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 Picket Pin Mountain Area part of MT Field Guides
This traverse through part of the Banded series provides relatively easy access to the upper half of the exposed part of the Stillwater Complex. The percentage of outcrops is high, and even in areas of soil or tundra cover, the float appears to be sufficiently immobile and concentrated to give an accurate picture of the underlying bedrock geology....The traverse begins in the Middle gabbro zone and terminates where Paleozoic limestone unconformably overlies the Upper anorthosite zone. Above the Middle anorthosite zone, it follows the Picket Pin section described by McCallum and others (1980) and Raedeke (1982b)....An optional traverse through some lithologically complex and intriguing rocks in the Upper anorthosite zone north of the Castle Creek fault is also described. Total traverse time, including the drive from Carter's Camp and return, is 8-10 hours. Elevations range from 9,300 to 10,000 feet.

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

Guide to the Benbow Area part of MT Field Guides
The Benbow area (named for T.C. Benbow, who first discovered chromite there) is at the east end of the exposed Stillwater Complex....Most of the localities described...are in the Ultramafic series....The features emphasized at these localities include the nature of the cyclic units, pegmatoids associated with chromite seams, evidence for slumping anbd slope instability in the Stillwater magma chamber, lateral persistence of some of the chromite seams, and the unconformity between the Banded series of the Stillwater Complex and overlying Cambrian limestone.

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

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 Hebgen Lake Earthquake Area, Montana and Wyoming part of MT Field Guides
Several high-angle normal faults bounding the west front of the Madison Range north of Hebgen Lake, recurrently active during much of Neogene time, reactivated catastrophically on August 7, 1959. Faulting was accompanied by largest historic earthquake within the Intermountain Seismic Belt. Unusual geologic features were formed–spectacular fault scarps, a large landslide, a deformed lake basin (Hebgen Lake), and a new lake (Earthquake Lake)–each of which demonstrates the destructive power of a large eathquake. These features are described in the context of the bedrock geology in this field guide.

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

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

Depositional Surfaces in the Eagle Sandstone at Billings, Montana part of MT Field Guides
Marine shelf sandstones and barrier bars are commonly formed by lateral accretion of sand bodies with time lines inclined to the formation boundaries. Shelton (1965) described low-angle inclined bedding in the lowermost sandstone unit of the Eagle Sandstone at Billings, Montana. He recognized these beds as shoreface accretion surfaces of a barrier bar and likened them to those found on present-day Galveston Island, Texas. The Billings location provides an excellent opportunity to examine an important hydrocarbon reservoir rock of the Northern Rocky Mountain region.

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

A Study in Contrasts: Archean and Quaternary Geology of the Beartooth Highway, Montana and Wyoming part of MT Field Guides
Nowhere in the U.S. are the oldest and the most recent aspects of geology as spectacularly displayed as along the Beartooth Highway. The Beartooth Mountains are a block of largely Archean bedrock uplifted along high-angle reverse faults of Laramide age. The Precambrian rocks (3400-700 Ma) contain one of the best records of the early history of the igneous and metamorphic basement of the middle Rocky Mountains. These rocks include granulite-facies supracrustal rocks proposed as products of continental collision, calc-alkaline volcanic and plutonic rocks generated along an Archean continental margin, and mafic dikes some of which were emplaced during continental rifting. This Precambrian record encompasses at least two extensive periods of crustal evolution and records more geologic history at one location than any other place in the Wyoming Province. Descriptions of two key areas showing the fundamental relations between the Precambrian rocks are included: the Quad Creek area and the Long Lake area. The Beartooth Plateau has been extensively sculpted by glacial processes during the Pleistocene. The highway crosses a classic locality of "biscuit-board topography"–plateau remnants partially dissected by cirques–as well as deposits left by glaciers that etched the plateau. Features to be seen include glacio-fluvial terraces showing downstream effects of the glacial system and extensive areas of periglacial features that postdate glaciation. Glacial features are described from the Red Lodge area, Rock Creek Canyon, the hairpin turns, the Beartooth Plateau, and the Clark Fork basin.

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

The Stillwater Complex, Southern Montana; A Layered Mafic Intrusion part of MT Field Guides
Layered complexes provide the most visible evidence of processes of magmatic differentiation; thus, they occupy a central place in the study of igneous petrology. Such complexes are not only scientifically rewarding, they are host to several types of mineral deposits, including copper-nickel, chromium, and platinum-group elements. The Stillwater Complex is one of the world's great layered mafic intrusions, distinguished not so much by its size as by the fact that it is tilted on its side, and erosion has exposed the layering to ready access. This fieldguide presents a summary of the geology of the complex in the Benbow and Mountain View areas. The Benbow area offers easy access to a variety of rocks from the ultramafic series and chromite deposits but only limited exposures of features from the banded series. The Mountain View area offers easy access to most of the banded series and the platinum deposits.

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