July 7: Marine Fossil Excavated on Monticello Mountain

By Fraser Neiman

July 7, 2026

The Monticello-UVA Archaeological Field School is in full swing (Figure 1). Field school students and archaeology staff are excavating just west of the North Pavilion, in advance of the construction. of more accessible paths. Every Friday, we hold an on-site seminar where our student teams summarize for the class the stratigraphy of the excavation quadrats they've been working on, the finds from each layer, and what these mean for our understanding of landscape change on the mountaintop. During a recent seminar, Taylor Newton identified a puzzling find by one of her fellow students, Avery Ross, as a piece of fossilized tabulate coral (Figures 2-3). It resembled specimens Taylor had encountered while working on the paleontological collection at the San Diego Natural History Museum. 

A bit of research back at the lab and consultation with colleagues in geology and paleontology confirmed her hunch: this is a tabulate coral of the genus Favosites. Tabulate corals originated in the Ordovician period (484-444 MYA) and flourished as reef-builders through the Silurian (444-419 MYA) and Devonian (419-359 MYA) before declining sharply; the group finally disappeared in the Permian mass extinction some 251 million years ago. Taylor's insight raises an obvious question: how did this ancient marine fossil make its way to the top of Monticello Mountain?

Jefferson discussed possible answers to a more general version of this question in his only book, Notes on the State of Virginia. He considered what processes might explain why marine fossils, most conspicuously shells, were found in upland and mountainous settings. It turns out that his discussion of this topic in Notes contains clues that help answer our archaeological question.

Jefferson's Notes offered answers to his versions of twenty-two "Queries" that French diplomat François Barbé Marbois sent to the governors of the thirteen American states in 1780. Jefferson's discussion of shells was a small part of his response to Marbois's Query VI, "A NOTICE of the mines and other subterraneous riches; its trees, plants, fruits, &c.?" Jefferson wrote, "Near the eastern foot of the North mountain are immense bodies of Schist, containing impressions of shells in a variety of forms. I have received petrified shells of very different kinds from the first sources of the Kentucky, which bear no resemblance to any I have ever seen on the tide waters. It is said that shells are found in the Andes, in South America, fifteen thousand feet above the level of the ocean."

Three lines of evidence support the hypothesis that our piece of coral may be one of Jefferson's "petrified shells." First, Jefferson's use of the word "petrified" suggests he recognized that the original material had been altered to stone. The process was understood, albeit imperfectly, by the time Jefferson wrote. It was first diagnosed by Robert Hooke in the seventeenth century; Hooke used the recently invented microscope to show that the cellular structure of petrified wood matched that of modern specimens. The original tissue and pores have been replaced by minerals. Reef-building corals like Favosites built their skeletons of calcium carbonate. When we tested our specimen with hydrochloric acid, we saw no hint of the fizzy reaction that free calcium carbonate would produce. This is evidence that the original shell material is no longer present in reactive form, because calcium has been replaced by another element, commonly silica. Our specimen is not recent shell; it is genuinely "petrified."

Second, Jefferson tells us his petrified shells look nothing like those he had seen from Virginia's Coastal Plain, or "tide waters." These would have included modern oysters, clams, mussels, scallops, snails, and whelks. But they would also have included the Miocene and Pliocene (23–2.6 MYA) fossils eroding out of the banks of the James River at Kingsmill and Carter's Grove, just outside Williamsburg. Those fossils are the ancestors of the modern groups and are morphologically similar to them. We know Jefferson was familiar with these more recent fossils. In 1768, he mused about building a grotto on the north slope of Monticello Mountain and decorating it with "beautiful shells from the shore at Burwell's ferry" at Kingsmill. Similar grottoes, decorated with shells, were standard issue in the ornamental gardens of English aristocrats across the eighteenth century. Like Jefferson's "petrified shells," our Favosites fossil bears no resemblance to these more recent forms. 

The third line of evidence comes from Jefferson's mention of the shells' provenience. The phrase "first sources of the Kentucky" turns out to be more ambiguous than it might seem, once we understand the paleontological and geological context. The Kentucky River has its origins in Lee County, where its North, Middle, and South Forks converge. They drain the uplands of the Cumberland Plateau to the east. A bit of snooping in geological maps reveals that this area is underlain by Pennsylvanian (323–299 MYA) and Mississippian (359–323 MYA) deposits that are exposed along the banks of the rivers. The overlying Pennsylvanian deposits include sandstone, shale, and coal layers. Tabulate corals had not yet gone extinct when these rocks formed. However, the sediments in these layers accumulated in settings on or near land (rivers, deltas, swamps) that are inhospitable to reef-building tabulates like Favosites (Aitkin and Flint 1995). 

