GEOLOGY OF CORRIDOR H Geological Society of Washington Fall field trip, 2014

Leaders: Callan Bentley (NOVA), Dan Doctor (USGS), Alan Pitts (Univ. of Camerino)

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Geologic provinces of the Mid-Atlantic region

Our region of the country is divided into 5 physiographic provinces (areas of the land

that have a common appearance or landscape). This topography is determined almost

entirely by the underlying rock units, which are where they are due to the course of the

region’s geologic history. You can think of the different physiographic provinces as each

representing a “chapter” in the “book” that describes the region’s overall geologic

history. It’s a long story that spans more than a billion years of time and tectonics.

A closer look reveals that one of the

physiographic provinces has two distinct

parts, each telling a very different part of

the story: the Piedmont province is

subdivided into the “metamorphic

Piedmont” (crystalline rocks of the

Appalachian mountain belt) and the

“Triassic rift basins” (Mesozoic

lowlands). By making this distinction, we can move from physiography to geologic

meaning. So from this point forward, we will refer to the geologic provinces of the mid-

Atlantic region (Virginia, Maryland, West Virginia, Delaware, and Washington, DC).

The overall list of geologic provinces, running from east to west at the latitude of

Washington, DC, is therefore: (1) the Coastal Plain, (2) the Piedmont, (3) the Culpeper

Basin, (4) the Blue Ridge, (5) the Valley & Ridge, and (6) the Alleghany Plateau (also

more broadly called the Appalachian Plateaus). The boundary between the Coastal Plain

and the Piedmont is known as the “Fall Line” or the “Fall Zone,” as that’s the area where

waterfalls and rocky rapids appear in the rivers that flow east to the Atlantic. All the

major cities of the east coast are built along this important feature: Washington, DC is

merely the most prominent, but a fuller list would include Philadelphia, Baltimore,

Fredericksburg, Richmond, and Raleigh/Durham.

While “east to west” organizes the geologic provinces in space, it does not organize

them in time. It is perhaps better to think about the provinces in terms of the

chronological order of their formation. In that case, we would go from oldest to

youngest: (1) Blue Ridge, (2) Piedmont + (3) Valley & Ridge [contemporaneous], (4)

Alleghany Plateau, (5) Culpeper Basin, and (6) Coastal Plain.

The map on the next pages shows how they relate to one another.

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Overview:

The rocks of the Mid-Atlantic region (Virginia, Maryland, Delaware, Washington,

DC, and West Virginia) record two complete “Wilson cycles” of supercontinent

assembly and break-up, interspersed with periods of passive margin

sedimentation and tectonic calm. The story begins in the Mesoproterozoic era,

about 1.1 billion years ago (Ga), with the accretion of the Grenville terrane to the

ancestral North American continent. This collision of an independent chunk of

continental crust with the larger ancestral North American continent (also called

“Laurentia”) was only the most recent event in a chain of terrane collisions that

had been playing out since Archean times. A map of North America’s craton (the

more or less stable Precambrian ‘nucleus’ of the continent) shows how these

terranes have accumulated over time:

Map

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Note that western Virginia originated at the time of the docking of the Grenvillian

micro-continent with proto-North-America. The Grenville Orogeny was an

episode of mountain-building that resulted from this collision. All around the

world, between 1.2 and 1.0 Ga, continents and micro-continents were colliding

with one another, assembling a supercontinent called Rodinia.

Rodinia lasted for a while, and the Grenvillian Mountains were subject to

weathering and erosion, ultimately exposing the high-grade metamorphic rocks

(gneisses) and plutonic igneous rocks (granites and granitoids) at the roots of

those ancient mountains. Deposited on top of these exposed mountain roots

were clastic sediments (mud, sand, gravel) that later lithified to become shale,

arkosic sandstone, and conglomerate. Then Rodinia began to break up. An initial

episode of extension occurred around 730 million years ago (Ma), and a final

episode of extension (rifting) occurred around 565 Ma, accompanied by vast

eruptions of mafic lava (basalt). Rodinia broke apart, and between its separating

fragments, a new ocean basin formed. North America moved off in one direction,

and the Congo craton and Amazonia craton moved off in the opposite direction.

Seafloor spreading filled the gap between them, and the Iapetus Ocean was

“born.”

