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Mike Stoever Geological Foundations of the Environmental Sciences Presentation: The Geology of the Washington, D.C. area April 18, 2016 (#3)

The Geology of the Washington, D.C. area

Slide 1—Welcome to D.C.! This week, I’m going to give you a tour of the geological underpinnings of our nation’s capital, Washington, D.C. We’ll start with a brief history of the geological processes that led to the formation of the area on which the city sits and then segue to a broader view of how the region is currently categorized, geologically speaking (this includes the Fall Line, the image shown on this slide which I will discuss in great depth shortly). Then, like any good tour guide worth his salt, I’ll take you along to visit the many wonderful sites that are to be seen in my fair city. We’ll visit Teddy Roosevelt Island; Great Falls Park; Rock Creek Park; and the Smithsonian National Zoological Park (because the critters love geology too). Finally, we’ll wrap things up by checking in the various building stones that were used to create D.C.’s famous monuments and historic buildings and leave you with some (digital) maps to take home as souvenirs. As anyone who’s ever been to D.C. knows, Segway tours, as dorky as they look, are constantly buzzing throughout the city (hey, they are efficient!). So, let’s all hop on our imaginary rides and hit the road, shall we? Slide 2—IMPACT! A Geological Formation Overview. Before we get to the sights and sounds of the city, I must first give you the promised background on the geological processes that led to the formation of the District of Columbia. These include the usual suspects of plate tectonics, erosion and deposition, cataclysmic impact, and sea level fluctuation. Since everyone loves a pneumonic device, the National Park Service has come up with a handy acronym that makes these events and the order in which they occurred easy to remember. Thus, I.M.P.A.C.T. was born. It stands for (and bear with me here), “Imagine the Movement of Plates that created both the Appalachian Mountains and the Atlantic Ocean basin along with the Crater from the Chesapeake Bay meteorite that helped shape the Washington, D.C. area over Time”. Let’s examine these events in the next couple of slides. Slide 3—You know the I.M.P., so let’s get to the A.C.T. Thanks to our work in this class, we should all have a good handle on how plate tectonics works (if not, please kindly pull off to the side of the road, step off of your Segway, and review Professor Schubel’s lectures). So, I’m going to skip over the I.M.P. (“Imagine the Movement of Plates”) part of the acronym here and instead focus on the letters A, C, and T. The letter A represents two important tectonic events that occurred in the area, the formation of the Appalachian Mountains and the Atlantic Ocean basin. While we’ve spent a good deal of time detailing the formation of the Appalachians, they play such an

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important role in the geology of the region here that I’d be remiss if I didn’t speak on them during this presentation. The life of this range began approximately 1 billion years ago during the Precambrian Period, when a collisional event named the Grenville orogeny took place. After a period of rifting and passive margin building lasting from 600 to 420 Ma, the formation of the Appalachians continued during the Late Ordovician and Late Devonian Periods, when the Taconic and Acadian orogenies took place, respectively, between 420 and 370 Ma. Finally, at around 270 Ma (during the Mississippian/Pennsylvanian Periods) the Taconic and Acadian orogenies found themselves caught in a vice as the continents of North America and Africa collided, resulting in the creation of a huge mountain range resembling the Himalayas of today. This event, known as the Alleghanian orogeny, is responsible for most of the folding and faulting in the central and southern Appalachains (where thrusting, folding, metamorphism, and intrusion occurred) as well as the deposition of a clastic wedge being deposited across western Pennsylvania, West Virgina, Kentucky, and Tennessee. Upon the cessation of the Alleghanian orogeny, the Appalachians found themselves in the interior of the supercontinent known as Pangea. Beginning around 180 Ma, rifting began to break up the supercontinent, leading to the creation of the Atlantic Ocean and a new passive margin that forms the east coast of North America, where Washington, D.C. is of course located. Slide 4—Chesapeake Bay Meteorite and Bay Formation. Now that we have a good understanding of the processes that formed the Appalachians and the Atlantic Ocean, we have only the “C” and “T” left to address in our acronym. The letter C stands for the “Crater from the Chesapeake Bay Meteorite”, which was formed by the impact of a meteorite one mile in diameter with the Earth 35 million years ago. The location of this impact was the lower tip of the Delmarva Peninsula, near what is now Cape Charles, Virginia. The resulting crater, known as the “Exmore Crater”, was 