Among the various types of National Park Service units are National Historic Trails. Many of these pass through fossiliferous areas, but stating that fossils are found on a given trail is not always easy. Unlike national parks and national monuments, where the NPS owns and manages parcels of land, the trails are not defined by boundaries, per se. In their absence, I like to look for reports of fossils made by users of the original trails, like the handful of fossils observed by Lewis and Clark, or John Strong Newberry's extensive description of the Santa Fe Trail. That at least shows a historical association.
Juan Bautista de Anza National Historic Trail, located in Arizona and California, recognizes the 1775–1776 expedition led by Juan Bautista de Anza to establish a Spanish presence in San Francisco Bay and an overland route to the area from greater New Spain, as the California coast was otherwise only accessible by water. If you're from the Southwest or have an interest in the Cenozoic paleontology of that area, you may have already recognized that name as part of California's Anza-Borrego Desert State Park, and you may have guessed where I'm going...
Anyway, one of the members of the expedition was Padre Pedro Font, a Franciscan missionary. Font was the expedition's chaplain, was responsible for taking latitudes, and also kept a diary (link to English translation). From this we can see that the expedition was not especially comfortable for him; a significant part of the trip he was ill with various maladies, and there were several bouts of bickering with his commander. (If someone asked you to lug around a psalterio and then never asked you to play it, or didn't give you access to the surveying tool you were supposed to be using, all while you were suffering from malarial fevers, you'd probably be annoyed too.) Also, of course, the reader should not expect that an 18th century missionary's first response to unfamiliar natives is going to be "These people are doing all right and we should certainly not try to change them in any way." That said, Padre Font displayed a canny eye for natural history in his entry for December 9, 1775:
"…On the road about a league after starting there is a salty lagoon without pasturage, and at about four leagues a little well of salty water which Father Garcés called El Rosario... On account of the unfruitfulness of these lands, so level, and of the aspect of the sand dunes, and especially of the abundance of shells of mussels and sea snails which I saw today in piles in some places, and which are so old and ancient that they easily crumble on pressing them with the fingers, I have come to surmise that in the olden time the sea spread over all this land, and that in some of the great recessions which the histories tell us about it left these salty and sandy wastes uncovered... It is not possible that people should have made such mountains of shells by carrying them from the sea so great a distance merely to bury them in piles. All of which, although merely conjecture, has a high degree of probability."
At this point the expedition had just left Laguna de Santo Olella, Site #44 on this map. It's southwest of Yuma in Baja California, west of the Colorado River, and well north of the Gulf of California. We know today that he was completely correct: the Gulf had been this far north, and even farther, thanks to tectonic shenanigans, and had left beds with marine shells. Excellent reasoning by the Padre, without a geologic or paleontologic framework to lean on. One slight catch for the National Historic Trail, though: as noted, this site is in Baja California. Did he observe any fossils in what is now the United States? On December 11, on the part of the trail that crosses the international border, he reported
"One sees along the way many piles of mussels and an infinity of sea snails, very small and spiral shaped, and in places as white as flour. This confirms my opinion that this is a sea beach, and although the sea has reached here sometime, yet no barrancas are found like yesterday."
We're closer, although technically it's possible he saw them entirely on what is now the Mexican side. A few days pass without more shells, then bingo. On December 19:
"...We set out from San Sebastián [Site 49] at one o'clock in the afternoon, and at half past four halted at a flat [Site 50] with some galleta grass but without water, having traveled some four leagues west by north. The road is level, with sandy but firm soil, having some mussel shells and sea snails..."
At this point we are well within California, actually just east of Anza-Borrego Desert State Park in the Ocotillo Wells area. Not only does Font get credit for an astute inference about natural history, but he also cinches Juan Bautista de Anza National Historic Trail as a fossiliferous NPS unit (and one of the few with a report predating the Declaration of Independence).