At this time, ancestral North

America was south of the

equator, and the future Mid-

Atlantic region of the

continent faced southwest

rather than southeast, as in

the modern day. The paleo-

geographic map by Ron

Blakey for 550 Ma shows this

well.

As the Iapetus Ocean

widened, oceanic crust grew

off the rifted edge of

Map

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550 Ma

Iapetus Ocean

Ancestral

North

America

Amazonia /

Congo

cratons

Ancestral

Europe

Iapetus Ocean

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ancestral North America. The “eastern” margin of what is today the Mid-Atlantic

region got further and further from the edge of the tectonic plate. It switched

from being an active continental margin (convergent @ 1.1 Ga; divergent @ 730-

565 Ma) to a passive continental margin. The cooling crust contracted (making it

denser) and subsided. This allowed the accumulation of passive margin sediments

(mature sedimentary rocks) atop the older crust. A worldwide rise in sea level

caused marine waters to transgress across the North American continent during

the Cambrian period of geologic time, flooding the continental crust to make an

epeiric sea called the Sauk Sea.

500 Ma

Sauk Sea

Iapetus Ocean

Ancestral

North

America

Map

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Meanwhile, the Iapetus Ocean had begun to contract. A subduction zone formed,

initiating a growing chain of volcanic islands. As subduction consumed the oceanic

crust on the fringe of the North American plate, the volcanic island arc got closer

and closer to the future Mid-Atlantic region.

This volcanic archipelago collided with the edge of the ancestral North American

continent occurred around 460 Ma (late Ordovician time). This collision, the

Taconian Orogeny (also called the “Taconic” Orogeny) built a range of mountains.

These mountains’ roots are marked in the Piedmont by typical signatures of

orogeny: partial melting, deformation, and metamorphism. Sediments eroded off

the top of the mountains were carried downhill by rivers and turbidity currents,

and deposited with volcanic ash in immature layers (e.g. graywacke) in the epeiric

sea to the “west” (in modern terms). This is the future Valley & Ridge province.

After the Taconian Orogeny, the future Mid-Atlantic region reverted to more

passive margin sedimentation, marked by the accumulation of more mature

sediments such as quartz sand and limy mud. Offshore, in the Iapetus Ocean, a

new subduction zone formed, and brought a small microcontinent closer and

closer to North America. This microcontinent, dubbed Avalonia, collided with

ancestral North America around 360 Ma (Devonian period) in the Acadian

Orogeny. Much of eastern New England and the maritime provinces of Canada

were accreted to the continent at this time. In the Mid-Atlantic region, vast

quantities of sediments from this second phase of Appalachian mountain-building

poured into the epeiric seas, filling them to the brim with clastic sediment, and

aggrading rivers (and floodplains) further and further to the west. Again, these

piled up in a nice stack to the west of the mountains, in the region that would

later become the Valley & Ridge.

The ultimate paroxysm of Appalachian mountain-building came with the final

closure of the Iapetus Ocean, when the continent on the other side, ancestral

Africa (the leading edge of Gondwana) collided with ancestral North America. This

continent-continent collision is known as the Alleghanian Orogeny. It began

around 300 Ma, and was pretty much over by 250 Ma (Pennsylvanian-Permian).

As the Iapetus Ocean “died,” a new supercontinent was born: Pangaea.

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Running through the middle of Pangaea was a Himalayan-size mountain chain:

the young Appalachians, finally complete. During this time, older rocks of the Blue

Ridge were metamorphosed, and the sedimentary strata of the Valley & Ridge

were folded into anticlines and synclines, as well as being thrust-faulted to the

northwest. The resulting fold and thrust belt is well exposed on Corridor H.

Pangaea was the largest supercontinent the world has ever known. Over geologic

time, partial melting of older rocks yields more felsic magma, which cools and

crystallizes to make granite, the stuff that continental crust is mostly made from.

Ancestral

North

America

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Ancestral

Africa

Ancestral

South

America

290 Ma

P A N G A E A Young Appalachian

Mountain Belt

Ancestral

Europe

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The total proportion of continental crust on Earth has increased over time, with

Pangaea being the largest accumulation of that crust … so far!