50 miles in diameter and thousands of feet deep (equivalent to the size of Rhode Island and the depth of the Grand Canyon!), and it formed the mouth of what would become the Chesapeake Bay. It caused the disruption of the natural flows of several separate rivers cutting across the Coastal Plain (an area that I will discuss in a coming slide), causing them to converge in the crater before continuing on their paths to the Atlantic Ocean. The divergence of these rivers led them to cut across the Coastal Plain at steeper inclines, resulting in deep canyons being carved into the soft sediments. As sea levels dropped during the last Ice Age (when the coastline of the Atlantic Ocean was 180 miles east of where it is today), these canyons became increasingly steeper. Approximately 18,000 years ago, the glaciers created during this last Ice Age melted and caused a rise in sea level of 600 feet, flooding the canyons and the area now known as the Susquehanna River Valley. The mixture of this seawater from the Atlantic Ocean and the freshwater of the rivers draining into the canyons and crater resulted in the creation of the largest estuary in the United States, the Chesapeake Bay. The Bay assumed its present shape about 3,000 years ago, and remnants of the ancient Susquehanna River still exist today as troughs forming a deep channel along much of the bottom of the Bay.

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Interestingly, the meteorite did not only affect waters on the surface of the Earth. Its impact destroyed many of the major aquifers in the area, leaving behind layers that now hold super-saline water. These so-called “salty wells” are reflected in the geologic record, where they show a peculiar ring pattern distinctive of the impact crater. Finally, the last letter in our acronym, T, stands for geologic time, a necessary component of any study or understanding of the geologic history of an area. The processes that I’ve described over these past two slides could not occur in “human time”; instead, they need up to millions of years or more to take place. Slide 5—Washington, D.C.’s Geologic Setting. Now that we have a good understanding of the processes that led to the formation of the D.C. area, let’s zoom out a bit and examine the geologic setting of the area itself. The D.C. area incorporates parts of four main physiographic provinces, the Coastal Plain, the Piedmont Plateau, the Triassic Lowland, and the Blue Ridge (traveling from east to west). I’ll describe each in a bit of detail now. The Atlantic Coastal Plain is a flat, low-lying area with a maximum elevation of about 300 feet. It is underlain by a bed of crystalline rock covered by southeasterly dipping wedge-shaped layers of sand, clay, gravel, silt, and marl that first began deposition 100 million years ago. The oldest rocks in the plain are poorly consolidated gravel, sand, silt, and clay that come from the weathering of rocks to the north and west and were carried to the plain by south flowing rivers. The younger rocks, which consist of sands and clays containing the minerals glauconite and mica, were initially deposited in estuaries and on the continental shelf back when the present Coastal Plain was covered by as much as 200 feet of water. The Piedmont Plateau spans the distance between the Coastal Plain and the Appalachian Mountains. It is home to the majority of the rocks in and near D.C. itself, and is composed of several types of crystalline metamorphic rocks that can be most easily seen in valleys throughout the area where the soil cover has been stripped away by erosion. Examples of these rocks include slates, schists, marble, granite, and veins of quartz and pegmatite (on its eastern side), and sandstones, shales, and siltstones layered over by limestone (on its western side). In many places, the Piedmont has been intruded by igneous rock, which, combined with the variety of rocks that I just described, all help to create a very diverse topography. Deposition of most of the crystalline rocks occurred on the uplands of the area about 550-600 million year ago. As time progressed, they have weathered to saprolite, a porous, spongy, red-brown clay-rich material that is 200 feet thick in some spots. The final product of the weathering process can be found near the surface of much of the Piedmont, where sticky, reddish-colored clay exists. The Triassic Lowland consists of red shales and red and gray sandstones and conglomerates, all of which were deposited 200 million years ago and weather to a reddish soil. In areas close to D.C. itself, these sedimentary rocks are as much as 5,000 feet thick and in some places have been intruded by trap rocks (which are resistant fine-grained igneous rock). The western edge of the Triassic Lowland is home to a series of

