NOVA Annandale | Geology | Bentley | Billy Goat Trail pre-trip readings
GOL 135: Geology of the Billy Goat Trail, including Great Falls, Maryland

Here, I've compiled some photographs and graphics to help bring you up to speed with what we'll be seeing on our trip. This website will also be available to you to use after the trip, as a resource to help you write your paper. If you would prefer to print it out on paper instead of reading it on the computer screen, it will be easier to print as a PDF file.

You also might want to check out the geologic map of the Great Falls area, by Scott Southworth (USGS), 2000. The Nature Conservancy published a cool audio tour of the Billy Goat Trail in 2011, and the sections focused on the metagraywacke bedrock, Marys Wall, Pothole Alley, and river "erratics" are well worth giving a listen to.

For the information below, I've arranged things chronologically, so the oldest rocks will be presented first:

Overview & setting

The Chesapeake and Ohio Canal National Historical Park is a long, skinny national park which is located along the north bank of the Potomac River from Georgetown, DC, to Cumberland, MD. It cuts across all the major geologic provinces in the mid-Atlantic, and provides a "transect" through Appalachian geology. On this trip, we will be focusing on rock exposures in the Maryland Piedmont, near Great Falls. Along the C&O Canal, the western boundary of the Piedmont province is located at Seneca Creek. The eastern boundary is located at Georgetown, where the C&O Canal debouches into the Potomac River. You can check out the larger geologic context with this online geologic map.

Part 1: Deposition of clastic sediments in a deep ocean basin

(Above) Cross section through the Iapetus Ocean, predecessor to the Atlantic Ocean. (And as the Atlantic is named for the Greek titan Atlas, so is the Iapteus Ocean named for Atlas's father, Iapetus.) In the vicinity of the Billy Goat Trail, "dirty" (impure) sandstones called graywackes (see below) were accumulating on the bottom of the Iapetus Ocean. The ocean basin immediately adjacent to the ancestral North American margin was closing, as a subduction zone consumed the oceanic crust and brought an offshore chain of volcanic islands (a "volcanic island arc," labelled as the Chopawamsic Terrane in the above image) closer and closer to North America. Eventually, the Chopawamsic Terrane would collide with North America in a mountain-building event known as the Taconian Orogeny.

Map view of the tectonic situation about 510 million years ago, in the late Cambrian period of geologic time. "C&O" locates the Chesapeake and Ohio Canal's location on the map. (The Billy Goat Trail is within C&O Canal National Historical Park.) Note the volcanic island arc located offshore, in the Iapetus Ocean, moving closer to North America as the intervening oceanic crust is subducted. Map by Ron Blakey, Northern Arizona University.

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

(Above) Three different kinds of sandstone, each indicative of a different sedimentary environment. (A) Quartz sandstone is a mature sandstone, consisting (as the name implies) of mostly quartz. It is deposited in beach and dune settings. (B) Arkose is an immature sandstone with large angular feldspar grains. It is deposited in continental basins (rift valleys). (C) graywacke is an immature sandstone, composed of sand mixed with mud. It has a dark color, indicating its deposition in the low-oxygen environment of the deep sea. Typically, graywackes are associated with graded bedding (see below), indicating that the sediments were deposited by a turbidity current -- again, indicating the deep ocean as the site of deposition. graywacke is the original source rock ("protolith") for the Billy Goat Trail's metamorphic rocks.

Graded bedding is a characteristic sedimentary structure associated with deposition of sediments in a turbidity current. Turbidity currents are dense, sediment-laden flows which flow along the bottom of the ocean, starting from a source area adjacent to a land mass. As they slow down, they deposit their largest grains first, and their finest grains last. Turbidity currents are found today in deep ocean basins. So, when we find graded bedding in graywacke rocks, we assume it formed in the past the same way similar structures form today: in the deep ocean.

(A) schematic of the turbidity current / graded bedding depositional system.

(B) model turbidity current in a test tank, NVCC-Annandale geology lab.

(C) graded bedding preserved in meta-graywacke, Great Falls Park, Virginia (across the river from the Billy Goat Trail). Penny for scale.

