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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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

Field Guide, Little Belt Mountains part of MT Field Guides
This field guide extends from Monarch to Utica across the northeastern portion of the Little Belt Mountains. The trip, via Hughesville, Yogo Peak and the Yogo sapphire mines, provides an overview of Laramide igneous activity in the Little Belt Mountains of central Montana. The mountains were formed as a large anticline in the Late Cretaceous to late Paleocene or earliest Eocene. The forceful intrusions in the Little Belt Mountains by felsic, hence viscous magmas, contrast sharply with the low-viscosity, basic, alkaline extrusives in the Highwood Mountains to the north.

Topics: Surficial geology, Resources, Fossils, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Central Rocky Mountains Foreland Province

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

Field Guide; Belt Butte and Tiger Butte part of MT Field Guides
This field trip extends east from Great Falls across rolling glacial plains to the summit of Belt Butte, where its formation and the collapse structure adjacent to it will be discussed. In addition, the Tiger Butte laccolithic intrusion, contact metamorphic effects, associated dikes, and structure resulting from the intrusion will be examined.

Topics: Fossils, Hazards, Resources, Surficial geology, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Field Guide; Little Rocky Mountains part of MT Field Guides
This fieldtrip examines deformed Paleozoic sedimentary rocks on the margins of the Little Rocky Mountains, Tertiary intrusive rocks (porphyrys, magmatic-hydrothermal breccias, and dikes), and associated mineralization.

Topics: Fossils, Resources, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Bones and Rocks of the Upper Cretaceous Two Medicine-Judith River Clastic Wedge Complex, Montana part of MT Field Guides
The purpose of this field trip is to provide an overview of the paleontologic, stratigraphic, and paleoenvironmental aspects of the Upper Cretaceous Two Medicine-Judith River clastic wedge in western Montana. Strata of the Two Medicine Formation yielded the first evidence of such dinosaur social behavior as nest construction and parental care, and have provided important information concerning the processes of bonebed formation in the fossil record. The correlative Judith River Formation in Montana has yielded an abundant and diverse dinosaur fossil record primarily preserved as transported and concentrated accumulations of vertebrate remains ("microsites"). The geology and paleontology of four fossil localities [Seven Mile Hill, Shield's Crossing, Willow Creek Anticline (Egg Mountain), and the Badger Creek/Two Medicine River confluence] are described, but precise details concerning the locations of the sites are not given in the field guide.

Topics: Fossils, Sedimentary rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

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

Stratigraphy and Sedimentology of the Paleocene Fort Union Formation along the Powder River, Montana and Wyoming part of MT Field Guides
The Paleocene Fort Union Formation in the Powder River Basin in Montana and Wyoming and adjoining areas contains a large number of very thick, closely spaced coal beds that make up some of the largest coal reserves in the United States. The main purpose of this field guide is to provide a knowledge of the stratigraphy, fluvial facies sequences, and depositional environments of portions of this coal resource at three main localities along the Powder River. Field data from measured sections and outcrop descriptions are synthesized into a depositional model for the origin of the Tongue River Member of the Powder River Formation.

Topics: Resources, Sedimentary rocks, Fossils
Geographic Location: Southeast Montana
Geologic Province: Montana Plains

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