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alluvial fans composed of masses of limestone, quartz, and quartzite, all of which range in size from grains of sand to boulders as large as one foot in diameter, which are cemented by calcite. These alluvial fans serve as a marker of where the mouths of ancient rivers in the area once existed. The Blue Ridge is a region of north and northeast-trending valleys and ridges underlain by folded metamorphic and igneous rocks that were formed more than 500 million years ago. Near D.C., these rocks consist predominantly of granite, greenstone, and quartzite. The latter has contributed to the formation of sharp, steeply dipping ridges that are resistant to weathering and rise more than 1,000 feet above sea level. The namesake mountains of this province were immortalized by the band Fleet Foxes in their song “Blue Ridge Mountains”, a lovely tune that I guarantee will be stuck in your head for the rest of the day. Slide 6—The Terraces of Washington, D.C. Washington, D.C. is built on a series of terraces (or plateaus) that extend like broad steps away from the Potomac River and range roughly from 40 to 400 feet in elevation, dating from about five million years ago to present day. Each terrace originally began as a wide river bottom where sand and gravel were deposited in fairly flat layers by a combination of the Potomac River, its tributaries, and their ancestors. As the river cut deeper over time in response to falling sea levels or local uplift, what was once the flat bottom of the river was now left high and exposed from the water. Thus, the higher terraces in the city (and the ones the furthest from the Potomac River) are actually older than the lower terraces nearest to the river. While most experts recognize at least four distinct terraces in the city, others count as many as seven. This discrepancy is due to the difficulty in identifying the older terraces, which have been cut through by rivers, eroded by wind and rain, shaken by the movement of the Earth, and modified by human development. Depicted on this slide is the view from the southern end of Meridian Hill Park looking north. Where the railing is seen towards to the top of the slide the park sits upon a 170-foot terrace and offers gorgeous views of the city (and hosts incredible drum circles every Sunday afternoon, a local D.C. tradition). The southern end of the park descends from this terrace until reaching the 100-foot level at Florida Avenue. Originally named Boundary Street, Florida Avenue formed the northern edge of the city limits when Pierre L’Enfant was planning it in 1791-1972. Looing northward, it is here that the noticeably flat part of the city (built upon the easily developed and formed soils of the Coastal Plain) transitions to the steeper and rockier terrain of the Piedmont. Slide 7—The Fall Line. A unique feature essential to understanding the geology of the D.C. area is the Fall Line, a line of rapids and waterfalls that mark where the various rivers traveling from the Piedmont cross from hard bedrock to soft sediments as they encounter the Coastal Plain (depicted by the figure on the right hand side of this slide). As a river travels east towards the Chesapeake Bay, it encounters and washes away these soft sediments that are characteristic of the Coastal Plain (that also overlie Piedmont bedrock themselves), carving a deeper channel in these sediments than it was carving before as it was traveling through the harder to erode Piedmont bedrock. This is where

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the Fall Line is officially found, as the difference in the depth of the channel results in the line of numerous rapids and waterfalls. This naturally occurring geologic boundary extends from New York to Georgia and can be roughly traced by today’s Interstate 95. Further, the Fall Line’s path through Washington roughly runs north-south along the entirety of 16th Street. This explains the contrasting topography of the city, with steep hills on the Northwest side (due to the metamorphic Piedmont rocks) and clear lowlands on the Southeast side (which is built on the soft sediments of the Coastal Plain). Looking at a map, other contrasts become readily apparent: Rock Creek in the NW (which is discussed in an upcoming slide) descends through a narrow valley carved in the erosion resistant rocks of the Piedmont, while the Anacostia River in the SE flows slowly through a flat, broad, straight channel courtesy of the easily erodible sediments of the Coastal Plain. Additionally, the Potomac River widens noticeably at Theodore Roosevelt Island where it crosses the Fall Line and encounters soft Coastal Plain sediments (shown on the left hand side of this slide and discussed in further detail on the next slide). In addition to D.C., cities such as Baltimore and Richmond, Virginia were built along the Fall Line to take advantage of the abundant potential waterpower generated by the falls. As we all know, these cities rose to prominence during colonial times as important commerce areas. This is due to the fact that colonial ships could not sail past the Fall Line and had to stop in these cities to transfer their cargo to canals or overland shipping methods. Slide 8—Teddy Roosevelt Island. One of my favorite hidden gems in the city is Teddy Roosevelt Island, which is located right smack in the middle of the Potomac