Here's another graded bed in the metagraywacke exposed along the Billy Goat Trail. Quarter for scale.
Turbidity currents deposit graded beds of graywacke in submarine fans (a.k.a. "abyssal fans"), which are essentially big piles of sediment in deep ocean basins. This is likely the original environment where the Billy Goat Trail's mud and graywacke deposits accumulated.
This buckled sidewalk in Washington, DC, serves as an analogy for what sedimentary deposits (like those we find along the Billy Goat Trail) tell us. All sediments have a source, a place where other, pre-existing rocks are weathered and eroded. Gravity, aided by currents of water and wind, transport those sediments some distance, and then deposit them in a low-lying (not easily eroded) area of the landscape. These areas of deposition are sedimentary basins. Our Billy Goat trail graywacke sediments were accumulating in the Iapetus Ocean basin, between ancestral North America and the volcanic island arc.

Here you can see some of the more interesting textures in the graywacke. Clasts (chunks of rock) of many different sizes are mixed together. You can see clasts of several different compositions and compositions. Some are dark, some are light. Are these chunks part of the original depositional evironment? (perhaps a submarine landslide?) Or did they result from a tectonic reshuffling of the rocks during later continental collision? Hand lens for scale.

Part 2: The Taconian Orogeny: collision between the island arc and ancestral North America

Tectonic plates are shaped kind of like UFOs. The continental crust (thick and buoyant) underlies the continents. It is surrounded by a "skirt" of oceanic crust (thin and dense). When tectonic plates collide, the weak oceanic crust is destroyed. The oceanic crust gets subducted into the mantle (see below), but the continental crust is too buoyant to subduct. It rides high, and may collide with another continental landmass (on another tectonic plate), but it won't go down the hatch into the mantle. When the continents drift apart again, new oceanic crust is built up along their edges, like a new skirt being knit from the waist outward toward the hem..
So as the volcanic island arc began moving closer to ancestral North America, the Iapetus Ocean basin was decreasing in size. The oceanic crust underlying the basin was being subducted to the east, under the volcanic island arc. As the crust subducted, it heated up and partially melted. This generated magma which rose to feed the volcanoes. At the site of subduction, a deep ocean trench developed, and the graywacke sediments were "scraped off" the top of the subducted slab, accumulating in a big jumbled mess called an accretionary wedge. (This is kind of like a bulldozer scraping up the top layer of soil as it rumbles along.) Included in the graywacke jumble were chunks of the oceanic crust itself -- knobs and sheets which projected high enough to be scraped off (dark chunks).

The Taconian Orogeny, as viewed from above, about 450 million years ago. The Chopawamsic Terrane has begun to collide with ancestral North America, adding the volcanic rocks (basalt, well exposed at Prince William Forest Park) and sedimentary rocks (graywacke, well exposed along the Billy Goat Trail) to the eastern margin of the continent. Map by Ron Blakey, Northern Arizona University.

