District of Columbia

AKA Washington D.C.

 

Geological State Symbols Across America           Geology of the National Parks Through Pictures


District of Columbia Geological Symbols
Type
Symbol
Year Established
State Rock
Potomac Bluestone
2015
State Dinosaur
Capitalsaurus
1998

 

State Rock: Potomac Bluestone

Potomac Bluestone

Potomac Bluestone from the Lockkeeper's house in Washington D.C. Photo by Ken Rasmussen (Earth Magazine).

Potomac bluestone is a more archaic term of the rock unit currently known as the Sykesville Formation, a formation that has had numerous designations over its history. The Potomac Bluestone was originally referred to and mined as a granite, however further research has shown that this designation was not correct. The Sykesville Formation was later identified as a gneiss, although that too wasn't entirely correct, since much of the Formation has varying degrees of metamorphism and protoliths (the original parent rocks). Currently, the Sykesville Formation is identified as a metagreywacke. A metagreywacke is a metamorphosed greywacke, which is a poorly sorted, course and angular grained, sandstone or conglomerate. Greywackes typically are formed in the deep marine from strong turbidity currents (underwater landslides). The metagreywackes of the Sykesville Formation contain various degrees of metamorphism and in many places original sediment and sedimentary structures can still be identified within the rock unit itself. These original sediments and sedimentary structures are intermingled with more common metamorphic minerals  and structures, such as original quartz pebbles mixed with deformed quartz pebbles and metamorphic garnets.

 

Sykesville Geological Map

Geological map of the Sykesville Formation, showing its relation to the Potomac River and Washington DC (Southworth and Burton, 2004).

The earliest stones quarried by settlers of the region were the schists and gneisses of the Piedmont, known locally as the Potomac Bluestone. The Potomac Bluestone, or Sykesville Formation, lies towards the northwest of Washington D.C., crossing the Potomac River. This region contains many heavily metamorphosed and faulted rock units and these rocks are thought to have been metamorphosed from Neoproterozoic to Early Cambrian diamictites and sedimentary melanges, which contained a wide range of rocks. Later the Sykesville Formation was intruded by Ordovician igneous rocks. Dating of the Sykesville Formation indicates that it was likely being metamorphosed, with temperatures up the the upper amphibolite facies, while the Ordovician igneous rocks were being intruded through the Devonian and into the Silurian. The Sykesville Formation itself is a light- to medium-grey medium-grained metagreywacke melange consisting of a quartz, feldspar, and a large mixture of pebble and boulder sized chunks of unmetamorphosed rocks (termed olistoliths). The Sykesville has a fracture pattern along the foliation plane and two mutually perpendicular joint sets. This fracture pattern results in the landscape breaking into a series of pyramidal protrusion, as seen in the Chain Bridge Flats area. These fractures aided in the the use as an early building stone for the District. Quarries along Rock Creek and Little Falls in Maryland provided Sykesville blocks for many early Washington D.C. projects such as the foundation of the White House, the Lockkeeper's House (stones pictured above), the foundation of the Capitol Building, and the foundation of the Washington Monument. (Thanks to Callan Bentley of North Virginia Community College for the assistance)

 

State Dinosaur: "Capitalsaurus"

Capitalsaurus

"Capitalsaurus" vertebral centrum identified as Creosaurus potens Lull, 1911. Collected by J.K. Murphy and is currently located in the Smithsonian National Museum of Natural History, item #V3049 (Smithsonian Institute).

The "Capitalsaurus" is an interesting dinosaur species mainly because no dinosaur species has ever officially been named "Capitalsaurus" (hence the quotation marks around the name when referring to it. Back in January of 1898, a vertebral centrum was discovered by J. K. Murphy during a sewer connection excavation on the corner of First and F Streets S.E. within the District of Columbia. The vertebra was associated with some other bones as well as some iron carbonate nodules. The vertebra was found in the Aptian (Early Cretaceous) Arundel Formation of the Potomac Group (~120 million years old). The Arundel Formation is a rock unit of blue clays and iron carbonate nodules, however proper identification as a "formation" is questionable, so the bone is typically just identified as being found within the Potomac Group.

