Field Guide, Little Belt Mountains

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Route

Start point

Monarch, Montana

End point

Utica, Montana

Roads

U.S. 89, Dry Fork Road

Total distance

78.6 miles

Geology

Summary

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.

Key Lithologic Features

  • Devonian Jefferson Formation
  • Mississippian Madison Group
  • Three Forks Formation
  • Big Snowy Group
  • Pilgrim Formation
  • kersantite-minette lamprophyre sill
  • Maywood Formation
  • Flathead Formation
  • Eocene intrusions
  • Mixes Baldy-Anderson Peak bysmalith
  • Butcherknife Mountain laccolith
  • Big Baldy bysmalith
  • Park Formation
  • Gold Run tuff
  • Hughesville stock
  • Barker Mountain laccolith
  • Clendennin-Peterson laccolith
  • Wolsey Shale
  • Yogo Peak stock
  • Dry Wood laccolith
  • Sheep Mountain pluton
  • Mission Canyon Formation
  • Kibbey Formation
  • Otter Formation

Structures

  • Little Belt anticline
  • Pioneer Ridge uplift
  • Great Falls tectonic zone

Landforms

  • glacial cirques

Other Features

  • Missile site A-5
  • St. Joseph Lead Company
  • Silver-Bell mine
  • Clendennin smelter
  • St. Joe Lead Company mill
  • Carter mine
  • May & Edna mine
  • San Miguel mine
  • Gold Bug mine
  • Blue Bick mine
  • Gold Nugget mine
  • Vortex mine

Reference

Baker, D.W., Berg, R.B., McBride, G., Dahy, J., 1991, Field Guide, Little Belt Mountains, in Baker, D.W., and Berg, R.B., eds., Guidebook of the central Montana alkalic province: Geology, ore deposits and origin , Montana Bureau of Mines and Geology Special Publication 100, p. 145-162.

Availability

Out of print; can be photocopied for a fee. For more information, search for this publication's record at the MBMG.