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

As the two pieces of crust collided, temperatures rose. As things got warmer, partial melting occured. Minerals present in the graywacke that had low melting temperatures, like quartz, potassium feldspar, and muscovite mica ("felsic minerals") all melted, and trickled out of the area. Minerals with higher melting temperatures, like plagioclase feldspar, augite, hornblende, and olivine, were left behind as solids. These minerals (the "mafic" ones) didn't melt. The magma, being less dense than the solid residue, rose to higher levels in the crust.
One of the most amazing things about the rocks exposed along the Billy Goat Trail is that they show this process of partial melting "caught in the act." These rocks are called migmatite, rock that has partially melted and then recooled. As such, migmatite straddles the boundary between "metamorphic" and "igneous" rocks. It's partially metamorphic and partially igneous. (A) the dark-colored rock is metagraywacke. The light-colored rock is granite that has been "sweated out" of the metagraywacke. Penny for scale. (B) Close up of migmatite, showing granite and "shreddy" wisps of mafic minerals left over from the metagraywacke source rock. Here you see the leftovers (the mafic residue) and the magma yielded from partial melting (light-colored granite which surrounds the residual shreds). This image reminds me of nothing so much as the process of making cheese. Curds and whey are put into cheese cloth and squeezed: the whey passes through the cheesecloth as a liquid, while the curds are concentrated in a solid mass inside the cheesecloth. Here, we can imagine the partially-molten migmatite being squeezed, with the solid mafic residue remaining in place and the granite magma oozing out to find space elsewhere. Penny for scale.
The granite derived from partial melting then mobilizes as big blobs of low-density liquid. Like the blobs in a lava lamp, the granite magma rises through denser rock, cutting across it and working its way upwards. Here, a dike of granite cuts across the migmatite that spawned it. Mobilized granitic magma rises until it reaches neutral buoyancy, or something stops it. At whatever level in the crust it stops, it cools and solidifies into solid granite. To sum up, granite is generated through partial melting at depth in the crust, and then rises to cool and recrystallize at a more shallow depth in the crust. Felsic instrusive rocks are one of the signature characteristics of mountain belts. Scale is in centimeters and inches.
Differently-shaped bodies of granite magma have different names. If it's a roughly-blob-shaped mass, we call it a "pluton" (named for Pluto, god of the underworld, since granites rise up from below). If it is a tabular mass, we call it a "dike" (from the old Norse for ditch or barrier). Examples of both plutons and dikes are found in the greater DC metro area. Here, we see a depiction of the geologic principle of relative dating by cross-cutting relationships. The granite must be younger than the sedimentary strata that it cuts across, because you cannot cut across something unless it exists in the first place. The cutter is always younger. Note also the "aura" of contact metamorphism which surrounds the granite in all directions -- as it radiates heat, it "cooks" the older sedimentary rocks all around it.
Granite dike cutting though metagraywacke, Billy Goat Trail. Penny for scale.
The importance of metamorphic foliation, as seen in a batch of delicious cookie dough. (A) My favorite kind of cookie dough, chock full of ingredients. Here, we see chocolate chips, walnuts, raisins, oatmeal flakes, all randomly oriented in a matrix of dough. (B) I squash the dough, with several important changes to the ingredients. The raisins squash flat in the direction in which I push on them. The ductile raisins therefore acquire a new shape as a result of being squished. The oat flakes don't change their shape, but they rotate into a new orientation, perpendicular to the direction in which I push on them. This is the only stable orientation for the brittle oat flakes. Overall, the cookie now has a "fabric" to it that it did not possess before squishing. Like a plank of wood splits more easily along its grain than across the grain, so too does the cookie now have a "stronger" direction and a "weaker" direction. Likewise, the graywacke and mudstones of the Iapetus Ocean attained a metamorphic foliation as a result of being squeezed in the Taconian Orogeny. This foliation is very obvious as you walk along the Billy Goat Trail. It shows up as an almost-vertically oriented "flakiness" to the rocks, indicating that they have been squeezed from the sides.
The metamorphosis of the sedimentary rocks the Taconian Orogeny produced foliated rocks. Elevated temperatures helped fuse the sedimentary grains together into solid rock, and tectonic pressures re-alligned the minerals into parallel orientations. graywacke was transformed to metagraywacke, and oceanic mud (low sand content) was transformed in mica-rich schist (pictured here). We can easily see big, scaley flakes of silver-colored muscovite mica, and we can note how all those mica flakes are oriented in the same direction (metamorphic foliation). Because our original sedimentary rocks (graywacke and mudtstone) were squeezed from the sides, the micas lined up in a vertical orientation that is easily observed in Billy Goat Trail rocks. Metamorphosis is one of the signature characteristics of mountain belts.

In certain areas protected from lichen growth and scuffling feet, you can observe some intense metamorphic fabric in the Billy Goat Trail's rocks. Downstream from the schist, we find a higher-grade metamorphic rock called gneiss. ("Gneiss" is pronounced the same as "nice.") This gneiss has coarse bands of alternating layers of mica and quartz. Here, we see that the gneissic foliation has been folded. This folding indicates at least two episodes of metamorphism/deformation for these rocks. The initial episode (green arrows) compressed the rock and formed the foliation. Later, a second episode compressed the rock from a different direction (orange arrows), folding the foliation. Photograph's field of view is about two feet.

Photograph by David Sloan.

