NOVA Annandale | Geology | Bentley | Geology of Washington, DC
as seen on Callan Bentley's GOL 135 course for NVCC

(You can print out this page as a PDF file, if you would prefer to read it as a hard-copy.)

Part 1: Deposition of sediments

Field trip stop #1, as viewed in an aerial photograph: Chain Bridge, in the westernmost corner of the District of Columbia. We park at the small parking area just north of Chain Bridge, adjacent to the Chesapeake and Ohio Canal. Our route to the riverside exposures is shown in yellow. Modified from a Google Earth image.
Meta-greywacke, the most common of the bedrock units in Washington, DC. Greywacke, the parent rock, is an immature sandstone, and sometimes it includes clasts (chunks) larger than sand-size. For instance, (A) shows quartz pebbles preserved in the finer-grained grey matrix, and (B) shows more quartz pebbles, plus a cobble of granite included in the rock. Both of these photos were taken on the small bridge which leads from the Zoo's "Amazonia" exhibit over Rock Creek, towards the Adams Mill Road business entrance for the Zoo. (The bridge is built out of local rocks.) Width of each photo is about six inches.
An outcrop of the meta-greywacke of the Sykesville Formation, exposed along the banks of the Potomac River just upstream from Chain Bridge. Note the variety of clasts included in the "dirty" sandstone. Differential weathering of these clasts is easy to observe at this locality: the dark-colored clasts (made of less stable mafic minerals) are generally recessed into the face of the rock. The light-colored clasts, on the other hand (made of quartz and other more stable felsic minerals), are generally high-relief, projecting out from the face of the rock. Handlens for scale.
Another outcrop of the meta-greywacke of the Sykesville Formation, exposed along the banks of the Potomac River just upstream from Chain Bridge. Again, note the variety of clasts included in the "dirty" sandstone. Width of photograph is about 3 feet.
Large clast of gabbro in the meta-greywacke deposits of the Sykesville Formation. Note the "halo" of rust which surrounds this clast, due to oxidation of its outermost iron-rich mafic minerals. Chain Bridge outcrop area. Key for scale.

This is an additional outcrop of the Laurel Formation, a meta-conglomerate exposed along Klingle Road (closed to traffic; some people call it "Klingle Park") in Northwest DC. Several large (multi-cm) clasts are visible here. The Laurel Formation is interpreted as being an accretionary melange (French for "mix") in the same style as the Coast Ranges of California, only more metamorphosed. Pen for scale.

More images of Klingle Road exposures available here.
On a rainy Saturday morning, students learn about meta-greywacke at an excellent exposure on the north bank of the Potomac River, just upstream from Chain Bridge. This is just about the westernmost point in Washington, DC. The rocks are metamorphic rocks, meaning that they have been altered by high temperatures and pressures, but before they were altered, they were sedimentary rocks. The sediments they were made from were a combination of mud, sand, and pebbles, deposited quickly in a deep-sea fan type of setting, perhaps similar to the Gulf of Alaska today. Callan Bentley and Fall 2005 GOL 135 students for scale.

 

