When one moves into a new area, you need to learn the surroundings, its people and infrastructure. The same goes when a geologist picks a new area to take a group trip for geologic education. Such was the case when Renfrew Institute of Waynesboro, PA selected Front Royal, Virginia area for their annual geology field trip in May. Planning actually started in the fall of 2014 to search out possible sites. The group has always enjoyed fossil collecting. After using several references of fossil sites in the area and using the knowledge of my close friend, Dick Copper, a fossil wizard, we traveled to various fossil sites in West Virginia and Virginia to find that special site. After 3 trips, we still had no site that would produce fossils for about 40 people. Now what should we do?
With some luck we stumbled onto a shale pit near Martinsburg, West Virginia, but it was raining too hard to check out the rocks plus we did not have permission to access the pit. Not to be filling empty, I put my resources to work and by going through the County Planning Commission in the West Virginia, we tracked down the owner, contacted them and gained permission to check the site out. And yes, the shale had fossils! Enough for 40 hearty collectors? Well, I wasn’t sure but I do know that 80 eyes are better than 2 sets of eyes. I used the one visit to this shale pit in Berkley County for an opportunity for a young lad from Mercersburg, PA area that wanted to shadow a geologist for several hours. The student joined me, had the opportunity while driving the Interstate 81 corridor to talk and then show him how a geologist maps a shale pit to gain the understanding of the rock and its fossils.
As a game plan started to develop for the May field trip, I decided to include several stops on the northern part of the Skyline Drive. How could you go wrong with that, several nice overlooks to the east and west and a look at some of the oldest volcanic rocks on the East Coast. Even the time for lunch worked right in for a stop at a visitors center on the Skyline Drive. With my preparation work, I learned on a niffy book “Geologic Guide to the Skyline Drive” written by Robert Badger. What a great resource this publication provided. The book is sold by Shenandoah National Park or sold at one of their Visitors Center. I had several email conversations with Dr. Badger before I could purchase the book on his recommendations for trip stops on the Skyline Drive.
Another story about planning this trip is related to a “now abandoned” roadcut of a colorful rock known as unakite (an igneous rock composed of epidote (green) and orthoclase (pink). Not only is the rock pretty but is one of the oldest rocks on the East Coast, being dated as 1.1 billion years old (bya). How could you host a field trip into an area where a 1.1 bya rock occurs and not collect a sample? I read about the location in a geologic report of the Front Royal area written in the 1960’s. The authors talked about this unakite rock cut, but upon us visiting the area, we learned that a new road was built later and the “published” roadcut was now in the woods (although the old bridge still existed). Ok, so we found some unakite and now to find the property owner. That took another later visit when I was just about ready to give up on that site and thought I would ask some neighbors who owns the property. I approached a kind gentleman at his mailbox and asked who owned the property down the road on the right. His response was “That is me.” I believe my eyes about popped out of my head and I felt like a superpower had guided me here at this exact moment. I had a nice talk with this German-raised citizen and he had lots of questions about geology. He allowed us access to the property, lined up parking for our coach bus and even met us there on the day of the trip. I managed to collect him a fairly large unakite sample which I placed in his vehicle. He ended our conversation by wanting a group shot of us by the bus, which he took. I believe we thought we were celebrities in a parade!!
So with all of the planning complete and writing of the guidebook for the trip, my wife and I boarded the coach bus in Hanover, PA, rode to Waynesboro and picked up our participants. The trip turned out to be wonderful and actually the weather was quite pleasant. O yes, our fossil site near Martinsburg. It turned out as I hoped, 80 eyes found more than our 4 did and nearly everyone ended the day with some nice shells, crinoids and bryozoan samples.
Below I included several of the stops that might be on interest. Stop 1 is located just south of Front Royal along Rte. 522 for the volcanic rock known as metabasalt. Watch for police here as they sometimes will ask you to move on although the shoulder along the road is quite wide. Stop 2 is the unakite site; Stop 3 is the first major overlook on the Skyline Drive with an interesting rock cut; Stop 4 is the famous metabasalt columnar jointing roadcut and Stop 5 is a classic limestone cut with fossils south of Strasburg, VA.