 

Capitalsaurus Court

Capitalsaurus Court at the corner of First and F Streets, Washington D.C. (Atlas Obscura).

The bone measured 6 inches long and 4 inches wide, producing an animal estimated to be over 30 feet long and weighing more than 2.5 tons. The identification of the vertebra was unknown at the time, but it resembled the vertebra of the theropod Allosaurus considerably, however not enough to be identified as such. Closer inspection identified the vertebra as a member of the genus Creosaurus, but as a new species Creosaurus potens (Lull, 1911). Ten years later, the bone was renamed because the name Creosaurus had become invalidated with the name "Creosaurus" becoming synonymous with Allosaurus. So, since the only known large meat-eating dinosaur from the east coast during this time period was Dryptosaurus, the bone was identified as "Dryptosaurus?" with the question mark indicating the author's uncertainty (Gilmore, 1920). However, that designation is dubious as well, and Gilmore hoped more complete specimens would be found to help with the identification. Sixty years later, the bone was again reexamined and determined to not be a Dryptosaurus, or an Allosaurus (Creosaurus), and therefore must represent a new species not identified before. In an April, 1990 issue of Washington Magazine, Peter Kranz suggested giving the dinosaur the name "Capitalsaurus" (Kranz, 2003). Unfortunately, this was not a scientifically valid way to designate a new dinosaur species, but that didn't stop the public from latching on. In 1998, Watkins and Smothers Elementary School proposed a bill to the City Council hoping to make"Capitalsaurus" the official dinosaur of the district, and they succeeded. Eventually, the street corner where it was discovered was renamed "Capitalsaurus Court", and they even have a yearly festival on January 28th where children celebrate "Capitalsaurus Day" to commemorate the day when the vertebra was given to the Smithsonian Institute. It's a whole thing. So, even if the name isn't official, it's official enough for the children.

 

References

https://pubs.usgs.gov/gip/70039206/report.pdf

https://www.mindat.org/min-49126.html

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=VACAsv;0

http://www.mgs.md.gov/geology/building_stones_of_maryland.html

https://www.ncptt.nps.gov/buildingstone/stone/potomac-bluestone

Burton, W., & Southworth, S., 2004, Geology of the National Capital Region: field trip guidebook, Circular: Reston, VA.

https://www.earthmagazine.org/article/travels-geology-touring-capital-geology-washington-dc

https://blogs.agu.org/mountainbeltway/2015/08/03/the-sykesville-formation-in-6-new-gigapans/

https://twitter.com/callanbentley

https://www.si.edu/sisearch/collection-images?edan_q=Capitalsaurus

http://www.dcwatch.com/archives/council12/12-538.htm

Gilmore, C. 1920. Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus. Bulletin of the United States National Museum 60: 1-154.

Lull, R. S. 1911. The Reptilia of the Arundel Formation. Maryland Geological Survey: Lower Cretaceous: 173–178.

Kranz, P.M., 2003, Dinosaurs of the District of Columbia: Washington D.C., 31 p.

Carpenter, K., Russell, D., Baird, D., & Denton, R., 1997, Redescription of the holotype of Dryptosaurus aquilunguis (Dinosauria: Theropoda) from the Upper Cretaceous of New Jersey: Journal of Vertebrate Paleontology, v. 17, p. 561-573.

Kranz, P.M., 1998, Mostly dinosaurs: a review of the vertebrates of the Potomac Group (Aptian Arundel Formation), USA: Bulletin of the New Mexico Museum of Natural History and Science, v. 14, p. 235-238.

https://www.atlasobscura.com/places/capitalsaurus-court


Geology of the District of Columbia's National Parks

Through Pictures

(at least the one's I have been to)

 

Franklin Delano Roosevelt Memorial

Visited in 1999 and 2002

 

I had visited Washington D.C. several times growing up as well as while I was in college. I visited the FDR Memorial shortly after it opened in 1999 with my Senior Class trip, which was actually one of the only pictures to have survived that trip.