Ultimately, Pangaea lasted for tens of millions of years, through the greatest

extinction event in Earth history, and into the early Mesozoic era. In the Triassic,

continental extension (rifting) had once again resumed, resulting in a series of rift

valleys that opened up amid the mountainous terrain. These basins filled with

immature clastic sediments and mafic lava, and eventually some of them

connected up to form a gradually-widening narrow ocean basin. The Atlantic

Ocean was born, and Pangaea “died” as its continental fragments drifted apart. It

was not a clean break: parts of what used to be “North America” stuck to Europe

(e.g., northern Scotland), and parts of what used to be “Africa” stuck to North

America (e.g., Florida).

The Appalachian Mountains were gradually worn down by weathering and

erosion, and by the Cretaceous, the mountains had been exposed to their deep

roots. Rivers stopped cutting down, and began depositing new layers of sediment.

As time went by, particularly during the Miocene, thick accumulations of clastic

sediment and carbonate material accumulated on the new passive margin. These

are the layers of the Coastal Plain.

During the Pleistocene epoch of the Quaternary period, global cooling (probably

initiated by the closure of the Isthmus of Panama and the ensuing diversion of the

Gulf Stream into the North Atlantic) caused the buildup of massive ice sheets atop

northern Europe, Siberia, and North America. These glaciers existed because of

removal of huge amounts of water from the world ocean, causing sea level to

drop by around 100 meters (~330 feet). The rivers which were until then

depositing sediment switched into erosional mode instead, cutting down

(incising) into the landscape and initiating the “falls” along the Fall Line.

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In summary, the timeline of key events for the Mid-Atlantic region is:

1.2-1.0 Ga Grenville Orogeny (assembly of Rodinia)

730-565 Ma Breakup (rifting) of Rodinia; opening of Iapetus Ocean

550-470 Ma Passive margin sedimentation; transgression

460 Ma Taconian Orogeny (collision w/ volcanic island arc)

360 Ma Acadian Orogeny (collision w/ Avalonia microcontinent)

300-250 Ma Alleghanian Orogeny (collision w/ Africa; assembly of Pangaea)

200-180 Ma Pangaea breaks up; Culpeper Basin rifting; Atlantic Ocean opens

180-100 Ma Erosion of ancestral Appalachian Mountains

100 Ma Deposition resumes; Coastal Plain strata begin to be deposited

35 Ma Impact of Chesapeake Bay bolide

2 Ma Pleistocene “ice ages” begin; lower sea level; Potomac incision

0.00000 Ma GSW field trip to Corridor H happens for the 1st time

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Overview of east coast tectonics over the past 1.2 billion years

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Valley & Ridge:

The Valley and Ridge province is the fold-and-thrust belt of the Alleghanian

Orogen. While the core of this late Paleozoic mountain belt was to the east (in the

Piedmont, with the displaced sliver of basement rock, the Blue Ridge province, in

between, the flanks of the young Appalachian mountains were in this zone of

“thin skinned tectonics.” That is to say, the rocks you see today in the Valley and

Ridge were deposited during the early and middle Paleozoic, and were then

intensely deformed (folded, faulted, and cleaved) during the late Paleozoic

Alleghanian Orogeny. Thus, the rocks of the Valley and Ridge province tell three

distinct stories: (1) a sedimentary story, about changing depositional conditions

over time, (2) a deformational story about what happened to those older layers

when Africa smashed into North America, and (3) a story of differential erosion,

which takes all those crumpled up strata and expresses them on the Earth’s

surface as valleys and ridges. There is also a part (4), unrelated to the rest: weird

igneous intrusions in the Eocene. That won’t be covered on this trip.

The depositional history of the Valley and Ridge province:

The sedimentary record of the Valley and Ridge stretches from the Cambrian to

the Pennsylvanian period. Dozens of different formations (almost 50 in our area!)

have been named and described, and a place assigned to them in the

stratigraphic sequence for the area (see stratigraphic column by Lynn Fichter,

next page). The names and character of the formations vary not only temporally

(i.e., in the one direction of time), but also laterally, both across strike of the

Appalachian Mountains (i.e., east to west) and also along strike of the

Appalachian Mountains (i.e., north to south). In southwestern Virginia, Permian

evaporite deposits may be found, for example, but both the Permian and the

evaporites are missing in northern Virginia, Maryland, and northern West Virginia.

Sedimentologists can read a great deal of information from these rocks. The

conditions under which they accumulated changed dramatically over time,

shifting from shallow Bahamas-like conditions (carbonate sedimentation in a

tropical climate) to a deep ocean basin smothered by turbidity currents, to a

pristine white sandy beach, to rivers meandering across an ancient floodplain.