Even if the metagraywacke wasn't heated to the point of partial melting (like in the migmatite pictures above), it was still messed up as a result of the Taconian Orogeny. Here, we see a fold in graded beds of metagraywacke along the Billy Goat Trail. Folds are one of the signature characteristics of mountain belts. Penny for scale.
Just to drive the point home, here's a few more fold photos. (A) Isoclinal fold ("isoclinal" means the limbs of the fold are parallel) in metagraywacke. Nickel for scale. (B) Folds in migmatite. Penny for scale.
There is one rock exposed along the Billy Goat Trail which is a bit more enigmatic. It is amphibolite, a metamorphic rock very rich in the mineral amphibole (a.k.a. hornblende), which shows up as black in this picture. The white portions of the rock are plagioclase feldspar. This is a very mafic metamorphic rock, with large crystals. Amphibolite occurs are large, folded tabular masses in the area of the Billy Goat Trail. An important question to ask here is "What was the protolith?" In other words, what were these amphibolites, before they were metamorphosed into amphibolites? Whatever it was had (1) a mafic composition and (2) large crystals. The answer which leaps to mind, I'm sure, is gabbro -- a coarse-grained mafic rock. Gabbro can occur as sills (large, tabular masses of mafic magma that squeeze between pre-existing sedimentary layers, in this case of metagraywacke), or it can occur as intrusive masses. If the latter, one place where we find a lot of gabbro is in the oceanic crust. If we interpret the amphibolite as being a metamorphosed slice of oceanic crust, then we are looking at the floor of the Iapetus Ocean here: the oceanic crust on top of which the graywacke was deposited! Penny for scale.

Part 3: the Acadian Orogeny adds a few intriguing structures to the region

The Acadian Orogeny was the collision of another landmass with the eastern seaboard of ancestral North America about 360 million years ago. This time, instead of a volcanic island arc (like in the Taconian Orogeny), North America crashes into a micro-continent, a block of continental crust the size of Madagascar or Japan. Geologists dubbed this chunk of crust "Avalonia," and today you can walk on the Avalon Terrane up in Newfoundland and Nova Scotia.
The Acadian Orogeny took place mainly to the northeast of here. (As the name implies, Acadian-aged rocks are well-exposed in Acadia National Park in coastal Maine.) Along the Billy Goat Trail, they left one set of important structures: a series of four parallel dikes filled with the igneous rock called lamprophyre. ("Lamprophyre" is a fancy name for what is essentially a mafic igneous rock, like more-familiar basalt or diabase). Biotite mica from these dikes has been dated using isotopes of potassium-40 and argon-40 to be about 360 million years old. It's worth noting that the mafic-composition rock which makes up the lamprophyre dikes is less stable than the older metagraywacke that it cuts across: as a result, it weathers out more easily -- making the dikes show up as a series of "slashes" in the surface of the rock face. This is a view of the Virginia side of the river, as seen from the Billy Goat Trail (which is on the Maryland side). Width of widest dike (#2) is about two feet.

This is a map view of the lamprophyre dikes, which are present on both sides of the river. Notice the pronounced lack of allignment from the Virginia side to the Maryland side. What is the reason for this offset?

(A) An early hypothesis, and still a favorite possibility, is that the dikes were originally straight, and later cut by a fault, which offset the Virginia side to the right, relative to the Maryland side. Such a fault might help to explain the extremely straight trend of Mather Gorge (see below). (B) Another possibility is that the dikes were not originally straight. After all, cracks in rock can take on many shapes; they do not have to be planar. Instead, hypothesis B suggests that their original shape was jagged, and they therefore the offset dikes do not show good evidence for a fault underneath Mather Gorge.

More on this controversy below, when we discuss the carving of the Potomac River valley.

One piece of evidence that suggests the dikes might be crooked is found by performing a detailed examination of their shape where they outcrop on dry land. These photos, courtesy of Aaron Martin (UMD-College Park Geology Department) show that the dikes are not, in fact, straight. Instead, the dikes branch and split.

Left image: the dikes on the Maryland side of the river, as viewed from Virginia. Notice how the central dike splits and branches into several "arms".

Right image: Close-up of a branching dike. One small "arm" goes off to the right, with a block of (older) metagraywacke hanging down and separating it from the larger branch of the dike on the left.

Just because the dikes branch and split doesn't mean there is no fault, but it is an important piece of evidence that dikes aren't necessarily tabular and straight.

A strong piece of evidence that supports the fault hypothesis is the exceptionally straight, narrow trend of Mather Gorge. The idea here is that the river has this anomalously straight trend because when the river was incising (downcutting), it found it easier to exploit weak, crushed-up rock along the fault zone, so the fault controlled the course of the river in this area.

