In Maryland, United States
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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.
Special thanks to Callan Bentley for his input and sources he provided during my research. http://vimeo.com/5642852
(No hints available.)
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
Last Updated: on 4/7/2013 10:36:11 AM Pacific Daylight Time (5:36 PM GMT)
Coordinates are in the WGS84 datum