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GOL 135: Geology of Sideling Hill and Paw Paw Region, MD

NOTE #1: Download the podcast so you can listen (and learn!) while driving out to Sideling Hill on Saturday morning.
NOTE #2: You can print this webpage out easily as a PDF file (click here).

Stop # 1: the Sideling Hill roadcut, Interstate 68, near Hancock, Maryland

Recommended reading: David Brezinski's (1994) website on the geology of Sideling Hill (also available as a printable PDF file)

Aerial photograph of the Sideling Hill area. Interstate 68 cuts through the mountain on its way west from Hancock, Maryland. The dramatic exposure of rocks at the roadcut inspired a geologic visitor's center, located on the eastern side of the mountain and the northern side of I-68.

Modified from a Google Earth image.

Close-up aerial photograph of the Sideling Hill roadcut (as seen in a more zoomed-out perspective in the previous image). Prominent here are the parking areas for both eastbound and westbound I-68. The pedestrian bridge crossing the highway is obvious in this photo, as are the four "berms" which are notched into the face of the outcrop.

Modified from a Google Earth image.

Here is what a small scale model of the Sideling Hill roadcut looks like. This model is on display at the Visitor's Center. Note the different "berms" on the face of the outcrop. We will be walking along the first berm.
And here is the outcrop itself, as viewed from the pedestrian bridge over I-68. Pictured here is the northern outcrop at Sideling Hill. (There is another one on the other side of the highway). The prominent down-turned fold (called a "syncline") is obvious because of the different properties of the rock layers. Some are lighter in color, some darker. Some are more resistant to weathering, and so stand out in high relief. Others are more susceptible to weathering, and so they are "etched" into the face of the outcrop.

Here is a geologic cross section showing what we will see on the northern (south-facing) roadcut. You will note that there are six main kinds of sedimentary rocks exposed at Sideling Hill, color-coded on the left. These are not their colors in real life! There are two "formations" exposed at Sideling Hill, both deposited in the early Mississippian period of geologic time: (1) the lower, older Rockwell Formation and (2) the upper, younger Purslane Formation. The Rockwell Formation was deposited at the edge of a shallow sea adjacent to an eroding continent. The Purslane Formation is mainly a terrestrial deposit, coming from rivers that were draining off of the higher-elevation lands to the east.

Drawn after Brezinski (1994), Figure 2

The Rockwell and Purslane Formations are likely different aspects of the same system. When they were being deposited, the area would have looked something like this as viewed from the west. The highlands to the east were shedding off sediments. Rivers carried those sediments to the west, to a shallow inland sea. At the same time the sea was depositing sediments, so were the rivers. As time went by, the pile of sediments moved westward, or sea level dropped, causing the rivers to start depositing on top of the area where the sea had deposited earlier.

This paleogeographic map shows approximately what North America would have looked like about 360 million years ago, just before Sideling Hill's sedimentary layers were deposited. It shows the area of elevated lands (Acadian highlands) to the east, and the shallow continental sea (Kaskaskia Sea) to the west. The Acadian Highlands are mountain ranges and uplifted lands that were raised during the Acadian Orogeny, a mountain-building event in the Devonian period of geologic time. Sediments have to come from some place else: these Acadian highlands were subjected to weathering and erosion, which caused copious amounts of sediments to be shed off in every direction. Some of those sedimentary particles now make up the rock strata of Sideling Hill. Sideling Hill's sedimentary layers were deposited right at the ancient shoreline that divided marine ("underwater") from terrestrial ("up on the land"). Map by Ron Blakey, Northern Arizona University.

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

This is a generalized look at the stratigraphic stack of the Valley and Ridge province, if it were all unfolded. (A) Note that the stack is thicker to the east, indicating that the source of most of these sediments was to the east, not the west. Sideling Hill's Rockwell and Purslane Formations are towards the top. (B) Here is the same stack of sediments, color-coded to show how they relate to major tectonic events on the east coast. You will note that from the Devonian period through the Mississippian period, all the strata are color-coded red, which corresponds to the Acadian Orogeny to the east. This orogeny built up mountains and highlands, which were then subjected to erosion. Erosion removed sediments from the mountains (in today's Piedmont) and brought them here to be deposited in various environments.