Part 2: Metamorphism and deformation

When greywacke is metamorphosed into meta-greywacke, the clay minerals in between the sand grains are no longer stable. At higher temperatures and pressures, they react to form the mineral garnet. Garnet is distinctive: cranberry-red, and shaped like little necklace beads. Garnets are pretty hard, and so they frequently bulge out of the rock face, in high relief. However, they are not chemically stable at Earth-surface conditions. Because they are rich in iron, frequently garnets will appear rusty on their surface, or they will be surrounded by a small "halo" of rust. (B) is a close-up of (A). Keychain for scale.
Larger porphyroblasts of garnet developed in Laurel Formation meta-greywacke, exposed in Piney Branch, eastern Rock Creek Park, northwest Washington, DC. Because the garnets are hard (they're one of the favorite choices of grit to give sandpaper it's "bite"), they stand out in high relief on the rock surface. While other minerals (micas, in particular) are preferentially weathered away, the hard garnets stand up tall to the onslaught of weathering forces. Width of photograph is about one foot.
Oftentimes, these metamorphic garnets occur in little clusters, rather than being evenly dispersed throughout the metamorphic rock. These little clusters appear where the necessary elemental ingredients (iron, magnesium, aluminum) were present in some pre-existing clast. The neighboring metagreywacke matrix lacks this combination of "ingredients," so no garnets are found there. Penny for scale.
Pyrite porphyroblasts in meta-greywacke. Two cubes of pyrite are visible in the upper left, and one more is visible in the lower right. The euhedral (perfectly-shaped) cubes of pyrite indicate that they are not sedimentary grains, but rather formed in place due to metamorphism, much as the garnets detailed above. Photo was taken on the small bridge which leads from the Zoo's "Amazonia" exhibit over Rock Creek, towards the Adams Mill Road business entrance for the Zoo. (The bridge is built out of local rocks.) Width of photo is about six inches.
Another thing that happens to rocks when they get deep down in the Earth, under terrific temperatures and pressures, is that they get deformed. Meta-greywacke is no exception, as these stretched clasts show. Much like the first picture at the top of this page, this package of meta-greywacke contained numerous large clasts. Subjected to tremendous "squishing" pressures, these clasts have been "smeared out" into long, pancake-like shapes. These are classic examples of deformed rocks from the Rock Creek Shear Zone. Keychain for scale.
Field trip stops in the Adams-Morgan area, as viewed on an aerial photograph. (A) smeared clasts (see above photo) of the Rock Creek Shear Zone, (B) metamorphic garnet porphyroblasts, (C) calcite stalactites forming over the Beach Drive Tunnel, (D) Cretaceous river gravels deposited on the unconformity above the Rock Creek Shear Zone rocks, (E) Nelson Horatio Darton's Clydesdale Fault, at the intersection of Adams Mill Road NW and Clydesdale Place NW. Location of the Amazonia "pit stop" is also noted. Modified from a Google Earth image.

Part 3: Intrusion of magma and hydrothermal quartz

A pink dike of granite intrudes darker rocks at the old quarry south of Massachusetts Avenue, where Montrose Park's and Rock Creek Park's valleys join. This is one of the many types of plutonic rocks that comprise the Georgetown Intrusive Suite, all injected into the crust about 464 million years ago -- a time that is coincident with the Taconian Orogeny on the east coast. Photograph shows an area about 2 meters tall.

The Georgetown Intrusive Suite's jagged and fractured contact with host rocks. Here we see a light-colored (felsic) intrusive rock, granite, intruding into a foliated (metamorphosed) dark-colored (mafic) intrusive rock (meta-gabbro or meta-diorite). As a magma, the granite shoved its way into the pre-existing rocks, like the meta-whatever. As the host rocks fractured , the granite magma squirted into the cracks and sealed them shut. Scale is in inches and centimeters.
A thick dike of granite (light-colored) intruded into the foliated rocks of the Rock Creek Shear Zone (dark-colored), exposed on the east side of Rock Creek Parkway near the Waterside Drive entrance ramp, northwest Washington, DC. An artificial wall of locally-quarried building stones is visible in the background, behind the tangle of vines. Width of photo is approximately 6 feet.
Cross-cutting relationships observed in granitic dikes, quarry near Montrose Park. The thinner, pinkish dike occurred first, and was later cut across by a larger, grey-colored dike. Pen for scale.
Three parallel dikes of granite, exposed near the quartz vein in the photo below. The common orientation to these three fractures, and the homogenous nature of the magma that filled them indicates that they all formed at approximately the same time, due to the same stresses on the host rocks. Scale is in inches and centimeters.
A vertical dike of intermediate-composition igneous rock in the old quarry south of Massachusetts Avenue in Rock Creek Park, near the border with Montrose Park. This dike is not as felsic as some of the granites observed in the Georgetown Intrusive Suite, even those only a few feet away. Furthermore, it is foliated, with its mafic minerals all lined up in the same direction, indicating that it experienced regional metamorphism sometime after it cooled into rock. Width of photo is about 1.5 meters.
The Kensington Tonalite, a light-colored intrusive rock (essentially, a kind of granite), which occurs in a broad swath running north-south through Rock Creek Park. The Kensington Tonalite is particularly well exposed in a waterfall along Broad Branch. It is dated as being 464 million years old, which is coincident with the Taconian Orogeny, a major episode of mountain-building that occurred on the east coast during the Ordovician period of geologic time. Keys for scale.
A vein of hydrothermal quartz exposed on the east side of Rock Creek Parkway, across from the Zoo exit. This quartz was originally dissolved in solution, deep in the ground. When the rock around it (meta-greywacke of the Rock Creek shear zone) split open in a crack, the quartz-rich solution flowed into the crack and precipitated quartz. It's kind of like sealing together a broken birthday cake with a "glue" of icing. Scale is in inches and centimeters.
Vein of quartz cutting a boulder of meta-greywacke in Rock Creek Park near the Montrose Park drainage. Pen for scale.

