Our story begins in the Cambrian period of geologic time, the period which begins the Paleozoic Era (and the Phanerozoic eon). The beginning of the Cambrian is often referred to as "the biggest punctuation mark in the entire geologic record," because it marks the first abundance of well-preserved animal fossils. Basically, the animals that fossilized had hard parts (shells, etc.), whereas in the Precambrian, there were animals, but they hadn't yet evolved shells and other hard parts.

This paleogeographic map shows what our continent would have looked like during the middle Cambrian, about 510 million years ago. Probably the most important thing to notice at this point is that the Appalachian mountains aren't there yet. 510 million years ago, the area that today we call "the Appalachians" was in no way mountainous. Instead it was a lowland, flooded by seawater where sediments were accumulating. Instead of Appalachian Mountains (source of sediments), we're looking at an Appalachian Basin (place where sediments get deposited). Our story starts in this shallow sea.

Map by Ron Blakey, Northern Arizona University.

Many more of Blakey's highly detailed maps are available for different geologic time periods here.

PHOTO OF OUTCROP PENDING

In these calm shallow seas, we see evidence of some very simple life forms: microbial mats. These layers of slime were buried beneath a fine layer of sediment each day, and grew upwards to escape it. Over time, their colonies bulged upwards into dome-like shapes called stromatolites.We find these stromatolites at the Mulberry Run outcrop (stop 1), on route 55 west of Interstate 81.

Hammer for scale.

There are also some nice spherical grains of calcite called ooids (or ooliths). These form in wave-influenced carbonate banks today, like the Bahamas. So what's the interpretation of this environment, then? Looks like a nice passive margin, far from any major clastic inputs (i.e. mud or sand). Warm tropical temperatures leading to the chemical precipitation of lime mud from seawater. Little ooids rolling were gently back and forth in the waves, getting a nice even coat of calcite, making them spherical. This tropical scene is preserved in the form of this oolitic limestone.

Quarter for scale.

At the Tumbling Run section (stop 2), NOVA geology Honors students point out the contact between two limestones: the more massively-bedded New Market Formation below, and the more finely-bedded Lincolnshire Formation above. Formations are distinct packages of rocks which originated through the same formative processes.

At Tumbling Run, there are four formations exposed: the Beekmantown (uphill from the bridge), and then the New Market, Lincolnshire, and Edinburg limestones going up-section (downhill) from there.

Fossils may be found in these limestones: here, for examples are some bryozoans (seen in cross-section). Bryozoans (the word means "moss-animals") are colonial animals that filter the water for food particles. While they still persist today, they were more common early in the geologic record.

Pen for scale.

Here's what you're looking at a cross-section of when you see these byrozoans at Tumbling Run. These are twig-like bryozoan fossils from near Brookville, Indiana. Like the fossils at Tumbling Run, they are Ordovician in age.

Penny for scale.

Photograph taken by Mark A. Wilson, Department of Geology, The College of Wooster. Released into the public domain by the photographer.

Another couple of cross-sectioned bryozoans.

Hammer (in background) for scale.

...And here's a few more.
You get the idea by now, I'm sure.

Notice how nice and "clean" the limestone is: very light-colored (at least on the weathered surfaces). This is about to change...

Pen for scale.

Here's a couple cool images from another group of organisms which are found as fossils in the Tumbling Run outcrops. These are pieces of trilobites, arthropods (like insects, crabs, and scorpions) which lived in the Paleozoic era of geologic time. Unlike byrozoans, trilobites are extinct in the modern world.

Click on either of the individual panels to enlarge it, then use your browser's "BACK" button to return to this page.

Images from Adrain (2005).

But what have we here? A layer that weathers away much more easily than the limestone above and beneath it! Upon closer inspection, this layer consists of a tan to yellowish clay, very crumbly to the touch. This is a bentonite layer, a layer of clay that has resulted from the weathering of a volcanic ash deposit.

In Europe, the layer is referred to as the "Big Bentonite." In North America, it is referred to as the "Millbrig bed."

By the way, bentonite is mined for various industrial uses.

The bentonite is very crumbly and weak. You can actually reach in and grab a handful of it, as shown here. This is why it weathers away so much more easily than the limestone above and below it. It's weak stuff!

