Initial Publication Date: January 13, 2025

FIELD GUIDE: Columbus to Red Lodge, Montana: Stillwater River Valley, West and East Rosebud Creeks, West Fork Rock Creek

David Mogk, Professor Emeritus, Dept. Earth Sciences, Montana State University

(Download to your mobile device or print the PDF of this Field Guide (Acrobat (PDF) 9.8MB Dec31 24) for information about Beartooth Country when traveling in areas where there is no cell or wi-fi access).

Overview


From Columbus, head south on highway MT 78 to Absarokee (14 miles). Three miles south of Absarokee take the Nye Road (#419) to Fishtail and Nye. This road gives access to the Stillwater River Valley, West Rosebud Creek, the main body of the Stillwater Complex and the main Beartooth crystalline massif to the south. The East Rosebud Creek can be accessed from Roscoe (13 miles south of Absarokee). Red Lodge is 20 miles from Roscoe. See the USFS North Beartooth Front Motor Vehicle Use Map (Acrobat (PDF) 1.2MB Dec24 24) for authorized motor access to the Picket Pin, Iron Mountain, and Benbow Mine areas. The USFS South Beartooth Front Motor Vehicle Use Map (Acrobat (PDF) 743kB Dec24 24) identifies authorized motor vehicle use for West Fork Rosebud Creek, East Fork Rosebud Creek, West Fork Rock Creek (numerous campgrounds), and Rock Creek area (south of Red Lodge).

See an overview of the Stillwater and East Boulder Mine operations from Sibanye Stillwater. "Sibanye-Stillwater is a multinational mining and metals processing group with a diverse portfolio of operations, projects and investments across five continents. The Group is also one of the foremost global recyclers of PGM autocatalysts and has interests in leading mine tailings retreatment operations. Sibanye-Stillwater is one of the world's largest primary producers of platinum, palladium, and rhodium and is a top tier gold producer."

An overview of the Stillwater Complex geology, geology of the Mountain View section (ultramafic zone through the lower to middle banded series), and a walking traverse across the Picket Pin section (upper banded series and anorthositic section) is provided by McCallum (2012) (Acrobat (PDF) 4.6MB Dec24 24), STILLWATER COMPLEX Geology and Field trip guide Second Conference on the Lunar Highlands Crust July 12-16, 2012, Bozeman, Mont

The 1985 Stillwater Complex volume that includes most of the field guides referenced below can be purchased from the Montana Bureau of Mines and Geology ($25): Czamanske, G., and Zientek, M., 1985, The Stillwater Complex, Montana: Geology and guide, Reprint 2002: Montana Bureau of Mines and Geology Special Publication 92, 395 p.


