 DC Rocks
A cache by ZeWrestler Hidden: 11/3/2009
Size:  (Not chosen) Difficulty:  Terrain:  (1 is easiest, 5 is hardest)
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Earth cache in Burnt Mills Park East. Take a hike down this part of
the Northwest Branch Trail.
People look at the Washington DC area and they think politics,
Smithsonian, Washington Monument and a plethora of history from
1790 when the city was founded. What people tend not to think about
is the history before that time. I’m not talking about the
colonial period of the United States or the time of the Native
Americans. I’m talking about the geological history that has
shaped land around the capital.
This Earthcache takes you to Burnt Mills Park East, part of the
Northwest Branch Trail. If you go to the specified coordinates you
will find one of DC’s little secrets. A small valley/gorge
cut by a river that feeds into the Anacostia River. At this part of
the valley you will see an outcrop of rocks rich in geological
history.
At this site you will see some very old rock. The outcrop itself is
primarily meta-graywacke. Graywacke is a variety of sandstone that
generally identified by its dark color and poorly-sorted, angular
grains of minerals including quartz and feldspar. Graywacke forms
in underwater avalanches called turbidity currents. This produces a
gradient effect when the rock hardens because of the way the sand
and the mud settle at different rates: the bigger sand grains
settle out of suspension first, followed later by smaller-sized
flakes of mud. The first deposits of graywacke formed on the bottom
of an ocean that predated the Atlantic Ocean, called the Iapetus
Ocean. The Iapetus Ocean was between the continents Laurentia,
Baltica and Avalonia about 600 and 400 million years ago.
Through plate tectonics, the Iapetus Ocean disappeared as the
plates converged together at a subduction zone formed during a
period known Taconic orogeny. The oceanic crust sank below the
North American continent and in the process some of the upper rock
was scraped off and formed an accretionary wedge, a jumbled pile of
oceanic sediments. The graywacke in this outcrop is most likely
from that accretionary wedge. The accretionary wedge also put the
rocks under a large amount of heat and pressure. Metamorphic
reorganization of the rock resulted, changing the graywacke into
meta-graywacke. The Taconic orogeny can be dated to the late
Ordovician period of geologic time, about 460 million years
ago.
We can see the evidence in the rocks through the bending of lines
of metamorphic foliation in some of the metamorphic rocks within
the park, such as the following image.
In this rock we see the gneissic banding, which bent due to the
heat and pressure that was applied to the rock from the subduction
or the accretionary wedge. Gneissic banding is formed due to the
restructuring the original rock’s chemical components into
layers that are mineral concentrated.
A well-known geologic structure that is found in many areas of
igneous activity are dikes. A dike is an intrusion that cuts across
pre-existing layers of rock. Dikes are principally intrusive in
origin. An intrusive dike is an igneous body that has a thickness
that is usually much smaller than the other dimensions. Because of
this dikes are younger than their host rocks.
Dikes can form at any angle, but tectonic deformation may later
rotate the dikes so that they may appear at other angles. The dike
in this picture is a small section of a large fracture that
propagated upward as it filled with magma.
As time flies other tectonic activity occurred in the world and
around the Washington DC area. But the evidence of that activity
has been lost to weathering of the rocks. This lack of recorded
activity is called an unconformity. Instead, we see a bare eroded
surface. This erosional plane is topped with river gravel and newer
sedimentary deposits that are formed like conglomerate rocks. These
rocks are formed much later: during the Cretaceous period, which is
about 360 million years after the Taconic orogeny. Conglomerates
are sedimentary rocks consisting of rounded fragments that are
cemented together over time, but have not received the heat and
pressure necessary to transform the sediments into solid
rock.
This rock has since changed its shape due to time and the effects
of the river. This is particular noticeable in the form of
potholes. Potholes are formed when a river causes a vortex of water
that erodes a hole into the rock. The abrasion is mainly caused by
the circular motion of the small sediments that are carried by the
water. These potholes indicate that where this outcrop is was once
covered by the river’s surface. In the image above we see
various examples of potholes showing that they come in all shapes
and sizes. Here we see five different potholes in the same
location.