The rivers also incise older Mississippian deposits. These include not only sandstones and shales, but also significant limestones that represent the shallow marine settings in which reef-building corals thrive. However, the current definitive study of the major limestone unit (the Slade Formation) records a coral fauna of rugose corals (another group that vanished in the Permian extinction) and a single tabulate genus, the pipe-organ coral Syringopora (Ettensohn et al. 1984). No Favosites specimens appear among them. While this does not absolutely rule out the occurrence of Favosites in these formations, or the possibility that our specimen came from them, it does motivate the question of where in Kentucky Favosites has actually been found.

We can begin to answer that question thanks to the Paleobiology Database. Favosites has been recorded from two areas of Kentucky. The first is the Brassfield Formation, whose eastern outcrop belt is cut by tributaries of the Kentucky River northwest of Lee County. The Brassfield is Silurian (444–419 MYA). It began as limestone formed in the kind of shallow marine setting in which reef-building corals thrive; the original deposit was later chemically altered or dolomitized when magnesium in groundwater replaced some of the calcium in the limestone (Peterson 2001). 

The second area is near the Falls of the Ohio River, around Louisville, about fifty miles downstream of the Kentucky River's confluence with the Ohio River (Greb et al. 1993). At the Falls, the Ohio has exposed limestones and dolomitic limestones of both Silurian and Devonian (419–359 MYA) age. In ascending stratigraphic order, these are the Silurian Louisville Limestone and the Devonian Jeffersonville and Beechwood limestones. Geologists recognize that the limestones at the Falls are among the best exposed, easily observed and accessible, and well-preserved fossiliferous deposits from the Devonian period in the world. These deposits have been studied for the past two centuries, during which over 200 taxa have been identified from diverse groups, including not just Favosites, but other tabulate coral genera, as well as sponges, brachiopods, and crinoids (Hendricks et al.1994). The first monograph on these fossils was published in 1820 by the eccentric polymath Constantine Rafinesque (Rafinesque and Clifford 1820)  

Based on our review of the Kentucky evidence, Favosites occurs far more frequently in Silurian and Devonian deposits than in the later Carboniferous ones (Mississippian and Pennsylvanian). This local pattern matches the global picture: tabulate corals in general, and Favosites in particular, declined in diversity and abundance at the end of the Devonian (Bridge et al. 2022). That may indicate that our specimen is more likely to have come from the earlier periods. 

It is clear, then, that Favosites does occur in Kentucky deposits. While it is possible that our specimen came from "the first sources of the Kentucky," there are other possibilities. Perhaps Jefferson was supplying a plausible-sounding but possibly inexact provenience for fossil shells that had reached him from different locations in Kentucky.

We could shed further light on the question if we could identify the person who sent Jefferson the "petrified shells." A search of Jefferson's papers quickly turned up the most likely culprit: George Rogers Clark. Throughout the Revolutionary War, Clark had been based in Louisville. In December 1781, Jefferson, by then no longer governor, and deep in the composition of Notes, wrote to Clark asking him "to procure for me some teeth of the great animal whose remains are found on the Ohio. Were it possible to get a tooth of each kind, that is to say a foretooth, grinder…" (Jefferson 1781). The "great animal" that Jefferson would later call a "mammoth" is the creature we now identify as the American mastodon. The genus-level difference had not been recognized in his day.

Clark replied from Louisville in February 1782 that he was working on procuring a "Thigh and Jaw Bone Grinders and Tusk." Then he added: "Their is very Curious Shells found in many parts of this Cuntry. I shall send you a few of them" (Clark 1782). The next surviving link in the correspondence is Jefferson's thank-you note: "I received here about a week ago your obliging letter of Oct. 12. 1783. with the shells and seeds for which I return you many thanks. You are also so kind as to keep alive the hope of getting for me as many of the different species of bones, teeth and tusks of the Mammoth as can now be found" (Jefferson 1783). 

This exchange sheds some light on the provenience question. First, Clark says the "Curious Shells" are found across the country, and modern geological evidence attests that he was right. Second, throughout the correspondence Clark remained based at Louisville and the nearby Falls where Silurian and Devonian deposits in which Favosites are exposed. Sadly, Clark's cover letter of October 12, 1783 that accompanied the shells has vanished, and with it any more detailed account of where the specimens came from. All we have is his earlier, locality-free remark that curious shells were to be found in "many parts of this Cuntry." Notably, the phrase "the first sources of the Kentucky" is Jefferson's, not Clark's. 

Where does that leave us?  Clark's letter is missing. It may have not included proveniences for the specimens. We cannot say with certainty where his shells came from. But the evidence points in a consistent direction. Favosites is concentrated in the Silurian and Devonian across the world and in Kentucky. Clark spent the war and ensuing years at the Falls of the Ohio, where the river slices through the very Silurian and Devonian limestones which two centuries of research demonstrates contain abundant, recognizable, and accessible marine invertebrate fossils, including Favosites.