STOP 1. VA ROUTE 522 CATOCTIN METABASALT
Keywords: metabaslt, metamorphism, chrysotile, epidote, joints, vesicles
Ready to go find some geology? Our first stop involves a rock that once was lava approximately 540-600 mya during the Proterozoic and Early Cambrian periods. Some of the earliest research on the dating of this rock had its age up to 820 mya but revisions of the dating techniques, overlying stratigraphy and fossils has refined the dates. The rock is now called a “meta” basalt since it was affected by heat and pressure (metamorphism) at least one time and probably twice in its history. The rock is dark-green in color due to its mineral content. Minerals such as chlorite, actinolite and epidote are present. Larger pods of epidote can occasionally be seen in this roadcut. Also veinlets and pockets of whitish chrysotile (fibrous serpentine) is frequently seen, The rock weathers to a greenish-brown color. The vertical cracks seen in the bedrock are known as joints, a structure formed as a result of movement after the lava became lava, during a mountain-building episode (known as an orogeny). You will notice how dense the rock is when struck with a hammer. Be careful of flying pieces or how close you are to someone when striking the rock. Often it is better to search out a piece of metabasalt that has already been displaced from the roadcut to crack. Make sure when collecting a sample, you get a piece exhibiting the dark green fresh color. You will also encounter in this roadcut some small quartz veins cutting through the metabasalt. Also look for small cavities in the rock. These are known as vesicles and were formed as gas escaped from the rock leaving a cavity and never filled in with mineralization. The thickness of the Catoctin Formation is 1,500-2,500 feet in this area (Rader and Biggs, 1976).
If the Catoctin Formation metabasalts sound familiar, we have visited this rock closer to home on past fieldtrips. Since the metabasalts are found only in the Blue Ridge Province, South Mountain in Pennsylvania contains a rock that looks fairly identical to the rocks here in Front Royal. Locations in Pennsylvania where we have visited this rock include Jacks Mountain tunnel, The PennDot shed on Pa. Rte. 16 near Carroll Valley and at the Specialty Granules quarry in Charmain.
Also, interestingly, its twin brother rock known as rhyolite which is found in the northern part of the Blue Ridge Province is absent in this area. Matter of fact, rhyolite is seen as far south as near Frederick, Maryland in the Blue Ridge but not any further south.
The metabasalt represents a period of rifting associated with the early opening of the Iapetus Ocean. Approximately 600 mya ago, a supercontinent known as Rodinia was breaking apart. The basalt formed as a result of oceanic rifting similar to what is occurring along the Mid-Atlantic Ridge in the Atlantic Ocean. The rhyolite reflects a period of rifting related to the continental crust being torn apart, but as indicated above, this rock is not seen in this part of the Blue Ridge province. You will see several classic outcrops of the metabasalt later at Stops 3 and 4.
STOP 2. VIRGINIA RTE 649 BASEMENT ROCK EXPOSURE
Keywods: Unakite, orthoclase feldspar, , metasomatism, Rodinia, Middle Proterozoic
So you want more excitement? We move to this more remote area at the base of the Blue Ridge Mountains to allow you to collect a rather rare rock, or at least in the Pennsylvania-Maryland area it is rare. Known as basement rock, this is the oldest rock known within the Blue Ridge Mountains and one of the oldest rocks known on the East Coast. The rock varies in composition from location to location, but is generally classified as a granite, grandodiorite, granitic gneiss or in this case for Stop 2 known as unakite. Along an abandoned road, examples of unakite can be found. Don’t expect to find much outcrop of the rock as weathering has taken its toll on the unakite. The best collecting is near or in the small stream at the concrete bridge or in the bank to the east of the bridge. Also, if you are careful, walk further north on Browntown Road pass the dirt driveway on the right to a small embankment on the right. There are some pieces of the unakite exposed here.
If you are going to collect along the abandoned roadbed in the woods, take notice to the terrain here. Look at the hillside as it drops in elevation from right to left. You can see two distinct drop-offs here. The higher portion to the right (or in the direction of where the bus is parked) is actually underlain by the Catoctin metabasalt (similar to what you saw at Stop 1). It is the lower portion of the elevation that contains the unakite (Raqder and Biggs, 1975). Weathering has taken a toll of the rock and you will have to break some pieces to find a fresh surface.