Part 4: Tectonics ceases, and erosion takes over

A chronolonogical sequence of events showing the development of an unconformity surface. (A) Initial deposition of sediments -- in our case, these would be mud and sand. (B) Folding and uplifting -- in our case this is caused by the Taconian Orogeny (and later the Alleghenian Orogeny, which we don't have good evidence for along the Billy Goat Trail). The folding would have been far more extensive along the Billy Goat Trail than the modest folding shown in this image (see photos above). It was accompanied by metamorphism and partial melting. (C) After the Appalachian Orogenies ceased, the mountains were no longer being thrust upwards by tectonics. Instead, weathering and erosion took over as the dominant processes. They worked slowly, but over a long period of time, to grind the mighty Appalachians down to their roots. (D) When the land subsided, deposition took over again as the dominant process. While this image shows sea level rising, the body of water that was depositing new sediments along the Billy Goat Trail was not the ocean -- it was a river. Perhaps it was the ancestral Potomac River, perhaps it was some other river. (See below for how we know this.) The surface that divides the older, folded and metamorphosed rocks below from the younger, flat-lying rocks above is called an unconformity. It is an ancient erosional surface, and it represents a period of "missing" geologic time. During this "missing time," nothing was being added to the Billy Goat Trail area's rocks, but stuff was being taken away.

The important thing to recognize is that there is a LONG time missing in the geologic record along the unconformity. The last thing we have recorded before the unconformity is the Acadian intrusion of the lamprophyre dikes (~360 million years ago). Then we jump ahead to the age of the new sedimentary layers which are laid down on top -- they turn out to be about 100 million years old. So the unconformity surface, while physically tiny, represents about 260 million years of missing geologic time.

The sedimentary layers which we find on top of the unconformity (post-Appalachian Orogenies) are made of rounded cobbles and rounded gravels, mized with sand and clay. In sedimentary rocks, rounding tells us important information about the distance that a clast (chunk) has travelled. If it is angular, then it must be close to its source area. If it is well rounded, that means it has been tumbled a long distance. Well-rounded cobbles like the ones we find deposited on top of the unconformity in the Billy Goat Trail area came from a long distance away. Though image (C) shows a conglomerate (a rock made of rounded clasts), you should note that along the Billy Goat Trail, the rounded clasts have NOT been stuck together into a solid rock like a conglomerate. Instead, they are loose and unlithified.
Here are two boulders of quartzite (metamorphosed quartz sandstone) found on top of Glade Hill, the big hill on the Virginia side of the river. Boulders this large require a powerful current of water to transport them. They are powerful evidence that the top of Glade Hill used to be the bottom of a river. Quartzite isn't found as part of the bedrock in this area-- but it is found to the west. So the ancient river which deposited them must have been flowing from the west to the east. Since the time these boulders were deposited, the Potomac River has cut down into the landscape, leaving them stranded -- high and dry, if you will. Elsewhere, in similar deposits, Cretaceous-aged fossils have been discovered, giving us a geologic date for these deposits. That's where the 100 million year date comes from. Pen for scale.

Three pictures of boulders found along the Billy Goat Trail, each originating from somewhere else.

(TOP) A boulder of the red-colored Seneca sandstone, which outcrops in the Culpeper Basin, about 10 miles upstream of the Billy Goat Trail. This distinctive sandstone originally accumulated in a rift valley environment, when Pangea was splitting apart during Triassic and Jurassic time.

Quarter for scale.

 

 

 

 

 

 

(MIDDLE) Close-up of a boulder of the Catoctin Formation, a metamorphosed basalt from the Blue Ridge province. These flood basalts were erupted when the early supercontinent Rodinia was breaking up (and the Iapetus Ocean was being opened) about 600 million years ago. Note the white nodules: these are amygdules, lava gas bubbles that have been infilled with mineral deposits. The rock became green when later metamorphism generated chlorite and epidote minerals.

Quarter for scale.

 

 

 

 

 

 

 

(BOTTOM) A boulder of the Antietam Formation, a metamorphosed quartz sandstone from the Blue Ridge. The Antietam Formation was originally deposited as beach sand in a barrier-island type of environment, perhaps like the modern-day Outer Banks of North Carolina. It is found stratigraphically above the Catoctin Formation (middle photo) in the Blue Ridge. The Antietam is a distinctive formation because it is peppered with many Skolithos worm tubes (the light-colored dots on the top of this boulder).

Quarter for scale.

 

 

All of these boulders are from geologic units which outcrop to the west. Therefore, the river which transported them to the Billy Goat Trail must have been flowing towards the east.

Look at the shape of this one! Like the boulder immediately above, this is composed of quartzite, probably of the Antietam Formation (but no wormy traces in this one). The shape -- almost a perfect oval -- tells you something about this boulder. Namely: it's well-travelled. When a boulder-sized chunk is first broken off from its source rock, it's jagged and angular. But the further a sedimentary particle travels (down a river, perhaps), the more likely it is to get rounded. Victoria Martin's foot for scale.