FDR Memorial

The FDR Memorial is built with a reddish-grey granite called Carnelian Granite. The granite was quarried from a quarry near Milbank, South Dakota.

 

FDR Memorial

Carnelian Granite is a course-grained igneous rock that Early Proterozoic in age (about 2 billion years old). The granite is a true granite (unlike many countertops) that contains large minerals of quartz, potassium feldspar, plagioclase feldspar, and biotite.

 

FDR Memorial

This rock was chosen for the memorial because of the similarity in color to the reddish grey field stones that were used at another National Park, the Roosevelt estate at Hyde Park, NY. The rock finish was created to make it look like these stones were "just-quarried" by giving them a rough edge.

 

References

http://www.dakotagranite.com/

https://www.mindat.org/locentry-1134912.html

https://www.coldspringusa.com/Building-Materials/Project-Portfolio/Franklin-Delano-Roosevelt-Memorial/


Lincoln Memorial

Visited in 1990, 1995, 1999, and 2002

 

Lincoln Memorial

 View of the Lincoln Memorial from circa 1990. The Lincoln Memorial is composed of six different principle building stones from across the country. 


1. The terrace walls and lower steps are comprised of the Milford Pink Granite from Massachusetts. Granite is the state building stone of Massachusetts. The granite is a 630 million year old, Proterozoic igneous rock located in and around Milford, MA, covering an area of ~100 km2. The granite was initially discovered in the 1870's and was noted for its pink quality, although there is variation within the body. The colors range from light-gray to pale orange-pink. 


2. The upper steps, outside facade, and columns contain marble blocks from Colorado, the Yule Marble, which is the state rock of Colorado. The Yule Marble is found near the town of Marble, Colorado. It is one of the purest marbles ever quarried (meaning it lacks impurities). The Yule Marble started out as the dark-blue Mississippian age Leadville Limestone. During the Tertiary, parts of the Leadville Limestone underwent contact metamorphism along the edges in contact with the uplifted granitic Treasure Mountain dome. After recrystallization, the Leadville Limestone was metamorphosed into the distinctive white marble known today. 

 

Lincoln Memorial

View of the Lincoln Memorial from the top of the Washington Monument.


3. The floor is pink Tennessee Marble. Also used in the Jefferson Memorial, the Tennessee Pink Marble is interesting in that it actually isn't a marble, it is a limestone, meaning it was never metamorphosed like the Yule Marble. Limestone is the state rock of Tennessee as well. The "marble" even includes such sedimentary structures as cross bedding, bryozoan fossils, crinoid fossils, and stylolites. The rock is part of the Holston Formation, which formed 460 million years ago during the Middle Ordovician, along the continental shelf of Laurentia (the northern continent). The Tennessee Pink Marble is found along the eastern part of Tennessee, near Knoxville. 


4. The interior walls and columns are made from Indiana Limestone, also known as the Salem Formation and also used in the Jefferson Memorial. The Salem Formation is a Middle Mississippian age (335-340 million year old) light-grey to bluish-grey pure calcarenite limestone that crops out between Bloomington and Bedford in the south-central portion of Indiana. Quarrying of the stone began in 1827 and has continued up to the present day with nine different quarries all mining the same formation. Indiana Limestone is a "freestone", which means that there is no preferential cracking, jointing, or splitting. This also means that blocks of the limestone can be planed, hand-worked, or otherwise manipulated without fear of the rock breaking in a preferential direction. The limestone is 97% pure calcite with microscopic foraminifera and bryozoan fossils found throughout. 