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The conditions changed so frequently over the 250 million years over which

sediments accumulated here that you would be forgiven if it looked at first glance

like a hopeless mishmash of ever-shifting conditions. However, upon applying a

regional perspective to the sequence, a pattern emerges (see figure below).

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The strata of the Valley and Ridge can be said to represent just four main

sedimentary themes: (1) passive margin sedimentation in the aftermath of

Rodinia’s breakup, (2) the “dirty” clastic influence of Taconian mountain-building,

(3) a return to passive margin sedimentation in the ‘tectonic calm’ between the

first and second pulses of Appalachian mountain-building, and finally, (4) a return

to active margin sedimentation (things get “dirty” again) during the Acadian

Orogeny.

Therefore, telling the story in chronological sequence would begin in the

Cambrian period of geologic time, after the Chilhowee Group (Blue Ridge)

recorded the Sauk transgression. While the majority of the North American

continent was flooded by this epeiric sea, the shoreline was in the middle of the

continent. Our region lay far, far from the clastic influence of the land. With no

sand and no mud to deposit, the only available sediment was the dissolved ions in

the ocean. Under warm, tropical conditions, limestone was deposited as calcite

precipitated from seawater. Ooids formed where the waves were able to reach

down and roll these little precipitated nuggets back and forth, giving them a nice

even coating of calcite. Here’s an example:

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Stromatolites (fossil microbial mats that “dome” upward toward the paleo-sun)

grew in profusion. Periodic tropical storms (cyclones/hurricanes) temporarily

increased the water energy, resulting in carbonate-rip-up clast conglomerates and

hummocky cross-stratification.

At the famous Tumbling Run outcrop south of Strasburg, we can observe today

how the depositional setting changed under the increasingly clastic influence of

the late-Ordovician Taconian Orogeny. Here, we transition from the intertidal to

subtidal sedimentary facies of the New Market limestone into the reefal mound

facies of the lower Lincolnshire formation, marked by much thinner bedding and

the presence of fossils. The middle Lincolnshire shows darker color and pyrite,

indicating deepening water conditions (and accompanying anoxia). It also

features distinctive black nodules of flint (derived from siliceous sponge spicules,

perhaps?). In the upper Lincolnshire, shale comes into increasing dominance, and

then bentonite (devitrified volcanic ash) appears. This marks the transition to the

overlying Edinburg Formation, which has a strong mud component and carbonate

turbidites, interpreted as a basin-edge environment (deeper water). It crops out

with a distinctive “cobbly” weathering pattern. To sum up these changes, please

see the summary diagram showing both (a) features of the Tumbling Run outcrop

and (b) paleo-depositional interpretations by Fichter and Diecchio (1986), next

page.

Shallow water features in the Conococheague

Formation (Cambrian limestone/dolostone):

stromatolites (left); rip-up clast conglomerate (right).

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The bentonites weather out in negative relief: if you reach into one of those

recessive overhangs, you will pull out a sticky, pulpy yellow mess. This is what 470

million year old volcanic ash looks like, long after the glass in the ash has reacted

to produce clay minerals. The bentonites of the Edinburg include the Millbrig bed,

a bentonite layer that has been correlated with a European bentonite of about

the same age. This European ash layer goes by the name “The Big Bentonite,” and

the correlated total unit has been interpreted by some workers as representing

the pyroclastic fallout of the largest volcanic eruption during the entire

Phanerozoic eon! However, recent geochemical fingerprinting of trace elements

(Mn, Mg, and Fe) in the benonites, show two distinct element populations, which

suggests that though they are near twins, they are in fact two separate events

which distinct feeder magma bodies.

The tectonic interpretation for this “muddying of the waters” was the collision of

ancestral North America with a volcanic island arc. Subduction was closing the

Iapetus Ocean at this time (See paleogeographic map on page 43), and eventually

it brought a small volcanic archipelago into collision with the continent. This

orogeny, the Taconian Orogeny, named for the modern-day Taconic Mountains of

upstate New York (where it was first studied systematically), was the first of the

three pulses of Appalachian mountain building that (a) closed the Iapetus Ocean

and (b) helped to assemble Pangaea.

While we can -observe the “roots” of the Taconian Mountains on the Billy Goat

Trail near Great Falls, in the Valley & Ridge we get a different perspective on the

same event: the sedimentary perspective of the basin adjacent to the rising

mountain range.

