Working on Tertiary strata in the Gravelly Range, southwestern Montana, is sometime daunting to do. The Lion Mountain Tertiary section shown in the photo to the right is one of those places that makes for a grueling day or several days of field work. The Tertiary section unconformably overlies various Paleozoic units, such as Mississippian Madison Group carbonates, Pennsylvanian-Permian quartzite, and Triassic carbonates and red mudstone. And the ascent from these pre-Tertiary rocks to the top of the Tertiary section is worth it – for both vertebrate paleontology and sedimentary features. Current work status in the project that I’m working on with the Raymond M. Alf Museum, Claremont, CA, is that the section contains vertebrates ranging in age from about 40 million years to about 31 million years in age. A tuff unit near the top of the section that we collected has an Ar/Ar age of 31.4+- 0.7 million years. The capping basalt (the dark zone on the top of Lion Mountain) has a reported K-Ar age of 30.8 +- 0.7 million years. Sedimentary features include massive aeolian units and some channeling near the top of the section. A basal surge deposit occurs about 25 m below the capping basalt, signalling the initial pulse of extensive basaltic volcanism in the Lion Mountain locale. Several photos of my most recent Lion Mountain climb illustrate the section’s features and are shown below.
