Hepburn's Mesa is the name for the black-capped bluffs on the east side of the Yellowstone River. Its cap is a basalt lava flow from a small local volcanic vent that has since eroded away, and geologically the flow is very young, perhaps as recent as 2.2 million years old. Large amounts of iron give the basalt its dark color, and some of that iron crystallized as magnetite, which is magnetic and in large concentrations can cause compasses to behave erratically. Beneath the basalt lie unconsolidated gravels over older light-colored fine-grained sedimentary rocks deposited in an ancient lake that once occupied this site. Those sedimentary beds contain abundant Miocene fossils, including ancient rodents, moles, and Merychippus, a proto-horse identified as the first equine to show the distinctive head of modern horses. Above the basalt on the mesa top lies glacial till of Pleistocene age, with a hummocky surface strewn with glacial erratic boulders. For generations, Native Americans drove bison off the mesa's cliffs to obtain food, hides, and other materials important to their way of life. The mesa was named for John Hepburn, a local rancher, rockhound, and amateur paleontologist who came to the Emigrant area in 1909 after many years of work in Yellowstone National Park. From 1935 until his death in 1959, he operated a museum beside Secondary Highway 540 displaying many geologic specimens and fossils he had found in the area; it remained a local landmark for many years, still stands beside the old highway, and is listed on the National Register of Historic Places.