Now that I have given you a short history about the area, it is
time for you to explore the outcrop and claim this EarthCache. To
log this cache, you must do the following:
1) Find a dike within this area and take a picture of it and your
GPS. E-mail the coordinates that this dike is located and post the
image.
2) Go to the coordinates N39 01.746 W077 00.163. There will see a
large rock surface with potholes in it. (Including the potholes in
the picture above) How many Potholes would you say are here.
(I’m lenient on the answer because some people may
characterize some portions as potholes and others may not.)
3) Go to coordinates N39 01.745 W077 00.144 and take a picture of
you with your GPS or just your GPS with the scenery in the
background of requirement 2.
Note: I will delete logs if all of the requirements are not
met.
Congrats to Serrabou for the FTF. Sources
Special thanks to Callan Bentley for his input and sources he
provided during my research. http://vimeo.com/5642852
http://www.nvcc.edu/home/cbentley/geoblog/2008/08/geological-excursion-in-silver-spring.html
http://www.see.leeds.ac.uk/structure/basic_structure/primary/index.htm
http://en.wikipedia.org/wiki/Accretionary_wedge
http://en.wikipedia.org/wiki/Conglomerate_%28geology%29
http://en.wikipedia.org/wiki/Dike_%28geology%29
http://en.wikipedia.org/wiki/Giants_kettle
http://en.wikipedia.org/wiki/Greywacke
http://en.wikipedia.org/wiki/Iapetus_Ocean
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1 user(s) watching this cache.
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Additional Hints (No hints available.)
(Decrypted Hints)
Find...
A Dike found in the outcrop of rocks
Metamorphic Foliation of Rocks in the Park
Several potholes within a large rock
View of the Outcrop
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Warning. Spoilers may be included in the descriptions or links.
Cache find counts are based on the last time the page generated.
 January 24 by cphug184 (2181 found)
What a wonderful spot. My daughter would have liked climbing all over these rocks! I had seen potholes before on the Mather Gorge Earthcache by flyingmoose so recognized them right away. Dikes?...not so much. Hope I got it right. Saw a women right after I took the picture downstream. Due to the terrain, she was right upon me before we saw each other. It seemed I scared the heck out of her. Ooops! Thanks for bringing me to this terrifc place. I appreciated the research.
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January 19 by Scarecrowe (119 found)
This was a great location and I'm hoping that when the weather warms up a lot more people will go explore this one (I saw that it was only published in November so the snow has been keeping traffic to this one down). The information provided in the cache detail itself was a lot of fun, pretty much creating an interactive tour as you walk down the trail. I definitely suggest this one to any cacher that enjoys the hiking types of caches more than the urban.I might rate the terrain higher than a 3 since I had to do a lot of climbing across the rocks themselves as there was rarely a trail once I got more than 100 yards or so in. Answers to the questions on the way via e-mail.
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December 2, 2009 by bondoon (387 found)
Kind of scary climbing on those rocks by myself, but I did it! TFTC!
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November 15, 2009 by WV Muddy Boots (5909 found)
What a fun earthcache! We loved climbing over the rocks and walking along the trail.
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November 15, 2009 by CacheDetectives (7309 found)
What a fun earthcache! WV Muddy Boots and I had a great time enjoying the wonderful weather walking along this trail. We passed the serrabou family coming out but didn't recognize them until they had passed us. We were simply amazed at the grace exhibited by the Mrs. as she climbed over all of those rocks with a baby. While we were there we were joined by jfigel and CRW in our search for a dike. We really enjoyed visiting this location and climbing all over the rocks. TFTC!
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Current Time: 2/10/2010 12:19:43 AM (GMT-08:00) Pacific Time (US & Canada) (8:19 AM GMT)
Last Updated: 1/24/2010 3:17:00 PM (GMT-08:00) Pacific Time (US & Canada) (11:17 PM GMT)
Rendered: From Database
Coordinates are in the WGS84 datum