 

Lincoln Memorial

5. The ceiling tiles are Alabama Marble. Marble also happens to be the state rock of Alabama. Alabama Marble, also known as Sylacauga Marble, is named for the town Sylacauga and has been called the "whitest marble in the world" for its purity. The marble formed during metamorphism associated with the Appalachian orogen (mountain building) and is approximately Cambrian to Ordovician in age. Besides the pure white sections of the marble, there are also sections that have green, pink, gray, black, and gold veins in the ~5.5 cubic mile deposit. Official descriptions of the marble state it as "white and pale-blue to light-gray calcite marble, locally containing interlayered dolomite marble and thin phyllite layers".


6. The statue of Lincoln is composed of 28 pieces of Georgia Marble that is also used in the Jefferson Memorial. Although there are several different "varieties" of marble within Georgia, they all seem to be variations of the same marble deposit, Murphy Marble. The Murphy Marble Belt runs from North Carolina down through Georgia and centers on Tate, Georgia, where the Georgia Marble Company mines the marble. It is the Georgia Mining Company that provided the marble for the Lincoln statue. The specific variety of marble for the Lincoln statue comes from the Cherokee White Quarry, which is mixed in among folded gneisses and schists within the Murphy Marble Belt. The Cherokee White Marble is a partly dolominitic but nearly pure calcite marble. These marbles have been extensively quarried since 1840. The Murphy Marble started off as a Lower Cambrian limestone that was eventually metamorphosed and folded numerous times during the lower Paleozoic from the Middle Ordovician through the Early Mississippian. 

      

References

https://www.nps.gov/linc/learn/historyculture/lincoln-memorial-construction.htm

http://coloradogeologicalsurvey.org/colorado-geology/colorado-points-of-geological-interest/colorado-yule-marble-quarry/

http://academic.brooklyn.cuny.edu/geology/powell/613webpage/NYCbuilding/TennesseeMarble/TennesseeMarble.htm

https://www.gettysburgdaily.com/lincoln-memorial/

https://www.ncptt.nps.gov/buildingstone/stone/milford-pink-granite

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MAZmgr%3B0

http://academic.brooklyn.cuny.edu/geology/powell/613webpage/NYCbuilding/IndianaLimestone/IndianaLimestone.htm

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=ALsgq;7

http://www.sylacauga.net/library/sections/Sylacauga%20Marble%20Fiestival/Marble%20Fiestival%202010.htm

https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/site_page/B-43.pdf

https://books.google.com/books?id=F8dRAQAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

https://scholarship.rice.edu/bitstream/handle/1911/15132/7522017.PDF?sequence=1&isAllowed=y


National Mall

Visited in 1990, 1995, 1999, and 2002

 

National Mall

Looking down the National Mall at the Capitol Building (circa 1995).

 

Originally Washington D.C. started out as a tidal flat wetland with three different sides surrounded by water: the Potomac River on the south, Rock Creek to the west, and the Anacostia River to the east. During the 1800's and 1900's the land was drastically altered by the construction of new buildings and roads, dump sites (such as the present location of the Lincoln Memorial), and eventually dredging and draining of the rivers to expand the city itself. Seawall construction along the Anacostia river in the early 1900's eliminated ~90% of the remaining tidal marshes at that time.

 

National Mall

Although most of the wetlands have been removed from the city by today, there are still a small percentage of wetlands scattered throughout the area.

 

References

https://doee.dc.gov/service/history-wetlands-district


Thomas Jefferson Memorial

Visited in 1990, 1995, 1999, and 2002

    

Thomas Jefferson Memorial

View from the front (circa 1990). Like the Lincoln Memorial, the Jefferson Memorial is comprised of at least seven different build stones from across the country.