So what is unakite? First discovered in the United States in the Unakas mountains of North Carolina from which it gets its name, unakite is an altered granite composed of pink orthoclase feldspar, green epidote, and generally colorless quartz. It exists in various shades of green and pink and is usually mottled in appearance. A good quality unakite is considered a semiprecious stone; it will take a good polish and is often used in jewelry as beads or cabochons and other lapidary work such as eggs, spheres and animal carvings. It is also referred to as epidotized or epidote granite. In some of the Blue Ridge occurrences, an epidotized augen gneiss is present exhibiting foliation structures (http://en.wikipedia.org/wiki/Unakite).
Granite is an intrusive igneous rock. The rock formed from magma buried deep within the Earth. The magma never reached the surface, solidifying into a rock probably 4-5 miles deep. The fact that the crystals in the rock can be seen with your eye tells us that the rock cooled slowly, such as a granite. You might remember that the metabasalt contains small microscopic crystals indicating that the rock cooled faster compared to the unakite. Later chemical alteration of plagioclase feldspar by heated water or other solutions created the epidote (metasomatism).
Famous unakite sites exist further south in Augusta and Roanoke counties, Virginia have yielded fine lapidary specimens and can be seen for specimens for sale at area mineral shows or swaps. A site near Vesuvius can still be collected and yields some fine colorful specimens.
What also makes this rock important to collect is its age. The unakite is believed to be 1.1 bya and represents an ancient mountain range formed as a result of a continental collision (such as today’s example of the Himalayan Mountain). This mountain range extended from Texas to Newfoundland. Some geologists believe the mountain range extended into Mexico (Badger, 2012). This rock was the crust on a part of Rodinia. Several larger exposures of the basement are visible along the Skyline Drive between miles 21-33 (Davis and others, 1958; Badger, 2012).
This basement rock is assigned to the Pedlar Formation which is assigned a Middle Proterozoic age on the geologic time scale. The Pedlar Formation appears to be diminishing out of the more recent works describing the older rocks and only being referred to as the “basement” rock.
STOP 3. SIGNAL KNOB OVERLOOK – SKYLINE DRIVE MILE 5.3
Keywords: Massanutten Mountain, Signal Knob, erosion, sandstone, conglomeratic sandstone
Welcome to the Skyline Drive, a 109-mile long stretch of roadway that runs on the top of the Blue Ridge Mountains from Front Royal to Waynesboro, Virginia. Along the way, many exposures of sedimentary, igneous and metamorphic rocks are found, which tell the story of the evolution of this famous mountain range, at least in Virginia. Should you want to learn more about the geology of the Skyline Drive, check out a publication “Geology along the Skyline Drive – A Self-Guided Tour for Motorists” written by Robert Badger (2012). The book is for sale in the gift shop at the Dickey Ridge Visitor Center (lunch stop). If you enjoy hiking and are in shape, there is an excellent 7.1-mile round trip hike on Old Rag Mountain north of Syria within the Shenandoah National Park. “A Hiker’s Guide to the Geology of Old Rag Mountain” written by Paul Hackley (2006) is also available at the gift shop. Ok, now that the commercial break is over let’s go and see some geology.
As a reminder, while on the Skyline Drive, we need to keep all hammers inside the bus. Collecting of samples are not permitted within Shenandoah National Park. In a way, that is good as the two exposures we picked out for this trip are good outcrops to look at and photograph but not for rock collecting. Anyway, you already have a sample of this rock in your possession.
Before crossing the road to examine the exposure, let us take a look at the overlook. It is hoped while writing this guidebook that Mother Nature is on our side and presents us with a great day to see the landscapes. Our elevation here is 2,085 feet above sea level (asl). From here, the next prominent ridge to the west about 8 miles away is Massanutten Mountain, located within the George Washington National Forest. The mountain bisects the Shenandoah Valley just east of Strasburg in Shenandoah County in the north, to its highest peak east of Harrisonburg in Rockingham County in the south. The geology of the Massanutten Mountains is dominated by Silurian Massanutten Sandstone, a lateral equivalent of the Tuscarora Formation in the Appalachian Mountains to the west, overlying the Ordovician Martinsburg shale. Erosion of the Martinsburg shale in some areas of the mountain caused the sandstone to break and slide to form talus slopes. The Massanutten Sandstone is folded in a synclinorium, and it outcrops at the ridge tops (http://en.wikipedia.org/wiki/Massanutten_Mountain).
At the north end of Massanutten Mountain is Signal Knob, a location of Confederate lookout and signaling post during the Civil War.