Overlying the Edinburg Formation is the Oranda Formation, a shale with a still-

significant limy component, and overlying that is the voluminous Martinsburg

Formation, a shale and graywacke package 3200 feet thick that is interpreted as

deep marine turbidites. At first, the Martinsburg records distal submarine fan

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deposits (far away from the source of the sediments), but as you work your way

up higher in the sequence, you will find coarser and coarser material, implying

that (a) the basin is filling up and/or (b) the mountains that supply the sediment

are getting bigger/closer. The Martinsburg strata are moderately calcareous in the

lower part of the formation, but they are entirely clastic (sand and mud) in the

upper part. The transition from passive margin sedimentation to active margin

sedimentation is complete! While mountains were being raised above

Washington, DC, Interstate 81 was at the bottom of a deep marine basin, with

turbidity current after turbidity current pouring in from the east.

Eventually, this marine basin was filled with what Alpine geologists dubbed

“flysch” (i.e., the turbidites of the Martinsburg Formation) and it was topped off

with river and floodplain deposits - so-called “molasse.” The Taconian molasse is

known as the Juniata Formation, a distinct package of redbeds. (It looks virtually

identical to the Hampshire Formation redbeds, so prominent on Corridor H.)

Graded bedding and planar

(high-velocity) laminations

in a graywacke turbidite of

the late Ordovician-aged

Martinsburg Formation,

South Page Valley Road.

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coarse

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The next unit in the sequence is a 2000’ thick formation of quartz sand, with

smaller amounts of quartz pebbles and shale. This is the Massanutten Sandstone

in the Shenandoah Valley. Somewhat ridiculously, it’s known as the Tuscarora

Formation in every other mountain range to the west of the Great Valley.

This clean, mature quartz sandstone signals that the Taconian Orogeny had ended

by the time of its deposition (Silurian). A new subduction zone had formed, but it

was still a long way out in the Iapetus Ocean, far away from disturbing the local

peace. As the sediment supply dwindled, the deposition shifted back to

carbonates again; the Tonoloway Formation and the Helderberg Group of

limestones and dolostones came next. These are relatively thin in the east, but

thicken to the west. In some of the formations, distinctive indications of shallow-

water conditions may be found, including stromatolites, mudcracks, and halite

casts.

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Quartz sandstone and quartz

pebble conglomerate of the

Silurian-aged Massanutten

(Tuscarora) Sandstone. This

unit suggests a return to

passive margin deposition.

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Overlying this Silurian to Devonian passive-margin

sequence is another clastic sequence, part of the

“Catskill clastic wedge,” a thick sequence of sand,

mud, gravel, and other sedimentary debris that came

off another mountain range that was raised to the

east. This time the mountain-building was due to the

Acadian Orogeny, an episode triggered by ancestral

North America’s collision with a microcontinent called

Avalonia. To judge by the sheer amount of sediment it

produced, the Acadian Orogeny was a much bigger

event than the earlier episode of mountain-building,

the Taconian Orogeny. It shows a similar signature:

deepening of the sedimentary basin adjacent to the

mountain belt, turbidity currents (flysch), redbeds

(molasse), and plenty of sandstone, conglomerate, silt,

and clay.

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Mudcracks in tidal carbonates of the

Silurian Tonoloway Formation, Baker

Quarry, old route 55, West Virginia.

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Salt casts in Tonoloway dolostone,

Corridor H, West Virginia

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The structural geology of the Valley and Ridge province:

The Valley & Ridge province is not merely a stack of Paleozoic sedimentary layers,

however. These strata were intensely deformed during the final (and biggest)

phase of Appalachian mountain-building, the late Paleozoic Alleghanian Orogeny.

This massive continent-to-continent collision between Africa and North America

(see paleogeographic map on page 13) started in the Pennsylvanian period and

lasted well into the Permian, from roughly 300 to roughly 250 million years ago.

This collision had multiple effects: it generated

huge thrust faults in the Piedmont, snapped off

the Blue Ridge and thrust it westward

(metamorphosing it, too), and folded, faulted,

and cleaved the strata of the Valley & Ridge.

These folds occur on the regional scale (e.g., the

Massanutten Synclinorium), which is so large

that you can’t ever see the whole fold, but it can

be mapped out through carefully tracking the

orientation (strike and dip) of rock layers

exposed in isolated outcrops, such as these

dipping layers of Massanutten Sandstone

exposed above Red Hole on Passage Creek.