 

1. The memorial itself is constructed of white Imperial Danby marble from Vermont. Imperial Danby marble is actually the Lower Ordovician, Columbian Marble member of the Shelburne Formation. The Shelburne Formation includes three different rock units; the Sutherland Falls marble, the Intermediate Dolostone, and the Columbian Marble. The Shelburne Formation formed from the metamorphism of Cambrian to Ordovician age limestones that were metamorphosed during the Early Ordovician. Located on Dorset Mountain in Danby, the Danby Quarry is the world's largest underground quarry and is one of the largest producers of marble in the world. The Columbian Member is roughly 500 to 600 feet thick and is a white, massive, medium-grained marble that is composed primarily of calcite. There are also small instances of pyrite, chalcopyrite, muscovite, and chlorite that give the marble some green or dark streaks. The marble usually weathers white, but can vary to dark grey. Due to the strong similarities between the Sutherland Falls Marble and the Columbian Marble, it is often difficult to differentiate between the two, but unlike the Sutherland, contorted forms are not conspicuous within the Columbian and the markings usually have a linear pattern.

 

2. The dome is constructed of Indiana limestone, also known as the Salem Formation and also used in the Lincoln Memorial. The Salem Formation is a Middle Mississippian age (335-340 million year old) light-grey to bluish-grey pure calcarenite limestone that crops out between Bloomington and Bedford in the south-central portion of Indiana. Quarrying of the stone began in 1827 and has continued up to the present day with nine different quarries all mining the same formation. Indiana Limestone is a "freestone", which means that there is no preferential cracking, jointing, or splitting. This also means that blocks of the limestone can be planed, hand-worked, or otherwise manipulated without fear of the rock breaking in a preferential direction. The limestone is 97% pure calcite with microscopic foraminifera and bryozoan fossils found throughout. 

 

Thomas Jefferson Memorial

View from the top of the Washington Monument.

 

3. The foundation and circular terraces are made from Georgia granite. Despite the enormous amount of data regarding almost any other piece of stone in this memorial, or most of the other Washington D.C. memorials, there isn't much information regarding which "Georgia granite" was actually used in the construction of the memorial. My assumption (and it is only an assumption) is that the granite in question is the Elberton Granite. Elberton Granite is a "monumental grade" granite, meaning that the granite has a "uniform texture and color, freedom from flaws and general suitability for polishing and carving as well as resistance to weathering". It is also one of the few granites to be actively quarried in Georgia during the 1930's, when the Jefferson Memorial was constructed. Elberton Granite is part of the Lexington-Oglesby Blue Granite Belt that extends southwest past Lexington, over an area 25 miles long and 15 miles wide. The granite has a blue-grey appearance and is predominantly made up of three minerals, white felspar, greyish quartz, and the black flecks of biotite. The Elberton granite formed from an igneous intrusion within the Georgia Inner Piedmont region of eastern Georgia during the Mississippian, ~320 million years ago.

 

Thomas Jefferson Memorial

 

4. The interior walls of the memorial are constructed of white Georgia marble that is also used in the Lincoln Memorial. Although there are several different "varieties" of marble within Georgia, they all seem to be variations of the same marble deposit, Murphy Marble. The Murphy Marble Belt runs from North Carolina down through Georgia and centers on Tate, Georgia, where the Georgia Marble Company mines the marble. It is the Georgia Mining Company that provided the marble for the Lincoln statue. The specific variety of marble for the Lincoln statue comes from the Cherokee White Quarry, which is mixed in among folded gneisses and schists within the Murphy Marble Belt. The Cherokee White Marble is a partly dolominitic but nearly pure calcite marble. These marbles have been extensively quarried since 1840. The Murphy Marble started off as a Lower Cambrian limestone that was eventually metamorphosed and folded numerous times during the lower Paleozoic from the Middle Ordovician through the Early Mississippian. 

 

4. The floor is made of Tennessee Pink Marble. Also used in the Lincoln Memorial, the Tennessee Pink Marble is interesting in that it actually isn't a marble, it is a limestone, meaning it was never metamorphosed like the Yule Marble. The "marble" even includes such sedimentary structures as cross bedding, bryozoan fossils, crinoid fossils, and stylolites. The stone is part of the Holston Formation, which formed 460 million years ago during the Middle Ordovician, along the continental shelf of Laurentia (the northern continent). The Tennessee Pink Marble is found along the eastern part of Tennessee, near Knoxville. 