Part 4: More deformation

Fold in a quartz vein, foliated meta-greywacke of the Rock Creek Shear Zone. Because two sides of this block of rock are visible, we can see the three-dimensional structure of the fold. Note, too, that the quartz is moderately boudinaged (see below for more details on boudinage). Pen for scale.
Offset quartz veins exposed in the old quarry south of Massachusetts Avenue in Rock Creek Park, near the border with Montrose Park. Two generations of cracking (and subsequent) sealing with quartz are visible here: a first generation of two parallel fractures (steeply dipping to the left) and a later fracture which cuts across and offsets the older veins, and then was itself filled in with vein quartz. Brunton compass for scale.
A terrific outcrop of igneous and structural relations, exposed in the old quarry south of Massachusetts Avenue in Rock Creek Park, near the border with Montrose Park. A granite dike (light pink/tan in color) cuts across older rocks (darker in color: mafic intrusives and the foliated rocks of the Rock Creek Shear Zone). A xenolith (literally, "alien rock") of the dark host rock may be seen in a central location in the main, horizontal portion of the dike. Note how angular this xenolith is: it has not undergone any significant thermal erosion (as an ice cube goes from cubical to rounded the longer it sits in a glass of water): indicating that the xenolith had freshly broken off not far from its current location, dropped into the already-cooling magma, and was stuck surrounded by granite for the past 460 (or so) million years. Furthermore, on the right side of the photo, the granite dike is sheared and boudinaged (see below), a process which took place sometime later. Brunton compass for scale.
Boudinage of a granite dike, quarry near Montrose Park. The granite dike has been broken into three segments as it was stretched along its length. The largest segment is in the upper left (parallel to the pen); the next-largest segment is in the lower right (notice its "pinched out" end), and the smallest segment is in the middle between the other two. Not only was this dike boudinaged (from the French word for sausage, a reference to its visual similarity to a string of sausage links), but it was also offset, with the right side of the photo moving down relative to the left side. Pen for scale.
Boudinage in meta-greywacke, as observed in a block of locally-quarried stone used in a retaining wall on northern Beach Drive, Rock Creek Park. Notice how the granite dike (top) is broken into segments (like sausage links). Width of photo is about eight inches.
Fault in rocks of the Rock Creek Shear Zone, in the woods on the east side of Rock Creek Park, south of the Taft Bridge and north of Waterside Drive. Photos get progressively more zoomed in, from (A) to (C). At first, intrusive rocks appear to make up both sides of the fault, but close examination reveals a small mushed lens of meta-greywacke present along a fault (crack in the rock), between two slices ("tectonic shingles") of granite.
Faulting cuts across a granite dike, offsetting the dike by about one inch. Outcrop is in the old quarry south of Massachusetts Avenue in Rock Creek Park, near the border with Montrose Park. Pen for scale.
Kink bands in the highly foliated meta-greywacke of the Rock Creek Shear Zone. Structures like this indicate that these rocks experience not one, but two episodes of deformation, at least locally. Note squashed clasts (light color) present along the foliation plane. This exposure is in the creekbed of Broad Branch, near the intersection of Broad Branch Road and Brandywine Street, NW. Width of photograph is about 1 foot.
Kinked telephone book as an analogy for how kink bands form: a highly layered structure (phone book or foliated metamorphic rock) is compressed from the sides, resulting in kinking in order to accomodate the shortening of the pages/foliation.
A close up of kink banding in meta-graywacke: The first episode was compression which reoriented all their mineral grains into the prominent light and dark bands which run from the lower left of this photograph towards the upper right. (So this compression would have been pushing from the upper left and lower right.) Then, later, a second squeezing episode of compression shortened the foliation from top to bottom, resulting in the kinked band which runs left to right across the wide axis of the photograph. This would be kind of like what would happen to a phone book if you compressed it parallel to its pages, while confining it from above and below (see next photograph). Exposure is in the creekbed of Broad Branch, near the intersection of Broad Branch Road and Brandywine Street, NW. Width of photograph is about 2 or 3 inches.
A single kink band developed in the foliated structure of the Yellow Pages -- a visual analogy for the kink bands we see in similar foliated structures made of rock (like the meta-greywacke of Broad Branch, NW DC).
The largest kink band at this particular exposure in the streambed of Broad Branch is a ~1 foot wide kink fold which offsets foliation in an orienation parallel to many of the smaller kink bands in this location. Within the large kink band, there are smaller, less regular kinks. Notice how the stream has taken advantage of the weakness in the folds of the kink to cut down into the rock as a stream channel parallel to the kink band. Shoe for scale.
A cracked pod of some unknown toothpaste-green material, as exposed in a boulder of the Kensington Tonalite, exposed along Broad Branch Road, NW. This sort of cracking is the first step towards boudinage: the "sausaging" of brittle rock as more ductile rock flows around the boudinaged pieces. Width of photograph is about 9 inches.
S-C fabric in the Kensington Tonalite. This is a boulder of the Kensington Tonalite used to make a wall above a dirt path in Montrose Park, just south of the Italian Embassy. Note the two planes of folation here: one approximately parallel to the pen, one approximately parallel to the slug. This is a indicator of past motions of the Rock Creek Shear Zone: If this boulder were in place (which it isn't), then the sense of motion would be that the top moved to the right. Pen, and slug, for scale.