The valley between us and Massanutten Mountain and the large valley west of Massanutten Mountain belong to the Great Valley Section of the Ridge and Valley Province. This area is underlain with Early Paleozoic-aged limestone, sandstone and shale. These rocks are softer in composition, allowing weathering and erosion to be accelerated compared to the harder rocks on the ridges. To the west, further out, is the first ridge of the Appalachian Mountain Section, another member of the Ridge and Valley Province. From here the ridge is 22 miles in distance.
Let’s cross the road to observe the bedrock!. Be careful crossing the road and watch for traffic while visiting the site. This stop is dedicated to our tour participants. Up to this point you have learned about metabasalt and unakite and some other interesting tidbits of geology. Here is your time to shine. Take a look at the exposure and try to theorize what you are seeing. We love to hear theories, as that is what makes science so interesting. It is not unusual to have 10 geologists looking at the same outcrop and you get 10 different stories. We will allow you to investigate and then share your ideas. There may be prizes involved here.
PLEASE DO NOT READ ANY FURTHER UNTIL OUR DISCUSSION IS HELD.
So here is the dirt on this stop’s rocks. Um, there is a hint, I used the word “rocks,” meaning more than one rock type. When you walked over to the exposure and stepped onto rock, you are standing on the top of a metabasaltic flow (Catoctin). Notice at about knee level, there is a different rock appearing. This rock has a gritty feel to it and looks different when weathered. This is a shaly sandstone and notice as you look upwards through the rock, the rock contains increasingly larger rounded rock fragments. At the top of the layer, about 15-25 inches thick, there are fragments of quartzite (metamorphosed sandstone). This is called a conglomeratic sandstone. In which part of this layer of sedimentary rock was a stronger water current involved? This part of the exposure tells us that following a period of volcanism, a period where a stream flowed on top of the basaltic lava occurred, probably lasting for several hundred to perhaps several thousand years carrying the sediment.
Above the sedimentary layer, we have another metabasaltic flow (Catoctin). This layer is several tens of feet thick and would have covered several square miles of area. Within both volcanic flows look for the cavities in the rock known as vesicles (Badger, 2012). These were gas pockets that broke leaving a cavity and not yet filled in with any minerals. You saw a similar structure at Stop 1.
STOP 4. INDIAN RUN OVERLOOK – SKYLINE DRIVE MILE 10.7
Keywords: Piedmont, columnar jointing
Moving to our southern-most point on this trip along the Skyline Drive, we visit an overlook that allows you to see to the east. If weather permits, you can look southeast and east to see out past the Blue Ridge Province which is the Piedmont physiographic province. Looking east-southeast at a distance of about 23 miles is Warrenton, Virginia. The Warrenton area is underlain by sedimentary and igneous rocks belonging to the Mesozoic Era. The valley in front of you is the location of Va. Rte. 522. The white water tower in the valley to the southeast is at Flnit Hill, a distance of about 5.5 miles.
We are going to walk north from this overlook a short distance and examine the rock on the west side of the road. We brought you here because this is another (yes, another) exposure of the Catoctin Formation metabasalt. But there is a special reason to visit this site!! At this locality you can see the best example of columnar jointing found on the Skyline Drive. Yes, there are several other nicer examples of columnar jointing in Shenandoah National Park, but they are along hiking trails.
What is columnar jointing? It is caused by the contraction of cooling magma. Upon cooling, the surface of a lava flow gradually contracts due to a decrease in volume as the liquid changes to a solid. As the magma contracts, cracks propagate outward from regularly spaced points; the angles between cracks are also fairly regular. This process is similar to the formation of mudcracks on a baseball field as water evaporates and the mud dries. The basalt continue to crack, and grow below the surface as the magma cools and crystallizes at depth. The result is the formation of five-to-six vertical columns that extend from the top to the bottom of the flow. Most often columnar jointing occurs in thin basaltic flows. Here the columns are 6-8 inches across, but have been observed within the park as wide as 30 inches (Badger, 2012).
If the lava flow is horizontal, the cracks will be vertical in appearance. However, if the underlying surface is not flat, then the two cooling surfaces – one on top and the other on the bottom, will not be parallel and the columns may curve. Columns may also curve due to slight forward movement of the flow during cooling.