 

Thomas Jefferson Memorial

The bronze statue of President Jefferson

 

6. The statue of Jefferson is composed of bronze and rests on a black Minnesota granite. Like the "Georgia Marble" listed above, there are no specific references for the "black Minnesota granite", especially considering that granite is very rarely, if ever, found "black". The best assumption I can make about which building stone was used for this (without seeing the rock in person) is that the pedestal is actually composed of a gabbro. Gabbro, like granite, is a course grained, intrusive, igneous rock, so in essence they will have a similar appearance. Unlike granite though, gabbro is composed of mostly dark colored minerals without any quartz, unlike granite which has mostly light colored minerals and is predominantly quartz (a light colored translucent mineral). Within the state of Minnesota, the main gabbro being quarried at the time of the construction is the Duluth Gabbro Complex. The Duluth Gabbro Complex began intruding into the Minnesota host rocks approximately a billion years ago during the Late Precambrian. The Complex is made up of several different rock types from repeated igneous intrusions over a ~200 million year time span. These rock types include anorthositic gabbro, normal gabbro, ferogranodiorite, and granophyre. The oldest rock of the complex is the anorthositic gabbro, which is made up of 75-90% the mineral labradorite, a black colored mineral, making this a very black rock indeed. It is my hypothesis that the Jefferson pedestal is composed of this anorthositic gabbro (until I am able to get more information).

 

7. The statue of Jefferson also has a gray Missouri marble ring surrounding the base. Similar to the Tennessee Pink Marble above, the Missouri Marble not really a marble, but a limestone. One of the ways that this can be identified is the presence of fossils and stylolites, both of which would have likely been destroyed if the rock were to be metamorphosed. Unfortunately the specific rock used in the Memorial is not identified and limestone is found all across the state of Missouri. However, the Missouri state capital building used limestone in its construction and it was noted that once the interior limestone was polished, it gained the distinction of being called "marble". Based on the time period of the Capital Building's construction (1915-1917) and this common misnomer of the rock, I feel that the same rock was used for the Jefferson Memorial. The Capital interior stone is known as "Napoleon Gray marble" and can be found not only in the state capital but also in the NY Stock Exchange and the Legion of Honor: The Fine Arts Museum in San Francisco. The Napoleon Gray is more commonly known as Phenix Limestone, from the nearby town of Phenix, or geologically it was called the Burlington and Keokuk limestones. These limestones (the Burlington and Keokuk) are difficult to differentiate and are typically grouped together as part of the Osagean Series. The Burlington and Keokuk limestones formed during the flooding of North America 325-360 million years ago during the Early Mississippian. This flooding produced a shallow inland sea, called the Kaskaskia Sea, which allowed for the deposition of many fossils including crinoids, brachiopods, corals, and bryozoans.

 

Thomas Jefferson Memorial

View of the Memorial from across the Tidal Basin. From this angle you can see straight through the memorial.

 

References

https://www.nps.gov/thje/learn/historyculture/memorialfeatures.htm

https://pubs.usgs.gov/gip/stones/stops28-33.html

https://www.nps.gov/thje/faqs.htm

https://dec.vermont.gov/geological-survey/resources-energy/minres/marble

https://dec.vermont.gov/geological-survey/vermont-geology/staterx

https://anrweb.vt.gov/PubDocs/DEC/GEO/Bulletins/CastletonZenBu25.pdf

http://academic.brooklyn.cuny.edu/geology/powell/613webpage/NYCbuilding/IndianaLimestone/IndianaLimestone.htm

https://egaonline.com/learn/granite-facts

https://quarriesandbeyond.org/states/ga/ga-photos_1ad.html

http://thesga.org/2009/08/granite-from-elberton/

https://www.jstor.org/stable/30062398

https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/site_page/B-43.pdf

https://books.google.com/books?id=F8dRAQAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

https://scholarship.rice.edu/bitstream/handle/1911/15132/7522017.PDF?sequence=1&isAllowed=y

