Tertiary Geology and Paleontology in the Gravelly Range, Southwestern Montana

Lion Mountain in the Gravelly Range of southwestern Montana. This area is federal land managed by the U.S. Forest Service.

A part of my recent geological field work includes working on high elevation Tertiary strata in the Gravelly Range, southwestern Montana. The Gravelly Range is located in southwest Montana, about 10 miles southwest of Ennis, Montana. For some background on this area and what my field work is about, see an older blog that I posted at Geopostings.

So – now that one field season is done and field data compiled, both my co-worker, Don Lofgren and myself have interpreted some of our data. We recently outlined our work at the Geological Society of America’s (GSA) Rocky Mountain section meeting in Calgary. Alberta. The abstract from our session is given below as well as the poster itself in both a jpeg format and as a link to our  GSA presentation.

“Tertiary strata exposed in four high elevation areas in the south-central
Gravelly Range yield significant assemblages of Late Eocene to Oligocene
mammals. The thickest stratigraphic sections of Tertiary strata are in the
Lion Mountain-Black Butte area. The Lion Mountain section age is based
primarily on American Museum of Natural History collections; the lower
part of this section is Duchesnean-Chadronian (39-33 Ma) and the
uppermost beds are Whitneyan (32-31 Ma). Age of the basal part of the
Black Butte section is Duchesnean-Chadronian based on Harvard Museum of Comparative Zoology collections. Recent collections that include Miohippus indicate a probable Orellan age for uppermost exposures. The Tepee Mountain section is notable for abundant brontothere remains and is probably Duchesnean-Chadronian (approx. 39-33 Ma). The Rapamys site is the oldest vertebrate locality and is late Uintan to early Duchesnean (42-38 Ma) based on recently recovered specimens of RapamysProtoreodon, and Lycophocyon.

The Tertiary strata in this part of the Gravelly Range include fluvial, aeolian, and tufa deposits that are most likely mainly associated with localized Oligocene volcanism. The Lion Mountain section is about 270 meters in thickness; the lower half of the section is largely aeolian, with fluvial units comprising much of the upper section. Based upon age data, the 140 meter Black Butte section correlates to the lower 50-70 meters of the Lion Mountain section. The basal 20 meters of the Black Butte section contain some fluvial features, but much of the remaining section is largely aeolian in origin. Paleosols and extensive burrowing also occur within the Black Butte section. Stratigraphic section thickness decreases rapidly away from the Black Butte-Lion Mountain area, with section thicknesses of about 20 meters for the largely aeolian Rapamys and Tepee Mountain sections. Tufa deposits are located along the west-central edge of the Gravelly Range where they are associated with previously mapped thrust faults. Leaf imprint assemblages of Eocene-Miocene age are contained within these tufas. Strata previously mapped as Upper Cretaceous-Paleocene Beaverhead Formation are now variously reassigned to the lower Cretaceous Kootenai Formation, southwestern Montana Cenozoic Sequence 2, and diverse Quaternary units.” From: Abstract from Geological Society of America Abstracts with Programs. Vol. 49, No. 5 doi: 10.1130/abs/2017RM-293156.

The poster presented at the 2017 Rocky Mountain GSA is available below as a jpeg and at GSA as a pdf.

Prospecting For Vertebrate Fossils On Hot Summer Days In Southwestern Montana

The end of July always brings The Webb School students who are interested in paleontology to southwestern Montana. That time is packed with prospecting a variety of Tertiary sites in the hopes of finding interesting vertebrate fossils. This year had its good finds along with persevering through some really hot days. Being on a surface of light-colored rocks under the intense sun while slowly looking for fossils such as rodent jaws, rabbit teeth, or even isolated horse teeth is a tough way to spend a summer day. Even prospecting for larger pieces of fossil vertebrates is no easy day, but the students hung in there. Here’s a few scenes from the prospecting adventures:

The late Eocene strata at Pipestone held the students attention for many hours
The late Eocene strata in the Pipestone area west of Whitehall held the students attention for many hours.


Late Tertiary outcrops north of Whitehall, Montana, yielded some interesting horse and camel remains.
Late Tertiary outcrops north of Whitehall, Montana, yielded some interesting horse and camel fossils.


A horse jaw and at least part of the skull was found in late Tertiary strata.
A horse jaw and at least part of the skull were found in late Tertiary strata located near Whitehall.


Still searching for that elusive fossil....
Still searching for that elusive fossil in the North Boulder Valley….


Late Tertiary canid dog skull fossil find from last season's Webb School  paleo students' efforts. Skull is now at the Museum of the Rockies.
Late Tertiary canid dog skull fossil find from last season’s Webb School paleo students’ efforts. The skull is now at the Museum of the Rockies in Bozeman, Montana.








For those truly interested in vertebrate paleontology, keep in mind that the renowned Raymond Alf Museum is also on the Webb Schools campus. The museum is definetly worth a visit.