The Montana-Yellowstone Geologic Field Guide Database
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Use this page to search our collection of educationally-useful geologic field guides and road logs in Montana and Yellowstone. You may search the database by entering a keyword to search or choosing one of the listed terms for geologic topic, geographic location, or geologic province.


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Guide to the Chrome Mountain Area part of MT Field Guides
The Chrome Mountain area lies at nearly 10,000-feet elevation in the west-central part of the Stillwater Complex between the main part of the Boulder River and the headwaters of the East Boulder River. At Chrome Mountain, the olivine +/- bronzite +/- chromite cumulates of the Ultramafic Series are relatively well exposed and stratigraphically broadly resemble those at Mountain View. In addition, a distinctive rock type occurs: a fine-grained dunite which appears to have formed as a replacement of the ultramafic cumulates.

Topics: Igneous rocks, Metamorphic rocks, Resources
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Road Log from Bozeman to Specimen Creek via Gallatin Canyon and U.S. 191 part of MT Field Guides
This trip leads southward through the Gallatin Range to Specimen Creek in Yellowstone National Park. It affords an excellent worm's-eye view of the structure and stratigraphy of this range as revealed both laterally and vertically through the quietly beautiful Gallatin Canyon. The route also borders the east margin of the Madison Range and its spectacular Spanish Peaks uplift. The Gallatin and Madison Ranges are geologically and topographically similar, an essential difference being the thick cap of andesitic lava and breccia that covers the high parts of the Gallatin Range.

Topics: Surficial geology, Hazards, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Gish Mine Area part of MT Field Guides
The Gish mine is located in the Boulder River valley near the west end of the Stillwater Complex. At Gish, the Peridotite zone of the Ultramafic series is exposed, and features of particular interest include an excellent exposure of discordant dunite and a major chromite seam which has been correlated with the G chromitite elsewhere in the complex. Other notable features include: (1) cyclic units which exhibit some lateral variation, and (2) a possible unconformity that separates the series of cyclic units (olivine-bronzite cumulates and bronzite cumulates), which the discordant dunite intrudes, from the major series of olivine cumulates which enclose the main chromite seam.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Stillwater Complex Exposed in the West Fork Area part of MT Field Guides
The parts of the Stillwater Complex exposed along the West Fork of the Stillwater River were the site of early chromite exploration, followed by Cu-Ni-sulfide exploration and, finally, platinum-group-element (PGE) exploration. The best exposures of Troctolite-Anorthosite zone I (TAZ I) anywhere in the complex and the discovery outcrops of the J-M Reef occur in cliffs on the west side of the river and are readily accessible. Parking for these outcrops is immediately beyond the bridge across the West Fork of the Stillwater River at the Absaroka-Beartooth Wilderness boundary. The lower chromite seams in the Peridotite-zone cumulates are well-exposed in the West Fork area but require nearly a full day to visit on foot. These exposures provide the opportunity to trace chromite seams and to study the changes in their internal stratigraphy and chemistry. The West Fork area illustrates quite well that not all members in Peridotite-zone cyclic units are continuous.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Road Log to the Picket Pin Mountain, Chrome Mountain and Contact Mountain Areas part of MT Field Guides
This road log describes the geologic features along the access roads for three additional traverses: Guide to the Picket Pin Mountain Area, Guide to the Chrome Mountain Area, and A Traverse Through the Banded Series in the Contact Mountain Area. "This trip requires nearly 2 hours without stops. Beyond 12.5 miles, the road is rough and steep in places; a four-wheel-drive vehicle is recommended.

Topics: Structures, Sedimentary rocks, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

A Traverse Through the Banded Series in the Contact Mountain Area part of MT Field Guides
This traverse through the Banded Series of the Stillwater Complex follows the section described in detail by McCallum and others (1980)....Unusual or particularly interesting features are highlighted by the 22 localities along the traverse. In view of the nearly continuous outcrop, however, there is much of interest to see between localities, and full appreciation of the entire section would require several days. Nonetheless, most of the important rock types can be seen on a 1-day trip.

Topics: Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Crazy Mountains, Montana part of MT Field Guides
This trip examines a variety of mid-Eocene alkalic (feldspathoidal) stocks, laccoliths, sills, and dikes emplaced into Cretaceous and Paleozoic sedimentary strata. These rocks include mafic and felsic varieties, and are both texturally and compositionally variable. Most of the mafic alkalic rocks are unique in the Montana Alkalic Province in having Na2O > K2O. They are unusual even among feldspathoidal rocks because they are stongly enriched in incompatible elements, and have Sr, Nd, and Pb isotopic compositions which reflect an ancient source having low Rb/Sr, Sm/Nd, and U/Pb.