Even though the modern bentonite is weak, you'll notice it's very thick. This implies a large volume of initial volcanic ash...

Photograph by Michael Wilpers.

This paleogeographic map shows the distribution of ashfall deposits from 454 million years ago: a massive blanket of volcanic ash which has since weathered to bentonite. The source of ash was one of the volcanoes erupting as a result of the closure of the Iapetus Ocean. The ocean basin was estimated to be 1000 to 2500 kilometers wide at that time.

The total estimated volume of this extraordinary deposit is therefore estimated by Huff, et al. (1992) at 1140 cubic kilometers of ash. (That is over 400 times the output of Mount St. Helens' 1980 eruption in Washington state.)

Huff, et al. (1992) prepared this figure correlating the "Millbrig bed " of North America with the "Big Bentonite" of Europe. Uranium-lead dating of zircon crystals in the bentonite confirm they are of identical age, 454 million years old. Detailed examination of the changes in mineralogy of the deposit (from bottom to top) suggest that it represents the ash output of a single sustained volcanic eruption that lasted 10 to 15 days.

The biozones ("zones") shown in this table indicate periods of time defined by the presence of a particular index fossil. This table focuses only on conodont and graptolite index fossils (from both continents).

So it's now reasonably well-established that the Big Bentonite represents an enormous volcanic eruption. But what was the effect on living things? Since volcanic eruptions are frequently blamed for Earth's mass extinction episodes, it's probably worth testing that hypothesis with an examination of how many species went extinct when this 'mother-of-all-eruptions' went off.

Huff, et al. (1992) scrutinized this idea by examining four major fossil groups: brachiopods, graptolites, conodonts, and chitinozoans. They found that the majority of the species in these groups sailed right through the eruption of the Big Bentonite without any problem (54 of 58, a total species survival rate of 93%). Only 4 species (in two groups) went extinct at the time of the Big Bentonite eruption (only 7% of the total species studied).

So it appears that the eruption which produced the Big Bentonite was not a major killer.

As we work our way up-section (down-hill) at Tumbling Run, we see the limestones get darker and darker in color, indicating a higher and higher proportion of clay and silt in them. This clay and silt has to be coming from somewhere, and the traditional interpretation of clastic sediments is that they are shed off of exposed landmasses. In this case, we think that the increasing flux of "dirty" sediment is being shed off the Taconian Orogeny, a late-Ordovician mountain-building episode that occurred east of here.

So the overall pattern seen at Tumbling Run is a transition from passive margin sedimentation towards active margin sedimentation. A tectonic bounary (subduction zone) draws near!

At the site of Taconian mountain-building itself, the Piedmont province (where I lead several other field trips), you can find all kinds of characteristic signatures of mountain-building: metamorphic rocks, folded rocks, boudinaged rocks, and granite intrusions. While all of those are signatures of mountain-building, they only tell what's happening at the roots of the growing mountain range. Sediments that are shed off the upthrust mountains accumulate in neighboring low-lying areas called basins. The Shenandoah Valley and Massanutten Mountain show sedimentary layers that were deposited in one of those basins during the time between the Cambrian and the Devonian.

Our second stop shows some of these land-derived sediments in the form of a transition from limestone to shale layers.

So part 1 of our story told about calm, clean sedimentation during the Cambrian and early Ordovician, but then that started to get dirtier during the late Ordovician, due to the onset of the Taconian Orogeny to the east. Eventually, the large volume of mud overwhelms the carbonate deposition, and we see a switch from limestone to dirty limestone to limey shale. Though these are different rock names, they represent a spectrum of types of sediment: 100% carbonate, 70% carbonate / 30% mud, 30% carbonate / 70% mud, and eventually 100% mud. The proportion of carbonate diminishes as time progresses; the proportion of mud (and eventually sand) increases. Here's a mudrock with a little bit of carbonate in it: the Oranda Formation (stop 3) is a calcareous shale. To a geologist, this indicates heavy clastic influence (mountain building), with a bit of carbonate (distance from the young, rising mountains). The mountain-building happening in the neighboring area is picking up, dumping increasing amounts of mud into this depositional basin.

Photograph by Michael Wilpers.

As we go higher in the sequence of sedimentary layers, we see that they get coarser and coarser, eventually losing all "clean" calcite and becoming 100% "dirty" (made of sand and mud). Here, you see the alternating beds of sandstone and shale that make up the Martinsburg Formation. These alternating grain sizes (mud vs. sand) indicate fluctuations in the strength of the water which deposited this sediment. When it was moving faster, it could carry (bigger) sand. When it was moving slower, it wasn't powerful enough to carry sand, but it could carry mud.

Pen for scale.

Much of the Martinsburg Formation displays graded bedding, a characteristic sedimentary structure that results from deposition by turbidity currents.

Cut and polished specimen from west of Bentonville, VA. Width of the sample is about 10 cm (4 inches).

Here is a turbidity current flowing underwater. Turbidity currents are dense, sediment-laden flows that are gravity-driven downhill. They flow along the bottom of the sea when a particularly large amount of sediment gets dumped into the system, starting from a source area adjacent to a land mass. This could be from a storm, perhaps, or due to a submarine landslide.

This turbidity current was produced in our experimental tank at the NOVA-Annandale geology lab.

The Buzzard Rock Trail stop (stop 4) showcases the Martinsburg Formation, though it's a wee bit overgrown with moss, and you can't get too close to it because of the stream immediately at its base...

Photograph by Michael Wilpers.

Here's the outcrop on the Buzzard Rock Trail (stop 4). It's also the Martinsburg Formation. Because the outcrop has a lot of lichens and moss growing on it, I've provided a cartoon field sketch of the same outcrop below. What you see here are alternating beds of sandstone (originally sand) and shale (originally mud). These have been interpreted to form from turbidity currents, as outlined above. This was far enough out in the sedimentary basin that it was mostly calm and only mud got deposited, but periodically, a turbidity current would come rushing in and dump a load of sand.

Nowdays, the shale is easier to erode than the sandstone, and it makes the sandstone layers stand out in high relief, while the shale is etched away.

Quarter for scale.

Another sedimentary structure that shows evidence of currents are cross-beds. Cross-bedding forms when currents of water build up little ripples of sediment on the bottom of the water. Viewed in cross section, they are small inclined depositional surfaces at a diagonal angle to the main bed. As the current brings in new sediment, it gets deposited in the slightly calmer water on the downstream side of the ripple. This new "drape" of sediment on the ripple is a new cross-bed.

Cut and polished specimen from the Buzzard Rock Trail outcrop (stop 4). Length of the sample is about 6 cm (2.5 inches).

How cross bedding can help us read ancient current direction: As the water current moves downstream, it picks up particles, like grains of sand. If they are exposed to the full force of the current, sand grains are likely to get picked up. However, if they fall into the "lee" of the ripple, they are not in the full force of the current. The water is calmer there, and sand is deposited, rather than transported. So the sand grains accumulate on the shallow slope of the ripple-mark, facing downstream. A succession of these "ripple fronts" is preserved as a series of cross-beds. Now take another look at the photo above, from the Martinsburg Formation exposures at Buzzard rock. Because the real cross-beds in the lower image are oriented the same way as the cartoon cross-beds at right, you now know that the current which deposited this sand was moving from right to left. The dominant direction of water flow during the early Mississippian period was from east to west, which agrees with our earlier idea about there being elevated highlands to the east (source of the sediments and the rivers).

WATCH AN ANIMATION OF CROSS-BED DEPOSITION.

Cross-beds and other current-indicating sedimentary structures are combined with the thickness of clastic sediments to arrive at this interpretive diagram. Here, four big "clastic wedges" create map-scale fan-shaped deposits as rock material is removed from mountain sources to the east, and transported westward by river systems. These rivers then dumped the sediments into the shallow sea that flooded the continent to the west, creating the mud and sand layers that characterize the Oranda and Martinsburg Formations.

Image from P.B. King's classic volume The Evolution of North America (1977).

As we work our way up through the stratigraphic sequence, and therefore forward in time, we find the next layer up is the Massanutten Formation, a quartz sandstone. Here, at the Blue Hole outcrop (stop 5), you see the beds of the sandstone tilted to the south, into the trough of the Massanutten Synclinorium.

This sandstone is interpreted as ancient beach deposits as the sedimentary basin produced by the Taconian Orogeny was filled in and topped off.

Parts of the Silurian-aged Massanutten Formation contain fossils of the earliest land plants (preserved as carbon films).

Parts of the Massanutten Formation contain particles too large to be considered quartz sandstone, and must therefore be called a quartz pebble conglomerate instead.

Quarter for scale.

Here's another shot of a boulder of the Massanutten's conglomeratic portion. It would have taken more energetic water currents to deposit this sediment than the sand which became the sandstone.

Penny for scale.

One feature that's reasonably common in the Massanutten Formation sandstone is the trace fossil known as Arthophycus. Arthrophycus are horizontal (bedding-plane-parallel) feeding traces left behind by (probably) a polycheate worm.

Hand for scale.

Another Arthrophycus trace fossil in the Massanutten Formation sandstone.

Hand for scale.

Here's the other major kind of fossil besides trace fossils: body fossils, which show the animal's actual body when it died. In this case, we've got some articulate brachipods who were buried in the Massanutten Formation sand, and then after the sand turned to sandstone, the brachs' shells dissolved, leaving external molds of those shells. Though these external molds are pretty lousy, you can see that the little holes in the rock have a "D" shape, with a long, straight hingeline where the brachiopod's two valves (shells) came together, and then the curved lip where the valves opened so the animal could feed.

Pocketknife (5 cm long) for scale.

Here's some nice cross-beds in the Massanutten Formation sandstone. Using the cross-bed current-reading skills you developed (above) when studying the Martinsburg Formation, you can deduce that the current that deposited these beds must have been flowing from left (south) to right (north).

Lens cap for scale.

Many parts of the Massanutten Sandstone show this cross-bedding (see the discussion above for the older Martinsburg Formation). Here, you see some of the cross-beds exposed at the Blue Hole outcrop (stop 5). They are oriented in different directions (see interpretive sketch below the photo), indicating shifting current directions. (From right-to-left, then left-to-right, and then back to right-to-left again.)

Penny for scale.

The upper part of the sequence at Massanutten consists of more terrigenous (land-derived) sediments. The source for this new package of "dirty" sediments is another orogeny to the east: the late Devonian-aged Acadian Orogeny, caused by the collision of the microcontinent Avalonia with the eastern seaboard. This tectonic convergence shoved rock masses up into the air (Acadian Highlands on the map), and those highlands were eroded, generating sediment. The sediment was transported westward, and deposited in a shallow inland sea, producing layers of siltstone and shale at Massanutten, and layers of sandstone and conglomerate higher up in the stratigraphic stack (not visible at Massanutten because they have been eroded away; you can find them outcropping to the west, at Sideling Hill for instance).

Map by Ron Blakey, Northern Arizona University.

Many more of Blakey's highly detailed maps are available for different geologic time periods here.

This outcrop of the Needmore Formation shows trace fossils (invertebrate burrows) highlighted with rusty coloration.

Quarter for scale.

Here's the Mahantango Formation, showing its massive silty appearance (not fissile like a proper shale), and a nicely defined horizon rich in external molds of brachiopod shells.

Quarter for scale.

Another trace fossil, in the Mahantango Formation. In this case, we are looking at the vertically-oriented feeding trace called Daedalus, thought to be a different aspect of the horizontally-oriented Arthrophycus (see above). This is half of a Daedalus, which is broken off in the middle. If it were complete, it would complete its crescent shape to the left.

Quarter for scale.

Above that, we see a new layer. Particularly well exposed near the burg of Seven Fountains, this layer looks a lot like something we saw earlier in the trip... Hints: it's fine-grained, kind of tan colored, and very crumbly. If that reminds you of the "Big Bentonite," you're on the right track! This is a younger layer of the same kind of rock. It's known as the "Tioga Bentonite," and it's a major stratigraphic marker bed throughout the Appalachians. This is a shot of the bentonite exposure on the Fort Valley Road near Seven Fountains, where it is found within the Needmore Formation.

The Tioga Bentonite has been dated to be about 391 million years old (Tucker, et al., 1998). That's in the middle of the Devonian period of geologic time.

Lens cap for scale.

This isopach map shows the thickness of the Tioga Bentonite over the mid-Atlantic region. Isopach maps depict the thickness of a particular stratum with contour lines. The Tioga Bentonite was originally deposited as volcanic ash during a large volcanic eruption 391 million years ago (according to Tucker, et al., 1998).

The map shows that the deposits of Tioga ash are thickes to the southwest, in particular at Seven Fountains, a village in the Fort Valley of Massanutten Mountain. Some authors have suggested the the way the thickest deposits "wrap" around Fluvanna County, Virginia, suggests that Fluvanna was the approximate area where the source volcano was located. Geochemical characteristics of the bentonite match geochemical characteristics of granites in Fluvanna County.

From: Dennison and Textoris (1971).

So, here's an overall look at the sequence of sedimentary layers (including bentonites) that we've seen so far. Stops on this field trip are noted on the far left in the light orange circles (some formations warrant more than one visit, because we want to see structures formed by later tectonic overprinting). The main part of the diagram shows the vertical "stack" of sedimentary layers in our field area. Periods of geologic time, and their beginning/end dates are shown to the right of that.

The overall tectonic story is shown in the boxes on the far right: initial passive margin sedimentation (various limestones) followed by active margin sedimentation from the Taconian Orogeny (sediments get dirtier, volcanoes erupt ash which is preserved as bentonite), more passive margin sedimentation (quartz sandstone and limestone), then the onset of a new orogeny, the Acadian, and a return to active margin sedimentation (more "dirty" sediments: mudstones and shale, plus more volcanic ash).

Now for the part of our story that takes this big sedimentary stack, and deforms it all to pieces. The latest and greatest mountain-building event in the Paleozoic era of geologic time was the Alleghanian Orogeny, a collision between ancestral North America and Africa. When these two behemoths collided, they deformed all the rocks along the suture zone, a mountain belt that today we call "the Applachians." This collision completed the assembly of the late Paleozoic supercontinent Pangea. Pangea folded up all the sedimentary layers we have discussed so far, and thrust most of Virginia's rocks up and to the west.

Map by Ron Blakey, Northern Arizona University.

Many more of Blakey's highly detailed maps are available for different geologic time periods here.

The "Great Valley" is usually called the Shenandoah Valley in Virginia, because the Shenandoah River flows north through it. Sitting in the middle of the valley is a mountain range, Massanutten Mountain. And in the middle of Massanutten, there is another valley, the Fort Valley. As you can see in this Google Map, Massanutten is a fence-like ridge separating the higher Fort Valley from the lower Shenandoah Valley. In fact, rumor has it that the name "Massanutten" is a native American term for "basket." This describes the overall shape of the mountain/valley quite well. It probably won't surprise you to learn that this valley-in-a-mountain-in-a-valley pattern is due to differential weathering of folded sedimentary layers. In fact, the entire Great Valley is one big downturned fold, a syncline. Actually, it's not a perfectly smooth fold -- there are some wrinkles and minor folds within the overall down-turned structure, so we call it a synclinorium.

This Google Map is "live": you can grab it with your cursor and drag it around to get a sense of how Massutten relates to the surrounding landscape. Try it!

The oldest rocks are therefore at the eastern and western edges of the Great Valley, and the youngest rocks are at the center of the Massanutten Synclinorium, up in the Fort Valley. It turns out that some of these rock layers are easily eroded, and some are tough. Of particular note is the Massanutten Sandstone (shown in yellow), a quartz-rich, well-indurated rock that is responsible for the ridges of Massanutten Mountain. It weathers away more slowly than the shales and carbonates (limestones) above and below it. Here's a cross-section view to show how the subterranean structure influences the surface topography. The map view up above (using Google Maps) and this cross-section also show the difference in landscape texture (and geologic cause) of the Blue Ridge province in the SE corner of the images.

Overall, Massanutten Mountain, the surrounding Shenandoah Valley, and the inner Fort Valley are surface features connected to one underlying geologic structure: they are all part of the Massanutten Synclinorium.

The Blue Ridge thrust fault may be seen to the east. Both thrust faulting and folding are the result of tectonic compression. These rocks got severely squeezed during the final phase of Appalachian mountain-building, the Alleghanian Orogeny (300-250 Ma).

Here's another view of the same outcrop, without human being and without interpretive arrows.

