etween the Cordilleran thrust belt--also called the Lewis Overthrust--and the continually shifting western shoreline of the Colorado Sea, which covered much of the interior of present-day North America during the Cretaceous, is a 3,600-square-mile, 2,000-foot-thick layer of non-marine sediments called the Two Medicine Formation. It contains the documentary history of a time span of roughly 12 million years.
Sometime between 144 and 65 million years ago—following the period parodied in the movie, Jurassic Park—this was prime dinosaur habitat, and today it ranks among the world's most productive fossil-bearing formations. The northern half is especially noteworthy for the nests and bone beds recently found there, which have provided paleontologists with valuable clues relating to dinosaurian nesting habits and herd structure.
During his trip up the Marias in June of 1805, had he not been continually on the lookout for signs of those hostile Blackfeet Indians other tribes had warned him about, Lewis just might have seen some fossils of marine shellfish. And on his more extensive trip in 1806, had he not had his gaze fixed on the horizon, watching for presumably hostile Blackfeet Indians, he just might have counted among his discoveries what would come to be known as a duck-billed dinosaur, a Tyrannosaurus rex, or some hypsilophodon eggs.1
Thomas Jefferson would have been elated!2
John R. Horner and James Gorman, Digging Dinosaurs (New York: Harper & Row, 1988), 36-49.
1. On the other hand, he did manage to pick up six plant specimens, five of them new to science, and carry them on his break-neck ride of July 27and 28. They are now part of the collection at the Academy of Natural Sciences in Philadelphia: Atriplex gardneri (Moq.) D. Dietr, moundscale; Euphorbia marginata Pursh, snow-on-the-mountain; Sphaeralcea coccinea (Nutt.) Rydb., red false mallow; Oenothera cespitosa Nutt. ssp. cespitosa, gumbo evening primrose; Sarcobatus vermiculatus (Hook.) Torr., greasewood.
2. The science of paleontology, new at the time, was one of Jefferson's favorite subjects. See Donald Jackson, Thomas Jefferson & the Stony Mountains: Exploring the West from Monticello (Norman: University of Oklahoma Press, 1981), 29, 39n. Jackson cites George Gaylord Simpson, "The Beginnings of Vertebrate paleontology in the U.S." Proceedings of the American Philosophical Society, 86 (1942), 130–88.