Photo by Victoria Martin.

So sedimentary particle size relates to the environment where it was deposited. Strong currents of water deposit big clasts, but carry away smaller clasts. Calm bodies of water deposit small grains, but big grains never make their way out to such a calm area (what would carry them?). Running the spectrum from large to small clasts (boulders, cobbles, gravel, sand, and mud), then, are the sedimentary enivironments of rivers, beaches, and deep ocean. The further away from the continents you get, the smaller the sedimentary grains are.
Here's a close-up of one of the cobbles from this unit. Like the boulders on Glade Hill, it too is a quartzite cobble. Note the long tubes that run through the rock. These are Skolithos worm tubes, a distinctive trace fossil from the Cambrian period of geologic time. In our local area, Skolithos tubes are present mainly in the Antietam Formation, one of the rock units exposed along the western side of the Blue Ridge. To find a cobble containing these fossils along the Billy Goat Trail indicates the source of those sediments -- they came from the west! Derived by the ancestral Potomac River from a spot perhaps close to modern-day Harpers Ferry, the cobble was then tumbled downstream (making it rounded) before being deposited atop the unconformity in our area. Long axis of cobble is about six inches.

Part 5: The Potomac River incises into the landscape

After grinding down to the roots of the Appalachian mountain belt, rivers established drainages leading both east and west off the mountains. The Potomac, of course, drains to the east, eventually feeding into the Chesapeake Bay and the Atlantic Ocean. (The New River of West Virginia is an example of a westward-draining river system which ultimately leads to the Mississippi and the Gulf of Mexico.)

At some point after these drainages had become established, the land uplifted and the rivers started cutting down ("incising") into the landscape again. This produced some interesting geomorphic features, like the prominent water gaps at Harpers Ferry. At first glance, water gaps might seem perplexing: after all, they are spots where a river flows across the trend of a large, linear mountain range. But when you consider that the river is older than the topographic relief of the mountains, they start to make more sense. The river was flowing across a flat landscape, then the mountains "rose up" on either side of it, while the river kept pace by downcutting. Another striking water gap along the Potomac is located at Point of Rocks, Maryland.

Another effect of the regional uplift which triggered the Potomac's incision into the landscape are 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.

How does a river actually deepen its channel?

One way is through nickpoint propagation, shown here as a series of river profiles. A nickpoint develops when the river's base level drops. (Base level dropping could be caused by sea level dropping, or the land uplifting, or other reasons.) On the river, you'd recognize a nickpoint as a waterfall or a series of rapids. Great Falls is a nickpoint along the Potomac, for instance, and so is Little Falls.

As water falls over the nickpoint, it excavates the rock which underlies the waterfall. As rock is removed, the nickpoint retreats in an upstream direction.

If base level were to drop again, perhaps due to sea level drop like seen here, it would cause a new nickpoint to develop close to base level. As time goes by, that nickpoint will work its way upstream too.

As each nickpoint propagates upstream, it carves a new channel, a new gorge. These gorges get longer because of nickpoint propagation, but they get wider due to mass wasting. The river chews its way downward, but that leaves large cliffs upsupported by the rock that used to hold them up. As a result, they tend to collapse into the river, which carries away the debris. So as time goes by, each nickpoint serves as the lead for a new valley carved in the middle of older, wider valleys. You could visualize these valleys as a nested series of "rowboat" shapes, each with a nickpoint at its bow. In the area of the Billy Goat Trail, Mather Gorge is the innermost and narrowest of these levels.

Note also the ancient riverbed deposits atop the high points (yellow).

Cross sectional cartoon that compares bedrock terraces ("straths;" bottom) versus traditional river terraces (top) cut into unconsolidated sediment. Same shape; different substrates. See a discussion of this terminology here.
Here's a cross-section of the Potomac River Gorge in the vicinity of the Billy Goat Trail. You'll note a "stairstep" effect to the landscape: each of these levels is one of the Potomac River's nested series of valleys. Geologists call these levels straths, and as you can see, each strath that can be traced over a significant distance gets a proper name. The Billy Goat Trail is largely on the Bear Island strath. Each strath level is the ancient bottom of the Potomac River -- that's why we find features like river deposits and potholes at elevations high above the modern (incised) level of the river.

