Educational Information:
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.