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