The most likely hypothesis is not that the Monticello specimen traveled from the headwaters of the Kentucky, but that it came from the coral-rich limestones at Clark's doorstep. In that case, "the first sources of the Kentucky" was Jefferson's tidy but imperfect formulation of a provenience Clark may have never supplied or that fit only some of the shells he sent. If so, we have solved the puzzle of how an ancient marine fossil reached the Monticello mountaintop. And we have uncovered an early trace of the struggle by amateur natural historians, Clark and Jefferson among them, to assemble the pieces of a larger picture: the geological history of the continent.

Acknowledgements

Archaeology is an interdisciplinary, collaborative enterprise. I am grateful for the geological and paleontological expertise of Professors Callan Bentley, Piedmont Community College, and Rowan Lockwood, College of William and Mary, and Dr. Karen Layou, opengeology.org, Special thanks to Professor Lockwood for putting me onto the Palaeobiology Database.

References

Aitken, J. F., & Flint, S. S. (1995). The application of high-resolution sequence stratigraphy to fluvial systems: A case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA. Sedimentology, 42(1), 3–30. https://doi.org/10.1111/j.1365-3091.1995.tb01268.x

Bridge, T. C. L., Baird, A. H., Pandolfi, J. M., McWilliam, M. J., & Zapalski, M. K. (2022). Functional consequences of Palaeozoic reef collapse. Scientific Reports, 12, Article 1386. https://doi.org/10.1038/s41598-022-05154-6

Clark, G. R. (1782). To Thomas Jefferson, 20 February 1782 [Letter]. Founders Online, National Archives. https://founders.archives.gov/documents/Jefferson/01-06-02-0150

Ettensohn, F. R., Rice, C. L., Dever, G. R., Jr., & Chesnut, D. R. (1984). Slade and Paragon Formations: New stratigraphic nomenclature for Mississippian rocks along the Cumberland Escarpment in Kentucky (U.S. Geological Survey Bulletin 1605-B, pp. B1–B37). U.S. Geological Survey. https://pubs.usgs.gov/bul/1605b/report.pdf

Favosites. (n.d.). In Wikipedia. Retrieved July 7, 2026, from https://en.wikipedia.org/wiki/Favosites

Greb, S. F., Hendricks, R. T., & Chesnut, D. R., Jr. (1993). Fossil beds of the Falls of the Ohio (Special Publication 19, Series 11). Kentucky Geological Survey. https://kgs.uky.edu/kgsweb/olops/pub/kgs/kgsxisp19reduce.pdf

 Hendricks, R. T., Frank R. Ettensohn, Stephen F Greb, & T. Joshua Stark (1994). Geology of the Devonian Strata of the Falls of the Ohio Area, Kentucky-Indiana: Stratigraphy, Sedimentology, Paleontology, Structure, and Diagenesis. Kentucky Geological Survey. https://kgs.uky.edu/kygeode/services/pubs/pub.htm?id=1552
 

 Hooke, R. (1665). Chapter 17: Of petrify'd wood, and other petrify'd bodies. In Micrographia. Wikisource. Retrieved July 7, 2026, from https://en.wikisource.org/wiki/Micrographia/Chapter_17 (Original work published 1665)

Jefferson, T. (1781). To George Rogers Clark, 19 December 1781 [Letter]. Founders Online, National Archives. https://founders.archives.gov/documents/Jefferson/01-06-02-0136

Jefferson, T. (1783). To George Rogers Clark, 4 December 1783 [Letter]. Founders Online, National Archives. https://founders.archives.gov/documents/Jefferson/01-06-02-0289

Jefferson, T. (1788). Notes on the state of Virginia. Documenting the American South, University of North Carolina at Chapel Hill. https://docsouth.unc.edu/southlit/jefferson/jefferson.html (Original work written 1781)

Jefferson, T. (2005). Memorandum books, 1771 [Manuscript]. Founders Online, National Archives. https://founders.archives.gov/documents/Jefferson/02-01-02-0005

Paleobiology Database. (n.d.). The Paleobiology Database. Retrieved July 7, 2026, from https://paleobiodb.org/

Peterson, W. L. (n.d.). Silurian System. In The geology of Kentucky: A text to accompany the geologic map of Kentucky (U.S. Geological Survey Professional Paper 1151-H). U.S. Geological Survey. https://pubs.usgs.gov/pp/p1151h/silurian.html

Rafinesque, C. S., and Clifford, J. D., (1820), Prodrome d'une monographie des
Turbinolies fossiles du Kentuky (dans l'Ameriq. Septentr.) : Gen. Sci.Phys. Bruxelles Annales, 231-235. https://archive.org/details/prodromedunemono00rafi