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Posts Tagged ‘Washington DC’

Being both a bicycle and monument enthusiast, recently I have been combining the two and taking a self-guided bike tour of my hometown, Washington, DC. Just for kicks I am putting together a personal collection of pictures of my bike with the monuments. Though I see the monuments often I still enjoy them and appreciate getting to share them with people for the first time. Yesterday I made my way over to a few memorials including the FDR Memorial along the Tidal Basin. I remember when this particular one opened up and enjoying it because it is a collection of waterfalls and statues, and that it brought a different approach than the other monuments in DC. While there, a few of  the rocks found in the walls jumped out at me; in particular this pegmatite dike.

*Bike as a sense of scale borrowed from Alan at Not Necessarily Geology

While the surrounding parent rock is pretty evenly distributed between minerals this dike is visibly dominated by feldspars, most notably the bright pink orthoclase. The other mineral looks to me like plagioclase with a green tint to it. I felt it might be frowned upon to scratch the memorial, but the two cleavage planes are apparent. Below a crystal of plagioclase has fractured and infilled with quartz.

Unfortunately I don’t know the locality where this rock was taken from, but I can appreciate the unique aspect and added beauty it brings to the memorial. Keep an eye out the next time you are visiting the monuments maybe there are some other hidden geologic gems to be found, and if you want a tour send me a message I am always willing to take another ride around the city.

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Yesterday after watching the greatest soccer match I have ever seen, Alan and I made way over to the Smithsonian’s Natural History Museum, specifically the geology exhibit. No bites on the private tour offers extended, but we had fun anyways. I wanted to share some of the pictures we saw there (some pretty extraordinary stuff):

Look at that psuedotachylite! I like to think of this as impact-breccia, as it formed during the impact of a meteorite. The pink rock is the original granite that broke off into melted rock (the black part) which melted from fracture friction.

Pretty sweet looking example of the Leesburg conglomerate. We have come across this formation during our excursions, but never got this great look of the interior all nicely polished. How much is it for a rock saw? It may be time to invest in one*.

*Vegas has the odds on me losing a finger at 4:1 (I take that as a compliment)

Here is Alan next to a cool display of columnar jointing. Gotta love those hexagonal shapes with their eye-catching 120 degree angles. Don’t look too long, buddy. Those are underage (~12 million years old).

I saved my favorite for last:

Holy garnite! This is the point where Alan and I jumped to the floor and pressed our faces against this rock (good thing there is a “Please Touch” sign), and some passer by quipped “You guys must be geologists”. Don’t be so snide, guy, we know you’re excited too. If you remember my recent posts on porphyroblasts, these ones dwarf my examples, about the size of a softball or grapefruit.

I highly suggest everyone go and check out the exhibit for themselves. There are also some great Burgess Shale fossils on the first floor, too.

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A few years ago I took a Geology Field Studies class through NVCC. One of the trips we took led us to a creek in Maryland just off the Capital Beltway. I believe it was Henson Creek, but it has been awhile and the memory is a bit foggy. There was no path so we rolled up our pant legs and tromped through the creek, where the glauconitic quartz sandstone, Aquia Greenstone Formation, is exposed nicely. It wasn’t until we had walked in about 150 meters that it was brought to our attention we were walking all over these guys:

That is Cucullaea Gigantea, a now extinct bivalve from the Paleocene ranging in age from 58.7 – 55.8 million years old. These guys were infaunal suspension feeders, meaning they burrowed into the seafloor and strained their food from the water. By now the shell had completely eroded away, but what was left is a perfect mold of the interior composed of the greenstone. When this creature died a small opening was left so that sediments could fill in and later harden into rock. As time passed the more easily weatherable shell went away and all that remained was the more durable greenstone. C. Gigantea is about twice the size of its relatives such as Cucullaea recendens averaging about 8 – 14 cm in length, while C. Recendens averages 5 – 8 cm. Finding marine fossils in a Maryland creek bed also shows that it was once a marine environment. Along with a plethora of C. Gigantea fossils I also found some Ostrea compressirosta fossils, which I will talk about in a future post.

So, if you ever feel like going fossil hunting in the Washington DC area, Henson Creek in MD is bound to get you a handful of these guys

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