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).
From this overlook, one can see huge talus slopes below dramatic
and rugged cliffs. These piles of eroded rock resulted from the
undermining of the Antietam Formation quartzite cliffs which are
harder and more resistant to erosion than the underlying
metasandstone and phyllites of the Harpers Formation which erode
more quickly. Much of the rock wall built at this overlook is
composed of quartzite from the Antietam Formation. Worthy of
special note are the white quartzite blocks that were once beach
sand deposited by the sea that covered this area after the eruption
of the Catoctin volcanics about 570 million years ago. What
makes some of these blocks especially interesting is the Skolithos
tubes that are fossilized vertical tubes resulting from burrowing
worms in the beach sand during that period. These holes quickly
filled with sand, but the filled burrows have a different texture
and color than the quartzite rock. These “trace
fossils” represent the earliest form of life found in
Shenandoah National Park. The worms lived at the edge of the
ancient Iapetus Ocean over 500 million years ago.
To log this cache:
Please locate one (of several near these coordinates) of the
quartzite blocks that display Skolithos tubes and answer the
following questions. Answers should be emailed to Techlines (the
cache owners) through our geocaching profile.
- 1. What color do the Skolithos “tubes” appear to
be?
- 2. How long are the longest of these tubes (estimate in cm or
inches)?
- 3. Talus slopes and cliffs containing the Antietam quartzite
are visible from this overlook; what direction(s) from the overlook
are these slopes?
- 4. The rock cliff at the roadcut on the opposite side of the
drive at this location contains sandstone that is from an older
formation, the Harpers Formation, and is quite a different color
than the quartzite of the Antietam Formation. What colors of rock
are predominant in the roadcut cliff?
- 5. What natural weathering process causes these rocks to change
color?”
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 this location and Skolithos tubes can be found in these
publications. More information about trace fossils can be found
here: http://en.wikipedia.org/wiki/Trace_fossil