Part 5: Weathering & erosion, deposition of younger sediments

An analogy for the sedimentary portion of the Rock Cycle (weathering, erosion, transport, and deposition), as seen in a section of sidewalk along Harvard Street NW, just outside of the lower entrance to the National Zoo. Plate tectonics has buckled the portions of the Earth's crust, shoving some up (right) and some down (left). The uplifted block is no longer at equilibrium with gravity, and so as water flows over it during times of rain, any loose sediment is washed away and carried downhill. Meanwhile, a short distance away, sediment-laden water pools in the "basin" of the down-dropped block, where the sediment settles to the bottom, eventually filling in the hole. The same things happen, on a much larger scale, in the real world. The taller a mountain is, the more actively it sheds sediment off. The deeper a basin is, the more room it has to accomodate deposits of sediment. Width of photo is about ten feet.
After the mountain-building (Taconian, Acadian, and Appalachian Orogenies) that built up the Appalachians, uplift and deformation ceased, and a period of weathering and erosion took over. These river gravels and cobbles were deposited by the ancestral Potomac River during the Cretaceous period of geologic time. Note the extreme rounding of the cobbles visible in the lower left -- this indicates that the individual clasts have been transported a long distance. Some of the individual cobbles can be recognized as pieces of the Appalachian Mountains: the Antietam sandstone, for instance, is spiked with distinctive Skolithos trace fossils (worm burrows), and some rounded chunks of the Antietam sandstone, complete with these "soda-straw" shaped fossils, are present in DC's Cretaceous river gravels. This unlithified deposit lies unconformably on top of the highly-metamorphosed rocks of the Rock Creek Shear Zone. Photo by Manu Malhotra. Gesticulating professor for scale.
Another environment that existed in late Mesozoic/early Cenozoic DC was a swamp. This cypress stump was preserved in a swamp that used to occupy the site that is now the National Mall in downtown DC. When construction workers were excavating for the construction of the Ronald Reagan Building (Pennsylvania Avenue and 15th Streets, NW), they found this petrified stump. Like the more famous "petrified forest" of Arizona, the wood of this tree was entirely replaced by silica. There are also small amounts of pyrite (fool's gold) visible on either side. The stump is currently located outside of the Rock Creek Park Visitor's Center near Miliary Road, NW. Photo by Manu Malhotra. Shoes for scale.
A large boulder of quartzite, about 2.5 meters in diameter, transported downstream by the Potomac River at flood levels, now abandoned on a higher strath level of the Potomac Gorge, just upstream from Chain Bridge. Width of boulder is about 2.5 meters.

