it? That is this earthcache lesson!
Since the dam was built to channel water to the power plant in the 1800's, the rocks on the south side of the island became exposed and you can see a large variety of the potholes.
They begin with the river dropping debris into cracks along the granite. As the debris wash around they produce a small pot hole that allows larger debris to drop in.
The strong current will roll the rocks and small boulders around and around causing a grinding away of the granite (and the boulder).
Some potholes are three to five feet deep. Look at the different shapes of the pothole and what is laying inside of them.
To prove you visited this area, answer the following questions.
1. What is the main color of the granite you are standing on.
2. The dark fissures in the rock tend to run in what compass direction?
3. List three other colors you see running through the rock.
4. How many years do you think it took the river to produce a 3 foot deep pothole?
5. Optional- take a photo of you and the potholes or rocks.
I hope you enjoyed your hike on the river!
Additional Hints
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