My next post about the Geology of the National Parks Through Pictures is about some parks visited a long time ago within Washington D.C.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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.
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.
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.
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.
The third of the marble facade backing stones is the previously mentioned Potomac Bluestone, which was also used as the foundational rock.
Burton, W., & Southworth, S., 2004, Geology of the National Capital Region: field trip guidebook, Circular: Reston, VA.
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