A rather famous site, Devil’s Tower National Monument in Wyoming has fabulous columnar jointing. In Pennsylvania, the only example known by your leader is at the Carbaugh Run Reservoir in Michaux State Forest, southeast of Caledonia State Park.
STOP 5. VA RTE 601 ROADCUT NEAR STRASBURG
Keywords: Bedding, dip, limestone, micrite, metabentonite, fossils, contact, Taconic Orogeny
We return back to the valley for our next venture. After a short drive from Front Royal westward to Strasburg, we will spend the remainder of the day in the Great Valley Section of the Ridge and Valley Province. The valley is predominantly composed of limestone (a sedimentary rock containing mostly calcium carbonate), dolomite (a sedimentary rock containing mostly magnesium carbonate) with minor amounts of shale and sandstone. These rocks in this area are Cambrian and Ordovician in age and reflect the building of a continental shelf off of the east coast of an ancient North America known as Laurentia.
This site is a classic site to study carbonate geology. Within this roadcut which measures about 900 feet long, are layer upon layer of limestone, a small amount of shale and several thin layers of metabentonite. You will be presented the chance to study the outcrop for a moment and answer these questions:
- In which direction is the rock dipping into the surface? Rocks are generally formed on a horizontal or near-horizontal angle. If the dip is steeper than that, this tells a geologists that some form of tectonic event has occurred to cause this.
- What is your estimate on the angle of dip? A vertical dip (90°) would be where the layers go up and down.
- At what end of the roadcut would the oldest rocks be found? A law of superposition states that the rocks on the bottom are the oldest, UNLESS the beds have been turned upside down due to tectonic movement.
- One last challenge. There are 3 different formations in this roadcut. These formations are all mostly limestone and can be generally divided by several small differences within the appearance of the rock. Below are several characteristics about each formation. Upon your inspection of the roadcut can you determine where one formation stops and another begins? From oldest to youngest, the formations are:
New Market Formation: Middle Ordovician in age
Composed of micrite. Micrite is a limestone constituent formed of calcareous particles ranging in diameter up to 4 μm formed by the recrystallization of lime mud (http://en.wikipedia.org/wiki/Micrite).
The rock is a medium gray, dense rock with medium bedding.
Thickness is about 55 feet with the base exposed in stream bed beneath the bridge.
Fossils present include bryzoan, ostracods, brachiopods and gastropods.
Lincolnshire Formation: Middle Ordovician in age
Thin-bedded dark gray limestone,
Limestone containing black chert,
Light gray cherty limestone and thin (up to 12 inches thick) layers of a buff-colored metabentonite (interpreted as volcanic ash).
Fossils include bryzoan, gastropods, ostracods, trilobite (Homotelus), brachiopods, and algae (Girvanella).
Thickness here is about 590 feet.
Edinburg Formation: Middle Ordovician in age
Cobbly, buff gray limestone,
Slabby to cobbly limestone,
Shaly,cobbly limestone,
Nodular to cobbly limestone,
Dense, black slabby limestone,
Chert,
Irregularly bedded medium-to coarse-grained limestone.
Fossils include cystoid (Echinosphaerites), algae and brachiopods.
Thickness is about 630 feet (Cooper and Cooper, 1946; Edmundson, 1945; Rader and Biggs, 1976).
Your guide will place yellow tape at the contact between the New Market/Lincolnshire formation contact and the Lincolnshire/Edinburg formation contact.
The best fossil collecting in your leader’s opinion is south of a dry wash filled with limestone about two-thirds of the way south. Several loose rocks with chalky weathering exhibits some darker impressions of fossils.
A little more detail can be told from these rocks. The New Market and Lincolnshire formations were deposited on a continental shelf during a rather quiet time off of the coast of Laurentia in the Iapetus Ocean. This continental shelf became well established by the Middle Ordovician Period. Shortly after the deposition of the Lincolnshire material, a mountain-building event known as the Taconic Orogeny was beginning to occur as a chain of volcanic islands were colliding along the East Coast. The orogeny is so named as the Taconic Mountains in New England were uplifted during this time. The Edinburg Formation exhibits some chemistry in the rock that tells us that the ocean was becoming muddy and more fine-to medium sized sediment was being introduced onto the continental shelf. The overlying Martinsburg Formation, which we don’t see here, formed from sediment that was being deposited within a subduction zone related to the volcanic islands