Stillwater Valley and Stillwater Complex

  • Stillwater River Valley: The geology of the Stillwater River Valley from MT 78 to Fishtail and then on to Nye is described in the field guide by Jenkins et al., (2024) (Acrobat (PDF) 146.1MB Dec10 24). East of the town of Fishtail the bedrock is the Paleocene Fort Union Formation (Tongue River, Lebo and Tullock Members). To the west, the rocks are Late Cretaceous Hell Creek Formation, mostly Livingston Group Sliderock Mountain Formation. Further west the rocks continue down stratigraphy to the Late Cretaceous Judith River Formation, Claggett Shale, and Eagle Sandstone Fishtail Butte and Nellie's Twin Buttes are cut by Late Cretaceous breccia dikes. Near the town of Dean, the Dean Dome is a Laramide (65 Ma) anticlinal structure that is rimmed by the Eagle Sandstone with the Niobrara Formation and Early Cretaceous Kootenai formation in the core. Oil and gas were produced on this structure until 2010. Information on camping and hiking in the Stillwater Valley is provided by the USFS.
    • The Stillwater Mine is located just west of the town of Nye. It has been in operation since 1986. This is the only mine in America that produces platinum and palladium. The mine facilities and tailings pond can be seen from the road.
    • The white cliffs across the Stillwater River show the Laramide fault that overturned Paleozoic sedimentary rocks over-ridden by overturned anorthositic units of the Stillwater Complex.
    • The Mouat Quartz Monzonite is a unique granitic body that is exposed in the outcrops just west of the Horseman Flats Road (next section) and also along the road to Woodbine Campground. This unit is significant because it was originally interpreted by Page (1977) to cut both the Stillwater Complex (2,71 Ga) and the Archean gneisses (2.8 Ga) south of the Mill Creek Fault Zone--thus it should have been younger than both. However, recent U-Pb zircon dating of this unit yields an age of 2.803 Ma (Wall et al., 2018) so it is ~100 million years older than the Stillwater Complex.
    • Woodbine Campground at the end of the road is also the trailhead to the Stillwater River Trail  (FS 24; see description from MontanaHikes.com ).  This trail traverses a narrow gorge through Archean granitic gneisses. There is a gorge bypass trail. However, these trails were highly impacted in the 2022 floods and access may be limited and difficult.  A hike about 3 miles upstream takes you to Sioux Charlie Lake, and the trail continues for another 23 miles to the headwaters of the Stillwater River to the trail head at Lake Abundance near Cooke City, MT (see: Cooke City Area Trail Guide). Side trails to the west provide access to Lake Pinchot, Wounded Man Lake and the Lake Plateau area.  
  • Horseman Flats Road (FS 2846) to West Fork Stillwater River--this road heads west off the Nye Road at the south end of the Stillwater Mine operations. As noted above, the outcrops to the south of the road are the Mouat Quartz Monzonite.  BE CAREFUL OF MINE TRAFFIC on this road and spur roads as the road can be narrow and steep.  Key features on this road are:
    • Outcrops on the road between the Nye Road and the Mountain View Road (FS 28466) are mostly units in the Banded Zone of the Stillwater Complex, that include varieties of gabbros (plagioclase-clinopyroxene), norites (plagioclase-orthopyroxene), gabbronorites (plagioclase-clinopyroxene-orthopyroxene), less abundant troctolites (plagioclase-olivine), and anorthosites (dominantly plagioclase). Igneous layering is evident in these rocks from meter- to tens of meter scale.  Many units have large "oikocrysts" (egg-crystals) that can be centimeters across of late-stage growths of either ortho- or clinopyroxenes.  A most notable outcrop is the "inch-scale layering" outcrop on this section of road that has developed doublets of green clinopyroxene layers separated by an inch thick layer of white plagioclase.  PLEASE PRESERVE THIS OUTCROP AND DO NOT SAMPLE! IT IS ONE OF THE IUGS DESIGNATED "FIRST 100" GEOSITES.
    • Mountain View Road: This road is gated and closed to public access. It is included in this description because this leads to the Mouat mine that extracted chrome from the ultramafic zone to support the WWII war effort from 1941-1943.  The road traverses the ultramafic zone with the famous "G Horizon" of chromite, the Lake Camp mining camp and ore crushing plants, and the contact with the lower Banded series and anorthositic units. The historical remnants of the Lake Camp are still present, the site that housed ~500 miners during the limited chrome mining operation during WWII. For more detailed information about this remarkable section of the Stillwater Complex refer to these Geologic Guides to the Mountain View Area, Stillwater Complex: 



  • West Fork of the Stillwater River Road: The end of this road is the location of the West Fork Adit and one of the 1974 "discovery outcrops" of the famous J-M (Johns-Manville) Reef that hosts the Pt-Pd deposits. Access the West Fork Stillwater Trail #90 Guide for further information.