http://academic.brooklyn.cuny.edu/geology/powell/613webpage/NYCbuilding/TennesseeMarble/TennesseeMarble.htm

https://conservancy.umn.edu/bitstream/handle/11299/57070/MGS_B_44.pdf?sequence=1&isAllowed=y

https://dnr.mo.gov/geology/docs/gcwinter6.pdf

https://www.us-parks.com/geology/geology-of-national-mall-and-memorials.html

https://dnr.mo.gov/pubs/pub2488.pdf

https://quarriesandbeyond.org/name_and_origion/p3.html

https://www.lakeneosho.org/Miss26.html

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MOMo%3B0


Washington Monument

Visited in 1990, 1995, 1999, and 2002

 

Washington Monument

View from Arlington National Cemetery. Like many of Washington D.C.'s monuments and memorials, there are multiple stones that were used for the construction of the Washington Monument.

 

1. Underneath it all is the foundation, which is comprised of the District's official rock, the Potomac Bluestone. Potomac bluestone is a more archaic term of the rock unit currently known as the Sykesville Formation, a formation that has had numerous designations over its history. The Sykesville Formation is identified as a metagreywacke, which is a metamorphosed greywacke. This type of rock is a poorly sorted, course and angular grained, sandstone or conglomerate. Greywackes typically are formed in the deep marine from strong turbidity currents (underwater landslides). The metagreywackes of the Sykesville Formation contain various degrees of metamorphism and in many places original sediment and sedimentary structures can still be identified within the rock unit itself. The earliest stones quarried by settlers of the region were the schists and gneisses of the Piedmont, known locally as this Potomac Bluestone. The Potomac Bluestone, or Sykesville Formation, lies towards the northwest of Washington D.C., crossing the Potomac River. This region contains many heavily metamorphosed and faulted rock units and these rocks are thought to have been metamorphosed from Neoproterozoic to Early Cambrian diamictites and sedimentary melanges, which contained a wide range of rocks. The Sykesville Formation was likely being metamorphosed from the Ordovician into the Silurian. The rock itself is a light- to medium-grey medium-grained metagreywacke melange consisting of a quartz, feldspar, and a large mixture of pebble and boulder sized chunks of unmetamorphosed rocks (termed olistoliths). The Sykesville has a fracture pattern along the foliation plane and two mutually perpendicular joint sets. This fracture pattern results in the landscape breaking into a series of pyramidal protrusion that aided in the the use as an early building stone for the District. Quarries along Rock Creek and Little Falls in Maryland provided Sykesville blocks for many early Washington D.C. projects.

 

Washington Monument

2. The outer layer of the Washington Monument is constructed of three different types of marble. The bottom 152 feet are built with "Texas Marble", named for the quarry in which it was mined in Texas, Maryland. However, the geological name for the marble is the Cockeysville Marble. The Cockeysville Marble is Late Precambrian (~600 million years old) in age and has many variations and layers within it making individual quarries of the marble contain significantly different rock types. The variations within the marble amount to differences in the amount of magnesium within the rock, where some areas have a metadolostone (high Mg content) versus a metalimestone (high Ca, low Mg content) varieties of marble. The Texas Quarry produces a course-grained marble that is a nearly pure calcitic marble (a high Ca metalimestone).

 

Following the initial portion of construction (1848-1854) the funds ran out for the project and the monument construction was stopped. This is where the color change takes place.

Washington Monument

View across the Tidal Basin.

 

3. Construction resumed 25 years later after discovering the foundation needed to be increased and repaired. Four rows of new marble were then added to the monument above the Texas Marble. This marble is the Sheffield Marble from the John A Briggs' quarry in Sheffield, Massachusetts. A slight color change can be observed at this point, however since the layers are so minimal compared to the size of the monument, it may be unobserved. The Briggs' Quarry marble is geologically known as the Early Ordovician, Stockbridge Marble. The marble is a white calcite marble interbedded with light grey dolostone.