The Rockwell Formation was deposited along the shoreline of this shallow sea, which flooded much of the North American continent from mid-Devonian to mid-Missisippian time. Continent-wide, it is known as the Kaskaskia Sea, but locally in the Sideling Hill area it is referred to as the Riddlesburg Sea. The Riddlesburg Shale is one layer in the Rockwell Formation which is definitely marine. The rock layers at Sideling Hill record the Kaskaskia Sea's apparent maximum extent to the east, and then it receded to the west. (Once the sea had receded, rivers from the east had to flow further to reach it.)

Fossils of marine-dwelling creatures called brachiopods (shown here) and bivalves are both found in parts of the Rockwell Formation. Plant fossils are much more common, however, indicating the proximity of the land.

Here's an image of some living brachiopods, sea-dwelling creatures that superficially resemble clams or scallops. Unlike those more-familiar creatures, however, brachiopods are connected to the bottom of the sea by a fleshy stalk. They filter the seawater for particles of food using an organ called a lophophore, which is shaped something like a spiralled feather.

One of the most intriguing rock layers at Sideling Hill is exposed only on the westernmost portion of the outcrop. Called a diamictite, this rock unit is unusual because it has several different sizes of sedimentary particle in the same rock. With most sedimentary rocks, they are better "sorted" than this: a sandstone, for instance, consists only of sand-sized grains, with none larger than sand, and none smaller than sand. A siltstone consists only of silt. Traditionally, geologists correlate sedimentary grain size with the "energy" of the water that deposited the sediments. Fast-moving water can carry bigger particles of sediment (gravel, etc.), so if we find big particles, that tells us the water must have been moving fast. On the other hand, really small particles would be carried away by a current of fast-moving water: small particles only get deposited when the water is low-energy (that is, when it is calm).

So what's up with this diamictite? It contains both big and small particles! Its origin is something of a mystery. Two possible ways to form diamictites both involve rapid dumping of large amounts of sediment without any longer-term "reworking" by currents. Option #1 is for a submarine landslide to dump a big package of mixed-grain-size sediments all in one massive event. Option #2 is for a melting glacier to dump that same package of mixed-grain-size sediments. North America was on the equator at this time, so it's more likely that it was a submarine landslide, since that doesn't require glacial ice. However, there was an Ice Age of sorts that just ended in the late Devonian (mainly centered on South America, which was then attached to Africa), so it wouldn't be impossible. Like many questions in geology, solving the origin of the diamictite will take further study and additional evidence. Scale (on notebook cover) is in inches and in centimeters.

Hand sample of diamictite, possibly the most interesting rock at Sideling Hill. As above, note the mix of large pebbles and cobbles (large white blobs) with much smaller sand and dark grey mud. Also note the rusty "weathering rind" which obscures part of the sample. Long axis of sample is 8.5 inches.

Brezinkski (2008) interprets this diamictite as evidence of late Devonian glaciation, and makes a pretty compelling case in this paper that this glaciation was a regional phenomenon, which makes it less likely to be a localized landslide.

