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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Quaternary Geology and Faulting of the Helena Valley, Montana part of MT Field Guides
This field trip will give the participants an overview of the Quaternary deposits and late Cenozoic faulting which have shaped the Helena valley. The Helena valley is a NW-trending graben surrounded by bedrock highlands. Most of the western half of the valley is a young alluvial plain formed of coalescing alluvial outwash fans issuing from drainages flowing towards Lake Helena. The western valley is ringed by older pediment surfaces sloping gently down from the valley margins. The eastern half of the valley is underlain primarily by later Tertiary silts, sands and gravels uplifted and segmented by normal faulting. Atop these Tertiary deposits along the southern valley margin, early Quaternary alluvial deposits are preserved as hilltop remnants of a formerly much larger deposit. Late Cenozoic faulting shaped the Helena valley, creating the modern topography and influencing sedimentation patterns; however, only a few faults show evidence of late Quaternary offsets.

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

Economic Geology of the Greater Helena Area: Helena to Montana Tunnels Mine, Lump Gulch, Grizzly Gulch, Fort Harrison and Marysville part of MT Field Guides
The Helena area has a long history of base- and precious-metal production from the Corbin-Wickes District on the south to the Marysville gold district northwest of the Capitol City. Gold has also been mined from the placers along Last Change Gulch now within the city limits. This field trip examinations the volcanic-hosted ore body at the recently opened Montana Tunnels mine south of Helena. From this mine, participants will travel north to Helena through an area of numerous inactive base- and precious-metals mines at the northern margin of the Boulder batholith and then onto the Marysville district northwest of Helena where the gold deposits are related to the Marysville stock.

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

Sapphire Deposits of the Missouri River Near Helena, Montana part of MT Field Guides
Sapphires occur with gold, both in terrace deposits along the Missouri River and in the active river channel. Both of these minerals have been recovered from these deposits beginning with recovery of gold in the latter part of the 1800's. More recently, up until the early 1940's, sapphires were also recovered from these operations for industrial uses. In recent years recovery of sapphires by recreationists for faceting into gem stones has become important. Five deposits open for fee digging will be visited during this field excursion. They are situated on the French, Eldorado, Spokane and Gruell's bars. Also, exposures of a sapphire-bearing andesite sill close to the Missouri River and considered a source for some of the sapphires found in the gravels will be examined.

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

The Middle Yellowstone Valley from Livingston to Gardiner, Montana: A Microcosm of Northern Rocky Mountain Geology part of MT Field Guides
The middle Yellowstone valley, between the Great Plains at Livingston and the edge of the Yellowstone volcanic plateau near Gardiner, is a complex palimpsest of lithology, structure, and surficial processes. It shares basement rocks with the continental interior, largely to the north and east; Paleozoic lithologies with the western interior, compressive tectonics with the Fold and Thrust Belt to the west; extension with the Basin and Range to the west and south; and Cenozoic volcanism and elements of its geomorphic evolution with much of the surrounding region. The geological exploration of this region serves as a microcosm of the evolution of the geological understanding of the American West.

Topics: Hydrology, Resources, Fossils, Environmental Geology, Hazards, Surficial geology, Sedimentary rocks, Igneous rocks, Structures, Metamorphic rocks
Geographic Location: Yellowstone National Park, Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Volcanism and Plutonism at Shallow Crustal Levels: The Elkhorn Mountains Volcanics and the Boulder Batholith, Southwestern Montana part of MT Field Guides
The Upper Cretaceous Elkhorn Mountains Volcanics (EMV) and Boulder batholith of southwestern Montana provide an example of a large-volume, epizonal, volcanic-plutonic complex whose deep level of erosion has exposed the cogenetic intrusive rocks while preserving sizeable portions of the volcanic field. Such a volcanic-plutonic association provides a unique opportunity for evaluation of many aspects of the evolution of a shallow-crustal magmatic system, such as geochemical relations of both the volcanic and plutonic rocks and the nature of intrusive-extrusive relationships at the present level of exposure.

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

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

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