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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showing only Rocky Mountain Fold-Thrust Belt Show all Geologic Province

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A Traverse Across the Northern Belt Basin From East Glacier Park, Montana to Bonners Ferry, Idaho part of MT Field Guides
This field guide examines differences between three segments of the Belt Basin along an east-west transect from East Glacier Park, Montana to Bonners Ferry, Idaho. Each segment is characterized not only by its structural style, but also by the suite of Belt rocks that comprise it. The easternmost segment consists of the Lewis thrust plate and associated thrusts as far west as Columbia Falls, Montana where it is terminated by the Rocky Mountain trench. This segment contains eastern Belt facies including: Altyn, Appekunny, Grinnell and Helena (Siyeh), Snowslip, Shepard, Mount Shields and McNamara formations. Fine sediments in these rocks, probably derived from a western continental source terrane, were deposited adjacent to the stable North American craton, where they were mixed with smaller amounts of coarse sand derived from an inferred coarse sand sheet that mantled the crystalline craton. The central segment extends from the Rocky Mountain trench to the Libby thrust system. It is characterized by broad, open folds that expose Belt rocks of the central part of the basin, including a thick section of Prichard, Burke, Revett, St. Regis, Empire, Wallace and Helena formations. These rocks are composed mostly of argillite, carbonate and fine-grained quartzite. The western segment includes the Libby thrust and Leonia fault system. It extends as far west as the Purcell trench. Exposed in this area are rocks from the Prichard through the Missoula Group. Rocks of the Ravalli Group and the Wallace Formation contain more fine-to medium-grained quartzite than those of the central segment, reflecting a western source. However, the Missoula Group rocks are finer grained and more calcareous than those of the type area, indicating that late in the Middle Proterozoic the western source terrane probably subsided and that the basin center shifted westward.

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

A Traverse Across the Eastern Belt Basin From Neihart to Townsend, Montana part of MT Field Guides
This road log focuses on Precambrian (Proterozoic) sedimentary rock deposited in the Helena embayment of the eastern Belt basin. It also includes a general description of the geology between exposures of Proterozoic strata, briefly describing occurrences of Precambrian igneous and metamorphic rocks, Paleozoic sedimentary rocks, and Tertiary igneous rocks. A stratigraphic column is provided for refeference.

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

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

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

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

Geology of the Butte Mining District part of MT Field Guides
The Butte mining district is one of the major mining districts of the world with continuous production from both underground and open pit mines for 119 years, from 1864 to 1983. During this time, mining activites at Butte have introduced many firsts in mining methods, techniques of mine mapping, and the detailed recording of geologic data. This field guide describes the geology of the mineral deposits, provides a geologic map of the district, and concludes with a tour of eight locations of interest, including Alice pit, Syndicate pit, Berkeley pit, the Emma and Orphan Boy veins of the Anaconda system, the World Museum of Mining, and the Montana Tech Mineral Museum.

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

Structural Geology of the Sawtooth Range at Sun River Canyon, Montana Disturbed Belt, Montana part of MT Field Guides
The Sawtooth Range is one of the best exposed examples of imbricate thrust faulting in the foreland fold and thrust belt of the western United States. It is an outstanding field classroom for the demonstration of stratal shortening and the various structural features associated with thrust belt terranes. This 7-stop traverse through Sun River Canyon offers excellent exposures of thrust contacts along the road and panoramic views of stacked thrust sheets and demonstrates the dependence of structural style on relative competence of units in the stratigraphic section.

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

Hydrologic Aspects of Metal Mine Impacts on Upper Clark Fork Superfund Sites, Butte - Warm Springs, MT part of MT Field Guides
This road log concentrates on environmental impacts of historic mining activities in the Butte Mining District, particularly with regard to water quality. The trip decribes the general geology between Bozeman and Butte and provides a detailed overview of the geology, hydrogeology, mineralization, and mining history in the Butte district. Specific topics included in this field guide are: water quality changes in underground mines and the Berkeley Pit, impacts and remediation of mine tailings and processing plants (mills, smelters, and reduction plants), and groundwater contamination and remediation. Detailed information from geotechnical and hydrogeological studies related to these topics is included.

Topics: Hydrology, Environmental Geology
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

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