Later deposits of the Rockwell Formation are dominated by "low-energy" deposits: silt and clay that were laid down in calm, quiet water. The small sedimentary particle size is preserved today as layers of shale and siltstone. These dark shale and siltstone layers are interbedded with higher-energy deposits of sandstone. The dark color of the shale indicates a high proportion of unoxidized organic material. These may have been deposited as mudflats right at the coastline of the ancient Kaskaskia Sea. Long axis of sample is about six inches.
As time went by, sea level began to drop, and the shoreline of the Kaskaskia Sea retreated to the west. As a result, the area which was below sea level was now above sea level. Geologists refer to a global sea level drop like this as a "regression." At this point in the Sideling Hill sedimentary record, we see a switch from the coastal Rockwell Formation to the river-deposited Purslane Formation. The Purslane is dominated by sandstones like this one. These sandstones were likely deposited by higher-energy river water. (Sometimes geologists use the term "fluvial" to describe river deposits.) Rivers only flow in one direction (downhill): that is, they have a current. We can read the ancient current direction from certain sedimentary structures in the rock called cross-beds. In this photo, the main bed of sandstone is oriented horizontally: it is parallel to the edge of the field notebook. The cross-beds are oriented at a diagonal to the main bed: they come up from the lower left and stop at the bottom of the upper bed in the upper right. Scale (on cover of notebook) is in inches and centimeters.
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 Purslane sandstone deposits at Sideling Hill. Because the real cross-beds 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-BEDDING BEING DEPOSITED.
Another indication that the current strength ("water energy") was increasing is deposits of conglomerate, a poorly-sorted sedimentary rock which consists of large rounded cobbles and pebbles set in a finer-grained matrix (in this case, the matrix is sand). Because pebbles are heavier than sand, this layer of conglomerate represents a period of high-energy water coming through this area, leaving behind pebbles as evidence of its passage. Scale is in inches and centimeters.
Coal is an important rock in the Appalachians, because it contains energy sequestered by ancient plants. Like modern plants, ancient plants living in swamps captured sunlight's energy via photosynthesis. If they were buried under low-oxygen conditions, that energy could be preserved over millions of years. We can now come along and dig up coal and reverse that ancient photosynthesis by burning the coal: energy is released (as well as some of the CO2 that the plants pulled out of the Mississippian period atmosphere). The coal we find at Sideling Hill is the compressed remains of an ancient swamp, probably a swamp similar to the bayous of modern-day Louisiana, right along the coastline. Long axis of sample is about four inches.
Further evidence of the terrestrial nature of the Purslane Formation is found in the many plant fossils it contains. These plant fossils include bark fragments and twigs. Here is a sample of the bark from a lycopsid, also known as a "scale tree" from the Mississippian-aged swamp forest which once occupied this area.
Reconstructions of some lycopsids from the middle Paleozoic era: Lepidodendron (far left, Late Carboniferous, ~50 m tall), Sigillaria (left, Late Carboniferous, ~40 m,), Valmeyerodendron (middle top, Early Carboniferous, 0.6 m), Protolepidodendron (top right, Middle Devonian, 0.2 m), Chaloneria (bottom middle, Late Carboniferous, 2 m), Pleuromeia (bottom right, Triassic, 2 m) and Isoetes (bottom far right, extant, 30 cm). Images are copyrighted © 2005, Dennis C. Murphy, from his incredible "Devonian Times" website. Check it out!

Though the sedimentary layers exposed at Sideling Hill were deposited as flat, horizontal sheets of sediment, they are obviously no longer in that shape. They have been folded. Geologists classify folds many different ways, but for our purposes, it is enough to note whether the fold is bent upwards (an "anticline") or downwards (a "syncline"). Because the Rockwell and Purslane Formations are bent into a downward-oriented fold at Sideling Hill, we refer to the mountain as a structural syncline. Note that synclines and anticlines are different aspects of the same thing: each fold shares a "limb" with its neighbor.

Folds like these are caused by compressional stress. In other words, the rock layers were squeezed from the sides, causing them to buckle up in folds. The folds were oriented perpendicular (at a 90º angle) to the direction of maximum compressive stress. Like other ridges in the Valley and Ridge province, Sideling Hill is oriented from the northeast towards the southwest. This indicates that it was squeezed from the southeast towards the northwest.

What was the cause of the compressional "squeezing" that folded up these rock layers? By about 315 million years ago, another phase of mountain building had started. This orogeny, called the Alleghenian Orogeny, would compress the rocks of eastern North America until about 250 million years ago. The cause of the orogeny was the collision of Africa with North America, an event which closed the Iapetus Ocean for good and completed the assembly of the supercontinent Pangea. Map by Ron Blakey, Northern Arizona University.

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

This folding was mainly accomplished by the many sedimentary layers sliding over one another. In other words, the layers deformed more like a bent phonebook, and less like silly putty. The evidence for this is preserved as mineral growths called slickensides which formed between the sliding layers. This sample of slickensides is from the Sideling Hill Visitor Center museum. Note that all the crystal fibers point in the same direction. Running your finger across this specimen from background of the photo towards the foreground would be easy because of this allignment, but it would be much rougher to try and run your finger across the sample from the foreground towards the background.

This kind of folding, where layers slide over the layers beneath them is called "flexural slip." Here, the same concept is illustrated with a stack of old issues of National Geographic. The right side of the photo shows that the edge of each magazine slides down towards the fold hinge, away from the edge of the magazine above it. The left side of the photo shows with even more layers (hundreds of pages, as opposed to six bound magazines), you don't have to have as much slip between each layer to get the same effect.

