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.

Paleosol features and grain morphology of the Pipestone Springs strata. A. Alfisols occur as rubbly zones that weather to become resistant units from approximately 12–40 m in the Pipestone Springs reference section. These paleosols are composite and are developed upon extensively reworked volcanic glass and non-volcanic grains. A vertisol occurs atop the 36.00±0.20 Ma upper lapilli tuff at Pipestone Springs. Backpack is 50 cm in length for scale. B. The Pipestone Springs basal lapilli tuff, with a 40Ar/39Ar age of 37.50±0.02 Ma, is a direct ashfall tuff and is partially overprinted by a vertisol paleosol. The vertisol contains pseudo-anticlines and micro-highs and lows. Scale for paleosol features is the circled field notebook at 19 cm in length and the shovel at 54 cm in length. C. Alfisols are the most numerous paleosols within Pipestone Springs strata. These paleosols have composite A–Bt1–Bt2– Bt3 profiles. Field notebook is 19 cm in length for scale. D. Alfisol A–Bt horizons are delineated primarily by the occurrence of abundant trace fossils. E. Backscattered electron microscopy image of sample P1a_7, taken from near the base of the Pipestone Springs reference section, shows volcanic grains (a glass grain indicated by arrow) mixed with detrital non-volcanic components (magnification ×160). A conspicuous rounded and pitted quartz grain is indicated by the white arrow. Overall grain morphology is rounded to subangular, with substantial smectitic clay overgrowth on most grains (indicated by arrow for one grain) (From: Hanneman, D.L , Lofgren, D., Hasiotis, S.T., and McIntosh, W.C., 2022, Priabonian, Late Eocene chronostratigraphy, depositional environment, and paleosol-trace fossil associations, Pipestone Springs, southwest Montana, USA: Acta Palaeontologica Polonica 67 (1): 5–20).

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.

Ichnofossils in the Pipestone Springs strata. A. Example of ichnofossils in the A horizon. B. Elongate ellipsoidal trace similar in morphology to Rebuffoichnus. C. Cylindrical to elliptical structure, partly walled, with meniscate backfill, similar to dung sausages of modern dung beetle nests. D. Helical constructed structure composed of pelletized sediment pressed together to form a wall with meniscate backfill or massive backfill, similar to Eatonichnus. E. Constructed spheres with an opening to a concave hemispherical impression, similar in morphology to Coprinisphaera and Feoichnus. F. Structures composed of meniscate backfills in a rosette pattern that is subsequently hollowed out into a hemisphere with architectural elements of Coprinisphaera, Eatonichnus, Feoichnus, and Rebuffoichnus (From: Hanneman, D.L , Lofgren, D., Hasiotis, S.T., and McIntosh, W.C., 2022, Priabonian, Late Eocene chronostratigraphy, depositional environment, and paleosol-trace fossil associations, Pipestone Springs, southwest Montana, USA: Acta Palaeontologica Polonica 67 (1): 5–20).

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.

Fluvial depositional environment of channel and overbank deposits near Three Forks. Tuff, 36.94 +- 0.04 Ma in age, underlies channel.

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.

Dung beetle burrows developed on lapilli tuff (37.50 +- 0.02 Ma) present at Pipestone Springs near Whitehall.

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.

Ischyromys skeleton (an extinct genus of rodent from North America) from the Pipestone Springs locality

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.