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.
The Dog Town Mine Tertiary fossil vertebrate locality is nestled on private property within the southern extent of the Elkhorn Mountains, southwestern Montana. The locality is about 20 miles southwest of Townsend, Montana, where Mesozoic and Paleozoic carbonate, quartzite, and red-colored mudstone, siltstone, and sandstone rocks underlie Eocene (Chadronian) strata. These unconformable Eocene strata contain the Dog Town Mine vertebrate fossil locality.
Earl Douglass (yes, that Earl Douglass of the Dinosaur National Monument fame) first collected at the site on Friday, June 27, 1902 (based on transcriptions from Earl Douglass’ journals done by Alan Tabrum and volunteers from the Carnegie Museum of Natural History). According to his journal, Douglass met a man from Toston, Montana, on horseback and this person told him about the Dog Town Mine, which was located on the divide between the Toston/Townsend and North Boulder Valleys. Douglass was headed to the North Boulder Valley anyways, so he rode to the mine where he found invertebrate fossils (brachiopods and bryozoa) in carbonate rock which was in contact with the ore deposit. A Mr. Allen, who he dined with that evening, told him that more fossils could be found a little ways west of the mine. After dinner Douglass rode a short way west of the mine and found banks along a ravine that looked like Tertiary White River beds. Here he found “Oreodont, Ischyromys, Palaeolagus, Titanotherium, and turtle remains” (June 28, 1902, Douglass Journal entry). This area is the present Dog Town Mine vertebrate fossil locality.
Tertiary strata at the Dog Town Mine are fine-grained, predominantly consisting of siltstone with minor fine-grained sandstone units. The deposits are probably of aeolian origin, originating from areal sediments rich in volcanic ash. These deposits are probably similar lithologically and in mode of origin to those Tertiary White River units found at high elevations within the Laramie Range and Medicine Bow Mountains (Evanoff, E., 1990, Early Oligocene paleovalleys in southern and central Wyoming: Evidence of high local relief on the late Eocene unconformity: Geology, v. 18, p. 443–446; Lloyd and Eberle, 2012, A late Eocene (Chadronian) mammalian fauna from the White River Formation in Kings Canyon, northern Colorado: Rocky Mountain Geology, v. 47, no. 2, p. 113–132).
Vertebrate fossils have been collected at the Dog Town Mine site for various museums since Douglass’ initial collection. The Carnegie Museum of Natural History in Pittsburgh, PA houses a collection from the site as well as the Museum of the Rockies in Bozeman, MT.
Most vertebrate paleontologists probably think of the spectacular dinosaur finds near Jensen, Utah, when the name Earl Douglass is mentioned. Douglass’s discovery of a partial Apatosaurus near Jensen in 1909 did spark the beginning of his long career with finding more dinosaur material in what we now know as Dinosaur National Monument. But Douglass began his quest for fossil vertebrates while he was in southwestern Montana – several years before he was summoned by the Carnegie Museum of Natural History’s director William Jacob Holland to find dinosaurs.
From the spring of 1894 to 1896, Douglass taught at a one-room school in the lower Madison Valley of southwestern Montana. The school house was located in the lower Madison Valley, directly west of the area known as the Madison Bluffs. These bluffs contain strata that range in age from probably as old as Eocene through the late Miocene. The strata are continental units that include alluvial fan to fluvial trunk stream deposits.
During his tenure at the lower Madison Valley school, Douglass spent much of his spare time exploring the Madison Bluffs. At the beginning of his teaching contract in 1894, he had very little knowledge of vertebrate paleontology and of the area geology. He initially considered the Madison Bluff beds as Cretaceous in age. But when he found a “tooth very much like a Protohippus” (Earl Douglass journal entry on May 12, 1894), Douglass knew that the beds were younger in age. As time passed, he began to find a significant quantity of fossil vertebrate mammal material within the bluff’s deposits. Consequently, he immersed himself into reading about comparative anatomy so he could readily identify the fossil material. Douglass eventually used his collected fossil material for his 1899 Master’s thesis at the University of Montana – ostensibly the first Master’s degree awarded by the University.
Douglass kept journals of his time in the lower Madison Valley, and often detailed both the area geology as well as his fossil finds. Alan Tabrum and volunteers from the Carnegie Museum of Natural History have transcribed many of his journal entries from southwestern Montana. I’ve included two portions of journal entries to illustrate his finding of a horse jaw from the bluffs (above diagram) and one of Douglass’s drawings of “Big Round Top” (an area in the bluffs near the one-room school house) as compared to that same area today in a photo that I took about a week ago.
It’s not difficult to understand how Earl Douglass became enthralled with the geology and paleontology of the Madison Bluffs. In addition to the fossil vertebrates, the bluffs contain many other fascinating geological features. Towards the central part of the bluffs (immediately south of the Madison Buffalo Jump State Park), calcic paleosol stacks mark the boundary between most likely Eocene and Miocene strata. The calcic paleosol stacks contain at least two generations of soil profiles (typically minus the A and upper part of the B horizons). Rootlets and burrows are commonly associated with these paleosols.
Volcanic tuffs also occur within the bluff’s strata, which is really handy for those of us who like isotopic age control for southwestern Montana Tertiary deposits. The tuffs could potentially help age constrain the paleosol stacks and sedimentation within the so far non-fossil bearing part of the bluffs. And with the help of the New Mexico Geochronology Lab, a group of us are working on just that aspect of Madison Bluff geology.
The Gravelly Range is located in southwest Montana, about 10 miles southwest of Ennis, Montana. Much of the range is covered by the Beaverhead-Deerlodge National Forest. The Axolotl Lakes Wilderness Study Area, managed by the Bureau of Land Management, is in the northern part of the Gravelly Range.
Our field group was interested in looking at Tertiary rocks, so we headed for the Black Butte – Lion Mountain area, the more south-central part of the range. A cold front had just swept through western Montana a few days prior to my field trip. That storm left some snow up on the range crest – yep, that’s right, snow in July. But it did melt off fast and it left vegetation along the Gravelly Range road (the main road that stretches along much of the top of the range’s extent) extremely lush. So it was a gorgeous drive from the Lyon Bridge crossing on the Madison River up to Lion Mountain and Black Butte. And as Black Butte is the highest peak in the Gravelly Range at 10,542 feet in elevation, it was not difficult to find our destination.
The Tertiary rocks of interest to us were primarily the Tertiary strata exposed on the west side of Lion Mountain. Fossil fauna from these strata have a North American Land Mammal Age of Whitneyan, and are approximately 29 to 32 million years in age. Carnivore, rodent, insectivore, and rabbit are some of the fauna of the fossil assemblage collected here by past workers.
It was a good workout to reach the top of Lion Mountain, but really was well worth the effort. The Tertiary strata had plenty of features to keep a sedimentologist like myself busy. And the views – just spectacular! To top off the trip – it was obvious that someone had been there before us because we found an aluminum ladder stashed is the trees near the top of the Tertiary exposures. None of us availed ourselves of its use, but maybe next time it will come in handy!
I thought that I’d post a link to really interesting work being done by Burke Museum researchers that revolves around this question: which came first – grass or the teeth to eat it??
Link to the study information at: Grasslands Evolution