Now swap out rock layers instead of magazines, and you get a picture of how Sideling Hill attained its synclinal shape!

Click here for an animation showing how flexural slip is acheived by sliding of layers. (Requires the Flash plug-in.)

Note that not all the rock layers ("strata") in the outcrop stand up equally well to weathering. In the space of less than 25 years, you can see a marked difference between the weathering styles of shale (marked with arrows) and sandstone. The shale is much less stable, and is easier to erode away. As a result, the shale layers are recessed into the face of the outcrop. The sandstone layers are enriched in the mineral quartz, which is very stable at Earth-surface conditions. The quartz-rich sandstone is therefore more resistant to erosion. Because it is tougher, it stands out in high relief on the face of the outcrop. Geologists refer to this as differential weathering. Different materials weather away at different rates.
The phenomenon of differential weathering is also important for understanding why the Sideling Hill syncline is preserved today as a mountain, rather than a valley. Because the Purslane Formation is richer in sand (and therefore super-stable quartz), it is "tougher," and more resistant to erosion than the rock layers underneath it. The Rockwell Formation is mostly shale, and so it would be more susceptible to weathering, if only it weren't sheltered by a "helmet" of strong Purslane Formation above it. The Purslane "protects" the Rockwell underneath it. The ridges of the Valley and Ridge province are made of resistant materials like sandstones. The valleys of the Valley and Ridges are valleys because they are underlain by weaker, more easily eroded rock types (like shale and limestone). The Appalachian Mountains used to be a mighty range, fully on par with the modern Alps or Himalayas. At their peak, when Sideling Hill's strata were folded, the Appalachians would have looked something like the top picture in cross-section. As time went by, however, mountain-building ceased and erosion took over. Rock types that were resistant to erosion rose up to become topographic high points (mountains), while nonresistant materials were etched away to make topographic low points (valleys). Note that the Blue Ridge is made out of different rocks than the Valley and Ridge sedimentary layers to its west (including Sideling Hill). Eventually, the Appalachians will be ground down to their roots, with very little topographic relief, much like the Canadian interior.
Because its topmost Purslane Formation was so resistant to weathering and erosion, Sideling Hill stood up above the landscape, making a long ridge oriented from the northeast towards the southwest. The ridge of Sideling Hill was an 80-mile-long obstacle to interstate transit. The old route around the mountain involved a dangerous hairpin turn. Compare it to the new roadcut in this aerial photo. Modified from a Google Earth image.

In 1983 construction began on a notch in the mountain. This notch allowed Interstate 68 to pass more directly from the east side of the mountain to the west, but it also exposed a great syncline full of interesting rocks!

The roadcut exposes about 450 stratigraphic feet of the Rockwell Formation, and about 350 stratigraphic feet of the Purslane.

The Sideling Hill roadcut cost about $20 million to build, and involved the use of about 5.2 million pounds of explosives. It took 16 months to finish the shortcut through the mountain, removing 10 million tons of rock in the process!

Stop # 2: the Paw Paw Tunnel, C&O Canal National Historical Park, Maryland

Recommended reading: "Paw Paw Tunnel led the C&O Canal under a mountain," Ann Sagi Ward, Baltimore Sun, August 18, 2005.

Just to the west of Sideling Hill is another dramatic geologic feature. This one was caused by regional uplift of the mid-Atlantic Region. As the land was uplifted, it triggered the Potomac River to incise (cut down) into the landscape. This produced a stunning set of entrenched meanders. This particular set, the most prominent along the Potomac's course, are called the Paw Paw Bends. They are located near the town of Paw Paw, West Virginia (whitish blob at bottom center). Rivers meander when they have established a wide floodplain, and are essentially "equilibrated" to their base level (the deepest level they can erode to). In other words, the meandering shape is a symptom of an old river. Yet deep channels, which we also see here, are a symptom of a young river. When the land is uplifted, base level drops, and the river starts incising again -- it deepens its channel. At Paw Paw, that deepening simply took place in the pre-existing river shape: the meander. These meanders are now hundreds of feet deep. They are no longer active meanders (which move from side to side): they are "locked in place," or entrenched. Geologists refer to rivers like this as being "rejuvenated" -- old rivers that have been made young again. Also: note the other entrenched meanders in this image: the Cacapon River to the east, and Town Creek to the west. Modified from a Google Earth image.