Part 6: Faulting of the unconformity

Recall that after the building of the Appalachian Mountains, an extended period of erosion ground the mountains down to their roots, exposing the igneous and metamorphic rocks we examined in the first part of the trip. Later, erosion ceased and deposition began, laying down the Cretaceous river gravels on top of this ancient erosional surface. Now, we see that surface (the "unconformity") broken by an episode of faulting. The faulting occurred sometime post-Miocene, since similar faults also cut Miocene sediments elsewhere in the city. Here are two photos from the 1930s (and published in 1950) showing these reverse faults in D.C. outcrops. This first photo is the fault exposure we visit on the field trip, before the enclosing "protective" structure was built. Hammer for scale.
This is a similar fault on Calvert Street NW, no longer visible. What's weird about these faults is that they demonstrate compressional stress, something that the D.C. area hasn't experienced tectonically since the Permian, about 300 million years ago. So what gives? The best explanation is that these faults are the result of isostatic readjustments of the Earth's crust. In other words, as the crust "pops up" when weight is removed (by erosion), certain segments pop up faster than others. Hammer for scale.

Part 7: Weathering & erosion in the modern exposures of rock

One of the things that makes the Chain Bridge stop on this trip so difficult to walk over is that the bedrock naturally outcrops in a series of "pyramid" shapes. These three-sided pyramids are repeated thousands of times over the Chain Bridge landscape. Here, their regular shape is emphasized by the river's water. Each pyramid consists of three tilted planes of rock: one tilted towards the photographer's vantage, one tilted to the right, and another tilted away from the photographer. These repeated patterns are three jointsets in the bedrock. (Joints are just cracks in the rock, and are among the most common geologic structures.) Jointsets are many joints in an area that all share a common orientation in space -- they're tilted in the same direction, in other words, by the same amount. Jointsets form in the crust due to tectonic or isostatic stresses. Now at the Earth's surface, the joints help make physically weathering easier by breaking out big blocks of metagreywacke along thier pre-existing fracture network. Middle pyramid is one foot wide at the base.
Physical weathering processes are concentrated in certain spots: like where swirling vortexes of sediment-laden water "drill" into hard bedrock. Here, a pothole in the metagreywacke at the Chain Bridge stop has drilled all the way through from one side to the other. Face of goofy geologist for scale.
An example of physical weathering, aided and abetted by biological forces. Along the south side of the bike path as it winds its way through the National Zoo's property is a great example of root wedging. The rocks in DC's Rock Creek Shear Zone have a well-developed foliation, a planar reallignment of minerals that impart a splitting property ("fissility") to the rocks. In short, they break into sheets pretty easily. The tree's roots have penetrated a crack between foliation planes, and as the roots swells (turgor pressure) and grows, it pushes aside the two blocks of rock. As a result, the plane of foliation for the left block (boulder) has been rotated about 60 degrees from its original position.
An example of chemical weathering, a natural geologic process, operating on an artificial structure. Reprecipiation of calcite, north end of Beach Drive Tunnel, near the National Zoo. Calcite is dissolved out of the mortar by rain, carried a short distance, then redeposited drop by drop into stalactites. A more complete description appears below.
Stalactites forming underneath artificial structures are a common enough scene in older US cities. The bridges over Rock Creek Parkway offer some nice examples, such as this collection of "soda straws" under the Whitehurst Freeway Bridge near Georgetown and Foggy Bottom. How do these structures form? The same way that stalactites form in the natural world (that is, in caves): water from rain percolates into the ground, dissolves away some calcite (a mineral form of calcium carbonate), carries the dissolved calcite some distance away, and re-precipitates it, drop by drop. Here, the calcite is being supplied by the mortar used to cement together the granite blocks which form the bottom of the bridge. Soot from auto exhaust has stained the stalactites a dirty gray color. Much better stalactites can be seen hanging from the Massachusetts Avenue Bridge: there is even some "curtain stone" there -- it looks like a drapery of stone.
A more domestic example of the same phenomenon as above. Tapwater frequently has low levels of various minerals dissolved in it. When the water precipitates on any structure, such as this showerhead, those minerals are left behind. Since this is the freshwater variety of limestone, we call it "travertine," the same rock name given to natural speleothems like the stalactites above.

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