River and serves as a good spot for us to take a break and stretch our legs. While small, it offers plenty of great hiking trails and serves as a wonderful escape from the hustle of city life that surrounds it. It is similar in this regard to Rock Creek Park, which I’ll be discussing in just a few slides. The island also serves an interesting geological purpose, as the Fall Line travels directly through it and as such, the island marks the geological boundary between the hard crystalline bedrock of the Piedmont and softer sediments of the Coastal Plain (shown by the red dots on this slide). In fact, the contact between these two can be seen below the Teddy Roosevelt Bridge, which cuts directly through the island. Slide 9—Great Falls Park. Another natural marvel found in the D.C. area is Great Falls Park. Located along the Fall Line about 14 miles upstream from Teddy Roosevelt Island, it is here that the Potomac River experiences its most dramatic elevation change, dropping 76 feet in less than a mile. I’ve hiked this park extensively and let me tell you, being able to see this drop and the change in rocks firsthand is incredible. When hiking the park, you can clearly see the diagonal cracks and fractures in the rock that are past indications of movement along fault lines. These weakened areas have been eroded over time by the force of the Potomac, changing the river’s course to its current position and forming the falls and another highlight of the park, Mather Gorge. The flooding that occurred following the end of the last Ice Age eroded deeply into the rocks and carried boulders far from where they were initially formed and as a result, Mather Gorge was formed as the Potomac eroded into the now visibly fractured rocks along a fault line.

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As we all know, geologic time never stops. Eventually, the erosive power of the Potomac River will move the falls of the park further upstream and past the Piedmont bedrock, where they will begin eroding away the softer Triassic Basin sediments found near Seneca Creek in Maryland. Encountering less resistance in the sandstone bedrock of the Triassic Basin than there was in the Piedmont, Great Falls is expected to change into a series of gentler rapids over the next 1-1.5 million years. So, there’s still time for everyone to explore them fully! Slide 10—Rock Creek Park. We have now arrived at my personal favorite location in all of D.C., Rock Creek Park. Yet another beneficiary of the Fall Line (which essentially bisects the park along the path traveled by the namesake creek), Rock Creek Park begins in Montgomery County, Maryland and runs along the entire length of D.C. It spans over 2,800 acres and comprises 7% of the District itself. Along with an incredible number of hiking and running trails, there are numerous rapids and falls to be found in the park, especially in the areas south of Military Road down to Boulder Bridge, as the Fall Line is the dominant geological feature affecting the landscape of the park. The park’s northern and western portions are home to many an exposed metamorphic rock from the Piedmont Plateau, including boulder gneiss, mica schist, and quartzite, while its southern and eastern portions are home to the fluviatile, channel fill, sand, gravel, silt and clay closely associated with the Coastal Plain. Due to the abundance of rocky outcrops, the park has been home to numerous quarries in the past. In fact, there are 10 major quarry sites that are still visible today. There are two named formations of metamorphic rock in the park, the Laurel Formation and the Sykesville Formation. The former underlies the majority of the park, while the latter occurs in its extreme western and southern reaches. Over time, both have weathered into loamy soils of poor fertility that primarily support Mesic Mixed Hardwood and Oak – Beech/Heath forests. Rock Creek Park is also underlain by and home to exposures of intrusive igneous rocks, the largest and most common of which is Kensington Tonalite. This rock is medium to coarse-grained and includes crystals of light and dark minerals (including quartz and biotite) that give it a granite-like appearance. Additionally, a large system of geologic faults, known as the Rock Creek Shear Zone, accompanies the Fall Line and assists in bisecting the park into its two distinctly different regions. This 1.8-mile wide area separates the rocks of the Laurel Formation on the east from all of the other rock units and affords the park, due to its location, a diversity of plants in a close proximity to one another that is rarely seen (such as those that prefer mountain environments and those that prefer coastal ones). Known by geologists as “the zone of mixed rocks”, the rocks in this zone have been sheared apart and are virtually unrecognizable from their parent material. Sheared jagged outcrops of mylonite, an abundant fault zone rock, can been seen throughout the park and it is the predominant bedrock that runs along and just west of the park near the National Zoo (which will be discussed in the next slide).

