Washington, D.C. is not generally ranked in the first order of fossiliferous areas. It can hardly be considered a bust, though. The "Middle" Cretaceous Potomac Group (due to a tragic geologic oversight, there is no formal Middle Cretaceous) has been reasonably kind for plants; see Fontaine (1889, 1896), Knowlton (1889), Ward (1895), Ward et al. (1905), and Sinnott and Bartlett (1916) for some of the gory details. Something you may notice from that list is that all of those publications are at least a century old. The obvious problem is that Washington is a city first and foremost, so it's not like there are a lot of outcrops for prospecting any more. The Potomac Group has also produced some scrappy dinosaur remains, and anywhere that the Potomac River once flowed is liable to have cobbles with Skolithos tubes, eroded from Cambrian rocks up in the mountains. The classic Potomac Skolithos cobbles are rounded pieces of orangeish quartzite with simple vertical Skolithos burrows, similar to skinny pencils and with a tendency to stand out from the host rock. Washington is also blessed with a profusion of fossiliferous building stone, particularly the inevitable "Indiana Limestone" (Salem Limestone). But I digress. In a city, we cannot come to the outcrop, so the outcrop must come to us. This is where subsurface explorations come in handy. We talked about taking cores from lake sediments a few weeks ago. The subsurface of Washington, like any major city, has been picked at innumerable times, uncovering fossils from places such as just north of the White House and near the Washington Monument.
Lafayette Square, on the north side of the White House, was cored in several places in 1976 by the U.S. Geological Survey for the purpose of checking out the distance to the bedrock through unconsolidated Quaternary and Cretaceous sediments (previous studies in the region suggested that the contact became abruptly lower over a short distance west to east; this might seem like kind of an esoteric thing to study, but you try significant construction work without taking into account the subsurface and see how well things go). The results confirmed this bedrock dive, but also turned up a layer of "dark gray sandy carbonaceous clay" in the Potomac Group sediments in three of the cores. A sample from 48 ft (close to 15 m) below the surface yielded plant bits (cuticle, wood fibers, etc.) and palynomorphs. Most of the palynomorphs were spores from bryophytes (mosses and friends) and pteridophytes (ferns, horsetails, and lycopods), with some rare pollen from flowering plants mixed in. These fossils are of terrestrial origin and indicate a late Albian age for the clay (about 100 million years old) (Prowell and Christopher 2004).
In addition to the Cretaceous fossils, an ancient peat bed was uncovered in the 1960s during excavations for the adjacent U.S. Court of Claims building (now the Howard T. Markey National Courts Building) (Knox 1969). The 1976 cores did not encounter it, and it is probably of very limited extent (even moreso now!) The peat bed was found about 10 ft (3 m) below street level and was about 3 ft (1 m) thick. The upper part contained a variety of fossils from marsh plants, including sedges, and proved to be more than 45,000 years old. The peat and underlying silt and sand turned up pollen from conifers and flowering plants, freshwater diatoms, and sponge spicules. Subsurface exploration indicates that the Pleistocene deposits butt up against a cliff of Cretaceous beds and overlying Pleistocene gravels, which paralleled H Street. This cliff was cut by the Potomac River at some time when its bed was higher and its course ran farther north, and after it departed, sediments filled in, including the marsh. Study of the pollen shows several switches in climate, from cool dry, to cold wet, to cold dry, to wet and warmer, to warmer and drier leading into the peat deposition. Dates are not available for most of the section, but the sediments are interpreted as dating from the last interglacial (Sangamon) into the Wisconsin glacial. Ice did not reach as far as Washington, but the flora responded to changes in temperature and moisture, and boulders in the river deposits may have been rafted out of the mountains by river ice.
The Lafayette Square marsh ties into a better-known roughly contemporaneous feature a short distance away, the "Walker Interglacial Cypress Swamp" of Knox (1966). Construction has turned up evidence of this swamp in multiple places across several blocks from Washington Circle to 16th Street along L Street, just to the north, most dramatically stumps of bald cypress, which is no longer naturally occurring in Washington. Construction along this corridor tends to reveal batches of cypress stumps around 20 ft (6 m) below street level. Apparently an old channel here was converted to a swamp (Knox 1966).
Turning to the south and the Washington Monument: Darton (1950) pulls together a number of subsurface samples from across Washington to describe the bedrock–sediment contact. The great monuments of the city are not excluded; actually, as shown in Plate 2 and Figure 9, sampling sites in Darton (1950) are all over the area between the Washington Monument, the Lincoln Memorial, and the Jefferson Memorial, but if fossils were found at any of these sites other than the Washington Monument, they were not mentioned. The subsurface of the Washington Monument was studied in some detail in 1930–1931. (One piece of trivia is that the foundation of the monument is 65 ft [almost 20 m] above bedrock.) Samples were taken in two rings around in the monument. In three of these holes, fossil wood was found between 47 and 59 ft (about 14 to 18 m) below sea level. Darton noted that fossil wood would be consistent with either Cretaceous or Pleistocene origin, generic cruddy ol' fossil wood not being all that age-diagnostic when you're talking about either option.
Darton, N. H. 1950. Bedrock subsurface of the District of Columbia and vicinity. U.S. Geological Survey, Washington, D.C. Professional Paper 217.
Fontaine, W. M. 1889. The Potomac or younger Mesozoic flora. U.S. Geological Survey, Washington, D.C. Monograph 15. Available
at https://pubs.er.usgs.gov/publication/m15, https://archive.org/details/potomacoryounger00font (text), https://archive.org/details/potomacoryounger01font (plates).
Fontaine, W. M. 1896. The Potomac Formation in Virginia. U.S. Geological Survey, Washington, D.C. Bulletin 145.
Knowlton, F. H. 1889. Fossil wood and lignite of the Potomac Formation. U.S. Geological Survey, Washington, D.C. Bulletin 56.
Knox, A. S. 1966. The Walker Interglacial Swamp, Washington, D.C. Journal of the Washington Academy of Sciences 56(1):1–8.
Knox, A. S. 1969. Glacial age marsh, Lafayette Park, Washington, D.C. Science 165:795–797.
Prowell, D. C., and R. A. Christopher. 2004. Data collected from USGS drilling in Lafayette Park, Washington, D.C. in November-December, 1976. U.S. Geological Survey, Reston, VA. Open-File Report 2004-1343.
Sinnott, E. W., and H. H. Bartlett. 1916. Coniferous woods of the Potomac Formation. American Journal of Science, 4th series 41(243):276–293.
Ward, L. F. 1895. The Potomac Formation. U.S. Geological Survey Annual Report 15:307–397.
Ward, L. F., W. M. Fontaine, A. B. Bibbins, and G. R. Wieland. 1905. Status of the Mesozoic floras of the United States. U.S. Geological Survey, Washington, D.C. Monograph 48. Available at https://archive.org/details/statusofmesozoic01ward (text), https://archive.org/details/statusofmesozoic00ward (plates).