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Flute casts on the bottom of a

turbidite layer, Brallier Formation

(Devonian), Corridor H, WVA.

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The rocks along Corridor H demonstrate folding and faulting on a more moderate

scale: the “meso-” scale of outcrops. We will spend the day examining these

strata and the structures which improved them.

Sideling Hill, Maryland,

is a famous syncline:

another example of the

profound deformation

that Africa’s impact

imparted to the

sedimentary layers of

the Valley and Ridge.

However, even good old

Sideling Hill is starting to

show its age, growing plants in profusion and falling apart. What Sideling Hill

looked like 20 years ago, Corridor H’s numerous outcrops look like today.

Anticline / syncline pair in

Tonoloway limestone, roadcut

along Corridor H, West Virginia.

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Bedding / cleavage relationships in

upper Edinburg Formation limestone

and shale/slate along Route 340, north

of Front Royal. Bedding dips

moderately to the right (southweast))

while sleavage is subvertical. Note the

cleavage deflection through layers with

different proportions of clay.

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A few words about differential weathering:

In the humid temperate climate of the mid-Atlantic region, shale and limestone

weather rapidly, producing valleys, while quartz-rich sandstone and conglomerate

weather more slowly, producing ridges. The simple facts of quartz’s (a) hardness

and (b) chemical equilibrium explain most of the topographic variation that we

see today in the Valley & Ridge. Almost every ridge is held up by quartz-rich rock,

while almost every valley is underlain by limestone or shale.

The outcrop pattern determined by this deformation is the primary control on

modern landscape in the Valley & Ridge province. For instance, the tough

sandstone of the Massanutten Formation weathers out as the distinctive ‘fence’-

like ridges of the Massanutten Mountain system:

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It’s also worth noting that extensive karst topography has developed in the

carbonate strata of the Valley & Ridge, both the Cambrian-to-mid-Ordovician

section and the late Silurian-to-Devonian section. Many of these caves are “wild”

while others have been developed as commercial destinations, such as Luray

Caverns. Natural Bridge is an example of what was once commercial, now

destined to be a new state park.

Examples of

Shenandoah Valley

karst:

(Left) Natural Bridge,

Virginia, a natural

bridge.

(Right) Luray

Caverns, with

Callan’s family for

scale.

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Alleghany Plateau:

The Alleghany Plateau is west of the Valley & Ridge province. Essentially, it

preserves slightly younger sedimentary strata (of Mississippian and Pennsylvanian

age) in a less-deformed state than the Valley & Ridge’s fold and thrust belt. We

will visit the very easternmost edge of the Alleghany Plateau, at the western

terminus of Corridor H. You’ll know it when you see it because the entire

escarpment is lined with massive white three-bladed wind turbines.

The Alleghany Plateau (as well as the other Appalachian Plateaux) is just a less-

deformed version of the Valley & Ridge. Its strata are not as folded or faulted, and

therefore were never uplifted to the extent of the Valley & Ridge. The uppermost

layers are subsequently younger in age.

Coal is an important sedimentary rock that formed at this time (the

“Carboniferous”), and mountaintop-removal coal mining is practiced just west of

our westernmost field trip stop.

This coal is the compressed remains of ancient terrestrial plants, preserved in

swampy “bayou” environments along the ancient shoreline. A quick search on the

area will reveal many plant fossils.

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In places, possible mass transport deposits can be observed (submarine

landslides), and marine deposits interfingering with terrestrial ones. These

features are all consistent with the relatively rapid small-scale sea level changes

that characterize the Carboniferous. These “cyclothems” were triggered by

glaciation switching on and off in southern Gondwana, and resulted in the

repeated drowning and burial of low-lying swamp vegetation along the coast.

These ancient climate changes are the proximal reason such a wealth of coal

formed around the world during the Pennsylvanian, an energy source that would

drive the Industrial Revolution hundreds of millions of years later.

Pennsylvanian-aged coal of

the Alleghany Formation

(overlain by sandstone),

Corridor H, West Virginia.

C.B

entl

ey p

ho

to

C.B

entl

ey p

ho

tos

(bo

th)

Plant fossils in the

Alleghany Formation,

Corridor H, West

Virginia.

28