  • Picket Pin Road, Picket Pin Mtn, Iron Mtn, Chrome Mtn, Contact Mtn--From Carter's Camp, head north on Stillwater Road (#420) for one mile to Limestone Road and continue west.  Limestone Butte is dominant on the skyline to the west and is capped by Mississippian Madison Limestone and Amsden Formation. Stay on the road towards Picket Pin and Iron Mountains.  Approximately 14 miles from Carter's camp, the Great Unconformity is exposed where Paleozoic sedimentary rocks (Madison Limestone) are unconformably deposited on the Middle Anorthosite Zone of the Banded Series of the Stillwater Complex. Drive with caution in this upper section of road, the Picket Pin Cirque, as there are blind corners, it has a very steep grade and narrow road tread, and this section of road may preserve a large road-blocking snow drift late into the summer season. 4-wheel drive vehicles are recommended.  Continuing along this road you can access:
    • Picket Pin Mountain, via a short hike to the north that exposes numerous units of the Middle to Upper Banded Series and Anorthositic units;
    • Iron Mountain and Chrome Mountain areas that expose the lowermost Basal Series and Peridotite Zone of the Stillwater Complex and the underlying contact metamorphic aureole to the south;
    • Contact Mountain area that exposes the upper Banded Series and Anorthositic Units.
    • For more geologic information: 


  • Benbow Mine Road--From the intersection with Highway 419 near Dean, MT, head south on the Benbow road to the historic Benbow Mine (~18 miles). From Dean to the Beartooth Front, the road initially traverses numerous Cretaceous units (Niobrara Formation, Eagle Sandstone, Judith River Formation). After the Stillwater Mine water processing plant (~4 miles), lower Cretaceous Thermopolis Shale rapidly transitions to older Mesozoic and Paleozoic sedimentary strata. Geraghty (2024) documents complex structures in this area that includes the main Laramide reverse faulting with both forethrusts and backthrusts. The Great Unconformity is also recognized in this area with Middle Cambrian Wolsey Shale deposited on the Stillwater Complex. Chromite was originally mined at the Benbow Mine in 1905, and again during WWI and WWII, but the quality of the ore could not compete with other foreign sources.  More information on the Benbow Area is provided by the USFS.  Benbow Area Trail Maps are available for the Stillwater Plateau Trail (#51), East Fishtail Trail (#37A), Island Lake Trail (#37B) and East Fishtail Spur Trail (#37C).

    For more geologic information on the Geology of the Benbow Mine area:

West and East Fork Rosebud Creeks, West Fork Rock Creek

  • West Rosebud Road--Access the West Rosebud area from Fishtail MT, taking the West Rosebud Road towards Mystic Lake. This road and trail head were severely damaged by the June 2022 epic floods so check ahead for road conditions and access.   This is the most common route to reach Granite Peak, the highest point in Montana (12,807 feet).  The bedrock geology is dominantly the 2.8 Ga granites similar to the Long Lake Magmatic Suite on the Beartooth Highway. This is also an area of spectacular glacial and periglacial landscapes. West Rosebud Trail (#19) provides access from the trailhead at the end of the road to Mystic Lake, and the Phantom Creek Trail (#17) over Froze to Death Plateau to the East Rosebud Creek are popular trails in this area. Access the West Rosebud Trail Guides. Other information on the West Rosebud Area is available from the USFS. 
    • Granite Peak: "Granite Peak is the tallest mountain in Montana (12,799ft). Two of the more popular routes to Granite Peak are accessed from either West Rosebud TH or the Phantom Creek TH, via Huckleberry Creek or from the Froze to Death Plateau. There are no maintained trails to Granite Peak. The mountain is remote and the
      terrain is very rugged."

 

From Roscoe to Red Lodge on Highway 78 (23 miles), the bedrock geology around Roscoe is the Late Cretaceous Judith River, Bearpaw Shale, and Hell Creek Formations, but these rapidly transition to the Tertiary Fort Union Formation (Tullock, Lebo and Tongue River Members).  There are spectacular views of the Beartooth Front from the highway with palisades of the near-vertical Madison Limestone cliffs in the foreground and Archean basement rocks on the horizon. Red Lodge Ski area runs are shown in this image.