Tuesday, March 05, 2019

Geological State Symbols Across America - District of Columbia

The next state up for the Geological State Symbols Across America is:

District of Columbia
AKA Washington D.C.


You can find any of the other states geological symbols on my website here: Dinojim.com (being updated as I go along this year).

                                                                             Year Established
State Rock: Potomac Bluestone                                    2015
State Fossil: "Capitalsaurus"                                        1998

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State Rock: 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. 

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

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