Stratigraphic layers in Shenandoah National Park can be classified into 6 geologic formations. The lowest layer contains the oldest rocks in the park which are 1.2 billion years old. These rocks are called the Basement Complex. They include the Pedlar and Old Rag Formations. The Basement Complex consists of metamorphosed granitic rocks and gneiss. Next is the Swift Run Formation which consists of metaconglomerate and metasandstone, followed by a thick volcanic layer, called the Catoctin Formation. The Catoctin consists of metabasalt (greenstone) which in places, exhibits spectacular columnar jointing. These formations are overlain by the Weverton Formation, another metaconglomerate and metasandstone; the Harpers Formation (formerly called the Hampton Formation), which consists of metasandstone and phyllite; and finally by the highest layer (youngest), the Antietam Formation (formerly called the Erwin Formation), which consists of metasandstone and quartzite. (Badger, 1999; Southworth, et.al., 2009)
At this overlook outcroppings from a volcanic flow of the Catoctin Formation are visible in the island area. Several features are visible in the metabasalt, including amygdules, zones of brown or purple oxidized metabasalt, striation grooves and pods of epidotes.
To log this cache:
Locate the following two features (at multiple places in the parking island area near these coordinates) and answer the following questions. Answers should be emailed to Techlines (cache owners) through their geocaching profile.
- As you look at the metabasalt rock outcropping from the paved area, the epidote pods are visible; they are oval or irregularly smooth shaped “spots” of quite an unusual color. These epidote pods were formed by cation exchange during metamorphism; calcium was transported by hot watery fluids from a portion of the basalt and deposited into pods at the same time that magnesium and sodium were transported from the emerging epidote pods into the surrounding basalt.
---a. What color are these unusual spots?
---b. How large are the largest of these spots? (estimate in centimeters or inches)
- At various places on the outcroppings at this location, grooved striations called slickenlines are visible. These parallel lines might be best viewed from the top of the outcropping, which can be accessed via several small paths. Beware of poison ivy growing on these rocks and the possibility of encountering snakes. These grooves were formed by the movement of one rock over another; during the formation of the Appalachian Mountains, folding, faulting and uplift of rocks provided the stresses to produce slickenlines
---a. In what direction do the slickenlines at this location run (north-south or east-west)?
---b. Why do you think all of the striations or slickenlines run in the same direction?
Other Educational Information:
“Geology along Skyline Drive. A Self-Guided Tour for Motorists” by Robert L. Badger and “Geologic Map of the Shenandoah National Park Region, Virginia” by Southworth, S. et.al., 2009 provided information for this cache. More information about epidotes, slickenlines and many other features along the Skyline Drive are available in these publications. The description of the formation of epidotes comes from Laboratory work by Jack Reed and Ben Morgan of the U.S. Geological Survey.