This article is written to provide an update to “Highlights of Geology Between Three Forks and Whitehall” (1950) by Eugene Perry that was part of the Billings Geological Society First Annual Field Conference (you can view the original article here with the embedded link) and appeared in the MAGMA summer 2025 edition, published by the Montana Geological Society. Perry discussed areal geology ranging in age from the Proterozoic to Quaternary. Much has changed both in the interpretations of this area’s geology and even in its highway system. The primary highway between Whitehall to Three Forks is now Interstate 90 rather than Montana Highway 2. As such, much of Interstate 90 now cuts through continental Tertiary strata instead of the Proterozoic to Mesozoic rocks that outcrop in the Jefferson River Canyon. Because my research focuses on Tertiary strata and Cenozoic basin formation in western Montana, I’ll update the Perry article by briefly summarizing in text and accompanying photographs how interpretations of Tertiary strata depositional environments and timing of major extensional events have changed since 1950.
Perry characterized Tertiary strata of this area as extensive lake deposits that partially fill major valleys. In actuality, large-scale Tertiary lacustrine deposits are not present in the Whitehall to Three Forks area. Instead, Tertiary depositional environments in this area encompass aeolian and fluvial depositional settings. Aeolian environments include massive silty mudstone units 8 m to 15 m thick, and thinner (0.5 m to 3 m in thickness), structureless beds that are separated from each other by pedogenically altered zones.
Paleosols present include vertisols, alfisols, and inceptisols that contain ichnofossils likely constructed by dung beetle (Coleoptera) adults and modified by larvae and pupae based on comparisons to modern and ancient traces attributed to this group.
Fluvial environments present in the Whitehall-Three Forks traverse include channel deposits that generally contain pebble to small boulder size fragments that are clast supported and have a sand to silt matrix. These coarser-grained deposits are typically flanked by finer-grained (sand to clay size sediments) floodplain material.
Also of note are numerous tuff beds that occur within both aeolian and fluvial depositional rock records in the Whitehall-Three Forks area. Age analyses of various sampled tuffs are ongoing but completed age analyses indicate tuff ages ranging from circa 37.5 Ma to 16.5 Ma.
Perry did note the existence of vertebrate fossil material such as horse, rhinoceros, and camel found within the Tertiary deposits of this area. The list of Tertiary vertebrates is very extensive as collection of these fauna has occurred since the late 1800’s. A well-known locality that yields a broad array of Tertiary fossil material is Pipestone Springs, located about 7 miles west of Whitehall. The faunal list for this locality is large and includes creatures such as snake, turtle, rodent, horse, various carnivores, insectivores, lagomorphs, oreodonts, brontotheres, and varied artiodactyls.
Perry also noted his belief that a middle Eocene mature land surface existed and was later disrupted by middle Tertiary large scale block faulting. My research of both surface and subsurface Cenozoic data indicates that extensive basin formation was underway by at least 45 Ma in the Whitehall-Three Forks area. Another phase of extensional activity did occur around 20 Ma, but this resulted in much less basin-fill than the earlier extensional activity.
In summary, much has changed with interpretations of Tertiary deposition and structural history over the past 75 years since Perry penned his article. I’m sure that continued research will continue to present more data and consequently more interpretations in the future.