GEOLOGY OF CORRIDOR H
Geological Society of Washington Fall field trip, 2014
Leaders: Callan Bentley (NOVA), Dan Doctor (USGS), Alan Pitts (Univ. of Camerino)
Geologic provinces of the Mid-Atlantic region
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 midAtlantic 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.
2
Map by Chuck Bailey, 1999
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.
3
Though this map is centered on
Virginia, it also includes relevant
locations that we will visit in
adjacent states and DC.
Overview:
Map by Ron Schott, 2014
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:
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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.”
550 Ma
At this time, ancestral North
Ancestral
America was south of the
Europe
equator, and the future MidAtlantic region of the
Iapetus Ocean
continent faced southwest
rather than southeast, as in
Ancestral
the modern day. The paleoNorth
geographic map by Ron
America
Blakey for 550 Ma shows this
well.
Iapetus Ocean
As the Iapetus Ocean
widened, oceanic crust grew
off the rifted edge of
Amazonia /
Congo
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cratons
Map by Ron Blakey, 2006
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.
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 @ 730565 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
Ancestral
North
America
Iapetus Ocean
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Map by Ron Blakey, 2006
Sauk Sea
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.
290 Ma
Ancestral
Europe
Ancestral
North
America
Mountain Belt
Ancestral
Africa
Young Appalachian
Ancestral
South
America
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.
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Map by Ron Blakey, 2006
PANGAEA
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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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
Washington Post, 1998,
courtesy of Bill Burton
Wilson Cycle #1
Wilson Cycle #2
In summary, the timeline of key events for the Mid-Atlantic region is:
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Marshak, 2009
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.
12
Lynn Fichter , JMU, 1991 (reformatted 1996)
13
C.Bentley diagram
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.
C.Bentley photo (EPCC pen for scale)
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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Shallow water features in the Conococheague
Formation (Cambrian limestone/dolostone):
stromatolites (left); rip-up clast conglomerate (right).
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.
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C.Bentley photos (both)
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.
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 stillsignificant 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
17
Fichter and Diecchio, 1986
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.
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.
Graded bedding and planar
(high-velocity) laminations
in a graywacke turbidite of
the late Ordovician-aged
Martinsburg Formation,
South Page Valley Road.
coarse
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.)
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C.Bentley photo
fine
Quartz sandstone and quartz
pebble conglomerate of the
Silurian-aged Massanutten
(Tuscarora) Sandstone. This
unit suggests a return to
passive margin deposition.
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 shallowwater conditions may be found, including stromatolites, mudcracks, and halite
casts.
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C.Bentley photo
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.
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Salt casts in Tonoloway dolostone,
Corridor H, West Virginia
C. Bentley photo
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.
Alan Pitts photo
Mudcracks in tidal carbonates of the
Silurian Tonoloway Formation, Baker
Quarry, old route 55, West Virginia.
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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C.Bentley photo
C. Bentley photo
Flute casts on the bottom of a
turbidite layer, Brallier Formation
(Devonian), Corridor H, WVA.
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.
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C.Bentley photo
C.Bentley photo
Anticline / syncline pair in
Tonoloway limestone, roadcut
along Corridor H, West Virginia.
C.Bentley photo
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.
C.Bentley diagram
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 lessdeformed 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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C.Bentley photo
Pennsylvanian-aged coal of
the Alleghany Formation
(overlain by sandstone),
Corridor H, West Virginia.
C.Bentley photos (both)
Plant fossils in the
Alleghany Formation,
Corridor H, West
Virginia.
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.
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