Overthrust Fault
This location highlights the intersection of the Piedmont and Coastal Plain provinces that contribute to the geography of the District of Columbia. This area also forms the junction of three of the four distinctive landscapes that can be used to describe the District.
Geological Provinces
The Piedmont Plateau and Atlantic Coastal Plain provinces form the basis of the geography of the District of Columbia. As the name suggests, the Piedmont province, with a formation period that extends to the Precambrian Era, has links to mountainous regions in the current Blue Ridge Mountains and is composed primarily of metamorphic rocks. The Coastal Plain is composed primarily of sedimentary rocks and dates its formation to the Cretaceous Period of the Mesozoic Era.
DC Landscapes
The District of Columbia can be considered to have developed across four distinctive landscapes:
- The Original City - built on the remnants of the coastal plain, land that was still considered to be unstable when construction was initiated in the 18th and 19th centuries
- Georgetown - primarily built on the metamorphic layer of the Piedmont province
- The Coastal Plain Bluffs - these bluffs were carved into the Coastal Plain by river flows during the Cretaceous period.
- The Anacostia Terraces
At this location, you find the junction of the first three listed landscapes. The Anacostia Terraces are located in the southeast side of the District, across the Anacostia River from where you are now.
Overthrust Fault
A thrust fault is a location where a crack in the earth's crust has enabled a layer of older rocks to be pushed across a layer of younger rocks. When the displacement angle is low (often considered to be 15 degrees or less) and the displacement is significant (often in the kilometer or more range), then the thrust fault can be referenced as an overthrust fault.
The overthrust fault highlighted at this location is at the southwest junction of a fault line between the Georgetown (Rock Creek) and Coastal Plain areas, extending in a northern direction from posted coordinates. Walk from the sidewalk up to the fence and look down into the valley behind the fence to gain a perspective of how the faults have formed the local landscape.
Back at the sidewalk, enclosed at the base of a tree is a micro representation of the macro overthrust fault. You can observe examples of both the metamorphic and sedimentary rock that are part of the layers and learn more about their relationships. In addition, the topsoil layer is visible.
Logging Requirements
Either by messaging through the site or sending an e-mail to the cache owner, please submit your responses to the following questions. While you can log your find prior to receiving confirmation, the cache owner reserves the right to delete logs that are not supported by a message with good responses.
- At posted coordinates, identify the two layers that are highlighted prior to fault formation. Also, identify the layers that are involved in the overthrust fault.
- Observe the geological features located within the enclosure. How does what you see reflect the thrust fault in the area?
- From your observations at the posted coordinates, which geological province forms the overthrust layer? Is this layer the older or younger layer?
- Post a photo that readily indicates that you (and anyone else logging the find) are at the location. You do not have to show your face, but the photo should be personalized by you or a personal item. Your background should include a view of the major fault. (You will be above the sidewalk - BEHIND the tree and the enclosure - and near the long fence for this photo.) NOTE: Per newly published Earthcache guidelines, this requirement is REQUIRED to claim the find.
First to Find
Congratulations to BAMA1986! The FTF here is all his fault!
Permission
No permission is required for this public location.
References
- DC Rocks The Geology of the Washington, D.C. Region
- Virginia Division of Geology Rocks of Virginia