A topographic map showing the southern area of the Paw Paw Bends with some specific details highlighting the C&O Canal's Paw Paw Tunnel. When building the Canal, engineers were faced with a difficult choice: To get from point A to point B, they could either follow the looping shore of the Potomac River (which would mean a longer Canal), or they could take a shortcut by tunneling through the mountain (which would be shorter but maybe a little more work). Like the later engineers who planned the Sideling Hill roadcut, the Canal's engineers chose the short, less curvy path.

However, It turns out to have been a lot more work. Because of the structure of the rocks in the tunnel area, constant collapses of rock and other structural problems meant the tunnel took 14 years to build! Though the Canal was six miles shorter, the rock was incredibly tough to tunnel through. They only managed to dig through 12 feet every week! They probably should have chosen to follow the river's bank, after all.

Contour interval = 20 feet

The decision to excavate the tunnel led Canal builders straight through a thick deposit of the Devonian-age Brallier Formation (somwehere between 380 and 360 million years old). The Brallier Formation is a shale, folded into anticlines and synclines like everything else in the Valley and Ridge province. Here, the Brallier outcrops as tilted beds at the upstream entrance to the C&O Canal's Paw Paw Tunnel.

The tunnel is 3,118 feet long (0.6 mile).

Here is the view at the downstream exit of the Paw Paw Tunnel: the Canal leaves the tunnel and enters a narrow valley called the "Tunnel Hollow." The reason this hollow exists is because of the unstability of the tilted beds of Brallier Formation shale. While they tried to make it into a tunnel, the roof kept collapsing, resulting in this long, linear artificial valley.
Here is a view of those tilted beds in the Brallier Formation shale, just downstream from the tunnel exit. The beds are here tilted at an angle of about 55º from the horizontal. When Canal workmen dug into these layers and removed their lower portions, the upper portions slid downward like an elephant coming down a playground slide! These rock slabs are big and heavy -- without support from below, they will collapse under gravity's influence.
Polished to a high shine by the folding process, these beds of Brallier shale gleam with reflected sunlight. Note the bolts (with square heads) which pin the heavy slabs of rock in place. This "polish" effect is due to the grinding effect of individual layers of shale sliding overtop of one another during folding.
Slickensides on the surface of Brallier Formation shale beds. Note their common orientation: they all point in the same direction. Running your finger across this outcrop from top to bottom would be very rough -- your fingertip would catch on each "step" of the slickenside crystal fibers. But if you were to run your finger up the outcrop from bottom to top, it would feel smooth. Your fingertip would glide off the lip of each slickenfiber onto the top of the next one. The block of Brallier rock which slid across this exposed layer must have moved in the "easy" direction: from bottom to top. This opposite block of rock (removed by the builders of the Canal) would have had slickensides oriented in exactly the opposite direction (see explanatory cartoon below). Pen for scale.
Here's a side-view of the processes that lead to the formation of slickensides. You start off (top) with a crack between two rocks. In the case of Sideling Hill and the Paw Paw Tunnel, this could be the disconnect between two layers of rock. As the top one slides to the right and the bottom one slides to the left (lower images), a small gap opens up. Mineral deposits fill this gap as it opens with growing slickenfiber mineral crystals. Orange is the oldest part of the crystal, and green is the youngest portion (right at the "step"). In the above photograph, the lightest-colored portions of the slickenfibers correspond to the green-shaded bits in this cartoon.
Because of the tilted beds of Brallier shale, the interior of the Paw Paw Tunnel could not be just exposed raw rock. That would likley have led quickly to collapse. Canal engineers built a supporting layer of bricks on the inside of the tunnel's walls. Six million bricks were used in this one tunnel!

The white lines trace out the bedding of the Brallier Formation which has been folded into an anticline.

Can you spot the fold in this picture?

Hover over the photo with your mouse to see the bedding plane drawn in with white lines.

This anticline in the Brallier shale is exposed at the downstream exit of the Paw Paw Tunnel.

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