The level of the Potomac River is not the same from season to season, or even day to day. (A) shows the "bathtub" ring of silt that shows up when the level of the river drops. (B) shows deposits of sand and freshwater clam shells that are found high above the river level. This mix of sand and shells gets deposited when the river is in flood stage, cresting above the level of the Bear Island strath. Penny for scale.

You can check out the current flow of the Potomac and compare it to the 75-year average by checking out the website for the USGS's river gauge at Little Falls, just downstream of Great Falls. (Scroll down to see the graph.)

A striking aspect of hiking on the Billy Goat Trail are the many potholes which are drilled into the rock. These are natural features that result when sediment-laden water swirls around in a vortex (same shape as a tornado, but made of water instead of air). The sand or silt carried in the water acts as an abrasive, and so the vortex is like a big drill, carving out these holes. Of course, when you get a bunch of potholes in a row (like in photo B), they act as a perforation in the rock. Just like the perforations at the top of your checkbook make it easier to tear out a check, so too do pothole perforations make it easier for the river to erode out larger chunks of bedrock. Also note in photo (B) that the drilling dynamic has been shut down in the closer of the two potholes, due to a large boulder becoming lodged in the pothole. Scale is in inches and in centimeters.
Potholes are frequently found along the sides of the many large rock protuberances in the Billy Goat Trail area. These rocky knobs have been likened by the geologist E-an Zen to "loaves of bread" because their shape is often strongly controlled by the metamorphic foliation -- making them long and "loaf" shaped. When the river floods, these "loaves" become islands, with the river's current diverging and wrapping around them. The water flows fastest out in the middle of the current, and flows slowest near to the rocky island's shore. As a result of this inequality, vortices (plural of "vortex") develop. Along the "river-right" side of the island, the vortices spin counter-clockwise. Along the "river-left" side of the island, the vortices spin clockwise. Together, they drill into the sides of the island, creating a "necklace" of potholes that will be visible once the river level drops again.
The culprit for hollowing out potholes must be very fine sand, silt, and grit, as shown by the texture of the pothole's rock surface. Quartz-rich layers stand up in high relief because quartz is a hard mineral, very resistant to scratching. Mica-rich layers are more easily eroded away because the micas are soft minerals. A large cobble (upper left) could not make the distinction between these two minerals: it is too blunt an instrument. A smaller grain like a piece of sand (small central circle) can easily fit in between the quartz layers to erode away at the mica layers.
Here's an aerial photograph of the Great Falls / Billy Goat Trail region. Note how wide the river is above the water diversion dam (start of the Washington Aqueduct). Note how skinny the river is downstream. in particular the very narrow, very straight section of the valley called Mather Gorge. Why is Mather Gorge so straight? One possibility is that a major crack in the Earth's crust, a fault, runs along the bottom of Mather Gorge. This fault would have crumbled up the rocks along its surface, making for an easy spot where the river could erode away rock. The offset lamprohpyre dikes (see above) are often cited as a strong piece of evidence in favor of the presence of a fault.
Great Falls is one of many waterfalls along the eastern seaboard where eastward-draining rivers have nickpoints. If you connect all these waterfalls up, they define a line of waterfalls called the Fall Zone. As you will note by comparing maps (A) and (B), the Fall Zone is also a clearly-demarcated geologic boundary. To the west of the Fall Zone are surface exposures of metamorphosed Paleozoic rocks, and to its east are surface exposures of post-orogenic, flat-lying late-Mesozoic and Cenozoic rocks. The Fall Zone is the boundary between the Piedmont province to the west and the Coastal Plain province to the east. All the major cities of the east coast are located on the Fall Zone: Atlanta, Raleigh, Richmond, Fredericksburg, Washington, Baltimore, Philadelphia, New York, etc. Geology determined these cities' locations: they were settled as far inland as was easily navigable on east coast rivers. But try sailing a boat up Great Falls! It can't be done, and so further inland navigation required the construction of a waterfall-less canal. This was the motivation for the construction of the Patowmack Canal (Virginia side) and the C&O Canal (Maryland side).
Lastly, here is a picture of the Fall Zone from a side-view perspective. Note that the Coastal Plain overlaps the Piedmont like a blanket. Note also that as the Great Falls nickpoint propagates upstream, the Potomac Gorge gets longer.

Finally, a nice summary of all of this in video form, courtesy of a student who took the class in 2009:

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