Rock hammer for scale.

The black nodules you see along bedding in the above image are flint nodules, very characteristic of the Lincolnshire Formation. If you get close to them, you'll find that they exhibit different mechanical properties than the limestone that surrounds them. They are more likely to break (brittle behavior) than flow (ductile behavior). On the other hand, the limestone which surrounds them is more likely to flow than to break. The two rock types exhibit a compentence contrast. The flint is more competent (stronger) than the limestone.

Quarter for scale.

Some of this folding took place on scales so large we need to be in an airplane (or look at a Google Map) in order to see the fold pattern. But sometimes we're lucky enough to see smaller-scale folds. Here's the small anticline in the Massanutten Sandstone, exposed along the Veach Gap trail (stop 8).

Another anticline, this one in (presumably) the Needmore Formation shale, at an outcrop in the southern Fort Valley near Mooreland Gap - a place that we will not visit on this trip. Like our final stop (stop 9, the "Pet Store Anticline"), this shows an excellent example of a cleavage fan in a folded sedimentary layer. Accentuating the folded shape is the fact that the underlying layer is easily-weathered, resulting in a "cave" effect.

Photograph by Aaron Barth.

Our final outcrop of the day (stop 9) is the "Pet Store Anticline," an outcrop of the Martinsburg Formation that can be found on the north side of Guard Hill, north of Front Royal, south of Interstate-66.

Quarter (center) for scale.

Here's a close-up of the strata at the pet store outcrop. The tilted and folded layers of the lower Martinsburg Formation (here a calcareous shale) have been overprinted by a strong sub-vertical cleavage. We observe here the rock cycle caught in the act of transitioning between a sedimentary rock (shale) into a metamorphic rock (slate). Shales break along their bedding planes. Slates break along their cleavage planes. One thing we can see here is the rock breaking into long thing blocks, a style of breakage known as "pencil cleavage."

Quarter for scale.

A satellite photo (false-color image) shows the distinct form of Massanutten Mountain in the Shenandoah ("Great") Valley (center), as well as the Blue Ridge Mountains in Shenandoah National Park (to the right of Massanutten). "Warm" colors represent high elevations; lower elevations are green. Massanutten stands up as a mountain because that's where the quartz-rich (erosion-resistant) Massanutten Sandstone outcrops. The Shenandoah Valley and the Fort Valley are places where less resistant rock appears at the surface (shale, limestone). The tougher rocks stand up high as mountains; the weaker rocks are eroded away to leave physiographic lowlands.

The texture of the Blue Ridge is markedly different from the mountains to its northwest. This textural difference is entirely due to the rocks the respective provinces are made of. The Blue Ridge is hewn from metamorphosed sedimentary and igneous rocks, whereas the Valley and Ridge province is a series of folded sedimentary layers, only lightly metamorphosed. Note also the pronounced meanders of the North and South forks of the Shenandoah River, on either side of Massanutten.

Image by NASA.

An interesting feature becomes apparent if we zoom in on the meanders of the North Fork of the Shenandoah River, just northwest of Massanutten Mountain. Note that these meanders have long, parallel limbs, and that those limbs are almost perfectly perpendicular to the trend of Massanutten Mountain's ridges. During Appalachian mountain-building, these rock layers were compressed from the southeast towards the northwest. The rocks responded to this compressive stress in two ways: (1) they folded, with the folds developing perpendicular (NE-SW) to the principal stress direction (SE-NW), and they extended laterally, opening up a series of fractures that were parallel (SE-NW) to the principal stress direction (SE-NW). In other words, as Africa crashed in from the southeast, the rocks of the Shenandoah Valley got squished towards the northwest, and popped apart towards the southwest and the northeast.

Later, the North Fork of the Shenandoah River took advantage of these fractures, which were easier to erode into than the unfractured rock next to them. The underlying rock structures help to determine the surface landforms. Though we don't observe the fractures themselves, the long limbs of the North Fork's meanders mark their location in an uncannily precise pattern.

Note one additional feature here: an abandoned meander where the North Fork has cut off one of these long detours to take a shorter, higher-angle downhill path. The long U-shaped valley to its northwest shows where the river used to flow before the cut-off happened.

Image modified from Melson (2004), Figure 39.