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Finally, did you know that Rock Creek Park is home to an endangered species that’s endemic to the park? That species is the Hay’s Spring amphipod, a tiny invertebrate that mostly lives in groundwater but comes to the surface in exactly five springs along Rock Creek. Growing to only 0.4 inches (10 millimeters), this critter is colorless and blind. Due to the diversion and contamination of the many springs that once ran through the park, the Hay’s Spring amphipod was listed federally as endangered in 1982. Little is known about its biology, population dynamics, or ecological community, and researchers are actually unsure whether it resides primarily in the flooded fractures of the park’s metamorphic rock or only in the saturated overburden above the bedrock, or both. Current research shows that the Hay’s Spring amphipod may spend its life in a shallow groundwater zone, where it moves in water that percolates among sand grains and gravel unless large volumes of water flush it up and out of a spring as an exit. Slide 11—Smithsonian National Zoological Park. Nestled right in the heart of the city alongside Rock Creek Park is the Smithsonian National Zoological Park. This gem of a zoo sits on hard Piedmont rock that’s been cut into a steep-sided valley by Rock Creek over the past million years, making for quite the descent from the Zoo’s main entrance along Connecticut Avenue to its back entrance a mile away. As the Zoo’s own website notes, “Rock Creek is also partially responsible for the impressive view northeast from the Connecticut Avenue entrance: In front of the Visitor Center, visitors can see about a mile across the valley, all the way to the Mount Pleasant neighborhood. The Zoo entrance and Mount Pleasant sit on what was originally the same 200-foot plateau; now Rock Creek bisects the plateau, providing the present view.” While Rock Creek and the Fall Line (on which the Zoo is located) dictate much of its geology, carving its steep valleys and causing occasional flooding, there is another familiar process at work. Within the Zoo, there are at least seven faults, most of which are hidden underground. However, the most famous one, which is located by the Zoo’s Adams Mill Road entrance on the southwestern side of the grounds, is set up as an exhibit of sorts and can be clearly seen by any passerby, most of whom have likely never paid it a thought or given it a second glance. I’ll go into much more detail on this wonderful piece of local geology on my next slide. Additional evidence of tectonic activity can be seen in the strikingly bent shape of Rock Creek at the Zoo. Here, the creek follows the joints in the underlying bedrock that have formed as a result of previous stress events, following the path of least resistance to erode away the crumbled rock rather than fight through the surrounding solid Piedmont rock. This results in the distinctive upside down U-shape of Rock Creek around the Zoo, where its arms are almost parallel (the apex of this is shown in the figure at the bottom of this slide). Slide 12—Darton’s Fault. Found adjacent/above the Zoo near its southwestern entrance alongside Adams Mill Road, these photos depict one of the greatest natural exhibits in the city, Darton’s Fault. Named after the geologist N. H. Darton, who had a cage put around the exposure of the fault in the 1920s to save it from vandalism and other destructive forces, the fault provides evidence of a thrusting event that occurred sometime within the

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last several million years. During this event, the old metamorphic rocks of the Piedmont were pushed over the much younger Coastal Plain sediments or river terrace gravels. While natural weathering over the past 90 years has partially obliterated the fault, nature has lent an explanatory assist to visitors in the form of a root from a gigantic (shown in the photo on the right) that helps delineate the now somewhat indistinct fault. Slide 13—The Building Stones of D.C. and a Brief History of the National Mall. As I mentioned during our time in Rock Creek Park, numerous rock quarries existed within its boundaries during the early 1900s. The variety of rocks underlying the area were quarried and put to