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.

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.

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.

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.

Yellowstone and Grand Teton National Parks and the Middle Rocky Mountains part of MT Field Guides
This trip is designed to show participants the granite-cored Laramide (Late Cretaceous-earliest Eocene) mountain ranges in the middle Rocky Mountains, and their various stages of burial by Cenozoic deposits and subsequent Quaternary exhumation. Mountain-flank structures involving Precambrian, Paleozoic, and Mesozoic rocks, the classic Heart Mountain detachment fault complex, and the rootless overthrust mountain ranges of the Wyoming-Utah-Idaho thrust belt are traversed.

The Yellowstone-Island Park Region part of MT Field Guides
The Yellowstone Plateau, at the center of one of the Earth's largest volcanic fields, spans the continental divide between the Northern and Middle Rocky Mountains at an average elevation of about 2.400 m. The eruptions of the Yellowstone Plateau volcanic field, entirely postdating 2.5 Ma, were exceedingly voluminous but are only the surficial expression of the emplacement of a batholithic volume of rhyolitic magma to high crustal levels. Although the latest eruptions were about 70,000 years ago, an immense hydrothermal system and a variety of geophysical characteristics indicate the continued presence of an active shallow magma chamber.

Road Log to the Picket Pin Mountain, Chome 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.

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.

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.

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.