Initial Publication Date: October 28, 2005

Heart Mountain Detachment Fault and Clastic Dikes of Fault Breccia, and Heart Mountain Break-Away Fault, Wyoming and Montana

Click for route map


Start point

First site is 0.5 mi due south of Silver Gate, MT

End point

Second site is 300 ft. east of the northeast entrance to Yellowstone National Park

Roads (and Trails)

From U.S. 12, the two sites must be reached on foot by climbing steep mountain slopes.

Total distance

not given



This field guide describes the geology of two sites near Silver Gate, MT (near Cooke City) where different features related to the Heart Mountain detachment are displayed. At the first site, several features pertaining to the origin of the Heart Mountain detachment fault can be examined, including: (1) severely deformed upper plate rocks in contact with undeformed lower plate rocks; (2) the character and composition of the fault breccia; (3) contacts of volcanic rocks with upper plate blocks and the Heart Mountain fault; and (4) dikes of carbonate fault-breccia injected into both upper plate blocks and overlying volcanic rocks. The second site is an exposure of the Heart Mountain break-away fault.

Key Lithologic Features

  • Tertiary (Eocene) volcanic rocks: Wapiti, Lamar River, and Cathedral Cliffs Formations
  • Paleozoic sedimentary rocks: Madison Limestone, Three Forks and Jefferson Formations, Bighorn Dolomite, Snowy Range Formation, and Pilgrim Limestone
  • carbonate fault breccia and related clastic dikes, some containing xenoliths and carbonized wood fragments
  • volcanic breccia


  • Heart Mountain detachment fault


Other Features


Pierce, W.G., 1987, Heart Mountain detachment fault and clastic dikes of fault breccia, and Heart Mountain break-away fault, Wyoming and Montana, in Beus, S.S., ed., Centennial Field Guide Volume 2: Rocky Mountain Section of the Geological Society of America , p. 147-154.


Out of print. Check Digital access to this publication is available via the links in the references section above.