 

After significant delays and problems obtaining the Sheffield Marble, the contract was canceled and the builders went back to the original stone, or at least as close as they could get to it.

 

4. Above the color change line encompassing the upper 2/3rds of the monument is a repeat of the Cockeysville Marble, however this time it is quarried from the Beaver Dam Quarry in Cockeysville, Maryland. The Cockeysville mine is located 1.5 miles from the Texas, Maryland mine where the lower section of the monument's marble is from. When the monument was being constructed the marbles were nearly identical and therefore it was assumed that everything would match. However, weathering has treated the two marbles differently, despite being from the same formation and from nearly the same quarry. This is because of the heterogeneity of the marble listed above. The marble at Cockeysville is finer-grained and has a much higher Mg content, more akin to a metadolostone. The smaller grain size and the increase in magnesium content results in the weathering producing a slightly different color for the marble over time, a feature that is pronounced when displayed on the scale of the Washington Monument.

 

Washington Monument

 

5. Behind the marble outer face are also multiple stones. One of the backing stones for the marble facade is red Seneca Sandstone. Geologically the red Seneca Sandstone is known as the Poolesville Member of the Manassas Formation. This is the same rock that was used for the construction of the Smithsonian Castle. The Manassas sandstone is part of a series of Triassic sandstone basins that extend from North Carolina to Massachusetts. These related rocks supplied much of the "brownstone" used in the NYC construction at the same time. The sandstone is primarily composed of quartz, alkali feldspar, and muscovite with ~5% Fe2O3 concentration, attributing to the strong rusty-red color.

 

Washington Monument

6. Another of the backing stones is the Maine Granite, which is from a quarry on Mount Waldo, Maine. The geological name for the granite, is surprisingly enough, the Mount Waldo Granite. The granite is a Devonian Age (390 million years old) intrusive igneous pluton. The pluton intruded within tightly folded Precambrian and Lower Paleozoic age schists, gneisses, quartzites, and migmatites. The granite is light to medium grey, course-grained, and porphyritic, which contains the minerals microcline, plagioclase, quartz, and biotite. The granite deposit was studied as a potential place for a shallow underground oil repository, however documentation of severe rockbursts have prevented the unit from being used as such.

 

The third of the marble facade backing stones is the previously mentioned Potomac Bluestone, which was also used as the foundational rock.

 

References

https://pubs.usgs.gov/gip/stones/stones4.html

https://www.mindat.org/min-49126.html

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=VACAsv;0

http://www.mgs.md.gov/geology/building_stones_of_maryland.html

https://www.ncptt.nps.gov/buildingstone/stone/potomac-bluestone

Burton, W., & Southworth, S., 2004, Geology of the National Capital Region: field trip guidebook, Circular: Reston, VA.

https://www.earthmagazine.org/article/travels-geology-touring-capital-geology-washington-dc

https://blogs.agu.org/mountainbeltway/2015/08/03/the-sykesville-formation-in-6-new-gigapans/

https://twitter.com/callanbentley

https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MDcm%3B4

http://www.mgs.md.gov/geology/geology_tour/stop6.html

https://books.google.com/books?id=n71FAQAAMAAJ

https://pubs.usgs.gov/bul/0744/report.pdf

https://books.google.com/books?id=5mGYCgAAQBAJ

Livingston, R.A .. Grissom, C. A., and Aloiz, E.M., 2015, Building stones of the National Mall, in Brezinski, D.K., Halka, J.P., and Om, R.A., Jr .. eds., Tripping from the Fall Line: Field Excursions for the GSA Annual Meeting, Baltimore, 2015: Geological Society of America Field Guide 40, p. 543-571, https://doi.org/10.1130/9780813700403

http://islandadvantages.com/news/2014/jul/31/the-history-of-maine-granite-runs-deep/#.XIccmSJKiHt

https://www.maine.gov/mdot/pnbo/about/

https://digitalmaine.com/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=1083&context=mgs_publications