use in the construction of the numerous monuments and buildings throughout the city. For example, most of the buildings in the area surrounding Rock Creek Park have what are known as “Rock Creek granite” foundations and chimneys, all quarried from the Kensington tonalite bedrock in the park (including the Zoo, where it is showcased in the Monkey House, Think Tank, and Beaver Valley). Additionally, the original Smithsonian Institution Building was built using the durable, red and reddish-brown sandstones from the Triassic Age found in Seneca Creek in Montgomery County, Maryland and the crypt and rotunda of the U.S. Capitol Building were constructed using the brown to light-gray sandstones from the Cretaceous Age found in Aquia Creek in Stafford County, Virginia. Many of the monuments and buildings ringing the National Mall and Capitol Hill are made of Potomac bluestone, which is derived from the Sykesville Formation, and the (infamous) Metro and many of the bridges in and outside of Rock Creek Park were built using quarried rock from within it. Speaking of the National Mall, the land on which it is located was originally swampland that actually extended south from the Lincoln Memorial and Washington Monument to the Potomac River. It was then reclaimed (as we humans tend to do to our natural environments) with material dredged from further down the Potomac. As anyone who has been to D.C. in the summer (which lasts from roughly May-September), the effects of being located on what was originally a swamp did not leave with the water… Slide 14—Fun with Maps! Before our tour ends and everyone returns their Segways to their rightful charging stations (are Segways electric? They seem like they’d be electric), I’d like to make sure everyone gets his or her promised parting gifts. On this slide you will find two handy geographic maps of the Washington, D.C. area. They both come courtesy of the USGS and the first was created in 1976 (just to show you how far we’ve come in map layouts), and the second is a portion of the most current version (in color no less!) where you can see the bend in Rock Creek referred to in Slide 11. You can use these to impress your friends and family the next time you’re traveling through D.C. (along with this presentation, of course!). Slide 15—Thank You! It is here that our tour of D.C. must unfortunately come to an end. I hope that everyone enjoyed themselves and learned plenty of fun facts about the wonderful geologic history of this amazing city. Please be sure to tip your tour guide accordingly. Bye for now!

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References: Slide 1: http://www.virginiaplaces.org/regions/fallshape.html Slide 2: http://www.nature.nps.gov/views/Sites/NAMA/HTML/ET_NamaTour1.htm Slide 3: http://www.nature.nps.gov/views/Sites/NAMA/HTML/ET_NamaTour1.htm; Marshak, S. 2015. Earth: Portrait of a Planet. New York, NY: W. W. Norton and Company, Inc. Slide 4: http://www.nature.nps.gov/views/Sites/NAMA/HTML/ET_NamaTour1.htm; http://www.chesapeakebay.net/discover/bayecosystem/baygeology Slide 5: http://pubs.usgs.gov/gip/stones/setting.html; http://www.chesapeakebay.net/discover/bayecosystem/baygeology Slide 6: http://nationalzoo.si.edu/AboutUs/History/beneathitall.cfm Slide 7: http://www.virginiaplaces.org/regions/fallshape.html, http://www.chesapeakebay.net/discover/bayecosystem/baygeology; http://nationalzoo.si.edu/AboutUs/History/beneathitall.cfm Slide 8: http://www.virginiaplaces.org/regions/fallshape.html Slide 9: https://www.nps.gov/grfa/learn/nature/geology.htm; http://pubs.usgs.gov/gip/stones/setting.html Slide 10: http://www.explorenaturalcommunities.org/parks-places/rock-creek-park; http://www.gonzaga.org/NetCommunity/Document.Doc?id=2696; http://www.rockcreekconservancy.org/rock-creek-parks/rock-creek-park/geology; http://www.fws.gov/endangered/bulletin/2002/01-02/08-09.pdf Slide 11: http://nationalzoo.si.edu/AboutUs/History/beneathitall.cfm; http://blogs.agu.org/mountainbeltway/2011/08/27/why-those-curves-rock-creek/ Slide 12: http://nationalzoo.si.edu/AboutUs/History/beneathitall.cfm Slide 13: http://pubs.usgs.gov/gip/stones/descriptions.html; http://www.gonzaga.org/NetCommunity/Document.Doc?id=2696; http://nationalzoo.si.edu/AboutUs/History/beneathitall.cfm Slide 14: n/a Slide 15: n/a