Topics: Structures, Metamorphic rocks, Igneous rocks, Sedimentary rocks
Geographic Location: South-Central Montana
Geologic Province: Central Rocky Mountains Foreland Province

Helena and the North Border Zone of the Boulder Batholith, Montana part of MT Field Guides
The geologic descriptions for this road log roughly follow the scalloped north edge of the Boulder batholith. The will be ample opportunity to examine the intrusive rocks, and to the north, the broad border zone of contact-metamorphosed Paleozoic and Mesozoic sedimentary rocks. The route will follow the highway eastward, with a stop in granodiorite of the batholith. The log then detours to the south into the mountains along Colorado Gulch to view skarn and hornfels developed in the Madison Group and Three Forks Formation in a "septum" (or "screen") of country rock caught between two lobes of the batholith. The log returns to Highway 12 and continues east to Helena's Reeder's Alley and Grizzly Gulch. After a visit to a talc vein and teh unconformity between Precambrian (Belt) and Cambrian formations, the trip leaves Helena southbound on Interstate 15. Near Montana City are stops at an outcrop of hornfelsed, stromatolitic Helena Formation (Precambrian Belt Supergroup) and at the contact with the Boulder batholith. The final stop is to examine the complex of intrusions, wall- or roof-rock inclusions and structures in the batholith's border zone. In this area, Precambrian, Paleozoic and Mesozoic sedimentary rocks dip southwest and south toward the intrusion. Within about one to two miles from the batholith, the sedimentary sequence is contact-metamorphosed, yet in most places sedimentary structures are remarkably well preserved.

Topics: Structures, Resources, Metamorphic rocks, Sedimentary rocks, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Geology of the Stillwater Complex Exposed in the Mountain View Area and on the West Side of Stillwater Canyon part of MT Field Guides
The part of the Stillwater Complex exposed on the west side of the Stillwater canyon has been the site of concentrated prospecting and geology activity since Jack Nye and Jimmy and Jonas Hedges found sulfide-rich rocks there in 1883. Readily seen in this area are well-studied and excellent exposures of: (1) most rock types, (2) ore deposits and mineralized occurrences, (3) structural features and (4) stratigraphic and age relations between the metamorphosed sedimentary rocks and the layered ultramafic-mafic stratiform intrusion. In addition, there are good exposures of a quartz monzonitic suite, which intrudes the complex and the metamorphosed sedimentary rocks, and of the Beartooth gneisses. Because the rocks in this area have been deformed by several episodes of faulting, a synopsis of the structural relations is given to clarify the geologic setting. Detailed overviews of the geology specific to the area are presented, and the geology of the more interesting exposures in the area is described in a road log that begins in the Ultramafic series, proceeds through the Basal series, the Banded series, and the quartz monzonite that cuts the complex, and ends in the Beartooth gneisses.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

A Traverse Across the Central Belt Basin From Bowmans Corner, Montana to East Hope, Idaho part of MT Field Guides
This road log highlights a variety of sedimentary rock types and structures along a transect of the Rocky Mountain Fold and Thrust Belt. The log follows Montana Highway 200, which crosses the central part of the Middle Proterozoic Belt basin from the Montana disturbed belt to Missoula, where it turns northwesterly into northern Idaho, traversing rocks of the western and northwestern Belt basin. The road log begins east of the Rocky Mountain front in the disturbed belt underlain by soft, Cretaceous shale and somewhat more resistant sandstone units. Next, the road log passes into the eastern thrust belt where thrust faults bring Belt rocks first over Cretaceous, then over Paleozoic rocks, and finally over Proterozoic rocks farther to the west. At Rogers Pass, the route crosses the Continental Divide and into the Ovando block where Cenozoic listric normal faults form the major structures. The leading edge of the western thrust belt is encountered at Bonner, MT. From Missoula, Highway 200 trends northwestward to northern Idaho and diagonally crosses the western part of the Belt basin. Changes in grain-size and sediment type observable from outcrops along this road log illustrate the evolution of sedimentary transport and facies tracts within the central part of the Belt basin.

Topics: Surficial geology, Structures, Fossils, Metamorphic rocks, Igneous rocks, Sedimentary rocks
Geographic Location: Northwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt