The Cretaceous Plants of Meridian Hill Park

Meridian Hill Park is one of the subunits of Rock Creek Park, in Washington, D.C., having joined it in 1910. It is a roughly rectangular parcel bounded on the north by Euclid Street NW, on the east by 15th Street NW, on the south by W Street NW (which is replaced by Florida Avenue west of the park), and on the west by 16th Street NW. The park gets its name from a geographic marker placed on the hill by Thomas Jefferson in 1804 to indicate the location of a longitudinal meridian (the White House Meridian). This marker no longer exists, but the name remains. The genesis of the park goes back to Mary Foote Henderson, who after unsuccessfully proposing the tract including the park as a site for a new White House, began the process of turning the tract into a public city park in the early 1900s.

Before there was a park, there had been an extensive outcrop of the mid-Cretaceous Potomac Group and overlying Cenozoic terrace deposits along 16th Street. It is mentioned at least as far back as Rogers (1875), which is also of interest for discussing the ubiquitous Skolithos-bearing cobbles of the Potomac region. Rogers described the exposure as showing the contact of underlying sandstone and overlying unconsolidated deposits, but noted that this had been obscured by grading and excavation after his visit in April 1875. The "Columbian College" mentioned in his description is what is now known as George Washington University, then located at Meridian Hill. Logically enough the site became known as the Sixteenth Street exposure in the literature. It didn't have much longer to last.

Cross-bedding in the exposure (Darton 1896).

The Sixteenth Street outcrop was investigated in detail during the 1890s by Lester Ward, William Fontaine, and other geologists (Ward et al. 1905). A stratigraphic section was taken at the site in May and June 1894, shortly before the outcrop was fully obscured (Ward et al. 1905:382–387). Beginning north of the intersection with Florida Avenue at the southwest corner of what is now the park, they recorded approximately 40 ft (12 m) of Potomac "Formation" rocks, composed of 25 ft (8 m) of cross-bedded sandstone followed by 15 ft (5 m) of clay, overlain by 20 ft (6 m) of stratified gravel and clay attributed to the Lafayette Formation, and a 5-ft (1.5-m)-thick cap of boulder clay at the crest of the hill. The upper Potomac clay proved to be paleobotanically productive. The geologists made a couple of collections in spring and summer 1893 (Ward et al. 1905:516). The plant fossils were found in clay rip-ups, indicating they had been reworked from an older part of the Potomac. Fontaine identified the conifers Athrotaxopsis tenuicaulis (sometimes spelled Arthrotaxopsis), Nageiopsis angustifolia, and Podozamites distantinervis (then thought to be a cycad), and the fern Thrysopteris angustifolia.

Here's another photo, this one from Ward et al. (1905). The composition is kind of odd, with the building just peeking over the top of the exposure.

The outcrop on 16th Street has generally been hidden since the 1890s, but during work for improvements to the newly created park in the early 20th century, some access was possible. Sinnott and Bartlett (1916) described plant fossils collected during an episode of access. They described the geology at their spot as the Patuxent Formation (part of the Potomac) beneath the Lafayette Formation. The Patuxent Formation outcrop they accessed included 5 ft (1.5 m) green sandy clay beneath 3 ft (1 m) of lignite-rich clay with leaf impressions. The plant fossils included charcoal, cones, poorly preserved foliage, and lignified logs. As identified by the paleobotanist Edward Berry, the foliage represented essentially the same taxa as identified by Fontaine: the conifers Athrotaxopsis grandis, Nageiopsis angustifolia, and Podozamites distantinervis, and the fern Onychiopsis psilotoides (Thrysopteris angustifolia is a synonym). The logs included examples of the conifers Podocarpoxylon mcgeei and new species Paracupressinoxylon potomacense. Interestingly, unlike the reworked foliage described by Ward et al. (1905), Sinnott and Bartlett (1916) thought the logs were likely buried in place.

Slides of Paracupressinoxylon potomacense from Sinnott and Bartlett (1916). 13, 14, and 18 are radial sections, 15 is a tangential section, and 16 and 17 are transverse sections.

Unfortunately, Sinnott and Bartlett did not specify what they based their new species on, so it is not clear if their type specimen (or specimens) came from Meridian Hill or a second locality, a paleobotanically productive deep excavation for Central High School (now the Cardozo Education Campus) a few blocks east. Nor did they specify anything else about the disposition of their fossils. Apparently their work was done at the Bussey Institution of Harvard and the USDA’s Bureau of Plant Industry, but Sinnott was based out of the Connecticut Agricultural College (now the University of Connecticut) and Bartlett was based out of the University of Michigan. When I was doing some research on the site a few years ago, I sent queries to several institutions, only to find that Harvard, the University of Connecticut, and the University of Michigan do not appear to have their material. (Also checked a bunch of museum databases, since why not. No dice.) I am not sure if the thin sections would have survived to the present, or would be informative or recognizable if they have survived. For that matter, the taxon Paracupressinoxylon potomacense has scarcely been used since 1916. I guess you can name something, but that doesn't mean anyone else has to notice.

References

Darton, N. H. 1896. Artesian-well prospects in the Atlantic Coastal Plain region. U.S. Geological Survey, Washington, D.C. Bulletin 138.

Rogers, W. B. 1875. Geological notes. Article II: On the gravel and cobble-stone deposits of Virginia and the Middle States. Proceedings of the Boston Society of Natural History 18:101–106.

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., 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. Text, plates.