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.

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.

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.

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.

Field Guide to an Archean Transect, Eastern Beartooth Mountains, Montana-Wyoming part of MT Field Guides
The eastern Beartooth Mountains of Montana and Wyoming contain a record of crustal evolution that spans almost 1000 Ma and culminates in a major episode of crustal growth 2700-2800 Ma. The earlier record is sparse and complex as a result of extensive magmatism and intense metamorphism associated with Late Archean activity. In general, however, it appears that continental material was present in this area by roughly 3600 Ma, and that a stable continental shelf accumulated quartzites, iron-formation, and lesser amounts of pelitic to psammitic units interspersed with small volumes of mafic to silicic volcanic rocks. This cycle of accumulation was apparently terminated by an episode of granulite facies metamorphism 3300-3400 Ma, perhaps as a result of continent-continent collision....About 2800-2900 Ma, a second major cycle of crustal growth began that bears some resemblance to those associated with modern continent-ocean subduction zones....The first igneous rocks produced during this cycle were andesitic or dioritic rocks, both coarse and fine grained, that were subsequently metamorphosed to amphibolite facies....This interval is restricted by the presence of a granodioritic series (Long Lake granodiorite) that was intruded late in the kinematic cycle... (2779 Ma)...and marks the lower limit for the last major episode of regional metamorphism. The last and most volumetrically important rock produced during the cycle was the Long Lake granite. This unit composes approximately 80-90% of the eastern portion of the range and engulfs all older rock types. It appears to have been intruded about 2740 Ma....This excursion will attempt to view the evidence of these two major cycles in four main stops: (1) Hellroaring Plateau, (2) Lower Quad Creek, (3) Upper Quad Creek, and (4) Long Lake. Late Archean mafic dikes (at Beartooth Lake) will also be examined.

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.

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.

Field Guide; Bearpaw 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.

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.