A detailed overview of the Bedrock of the Sheridan District was published by H Robert Burger III in December  1967.

 Click Bedrock Geology of the Sheridan District  to view the entire document

If you would like an original of this you can order one from the Montana Bureau of Mines and Geology

The following is a brief description of the surrounding geology of the Ruby Valley.

Sheridan, Mt (45^°27'22.95" N 112^°11'5121"W) is located in the heart of the Ruby Valley.  The valley consists of rocks and soils that have come from the surrounding mountains. The valleys are ringed by alluvial fans (basically mountain streams that have eroded the mountains and deposited the rocks and sands in a fan shape - in this area these have merged together to give the impression of a continuous apron at the base of the hills. These rocks and soils are primarily made up of materials that come from the Tertiary (65 - 1.8 million years ago), Oligocene (33.7 - 23.8 million years ago) through Pliocene (5.3 - 1.8 million years ago) periods.

Due North and South the underlying rocks include sedimentary rocks. Sedimentary rocks are formed from eroded or deposited sediments. This particular formation is limestone (generally made up of calcium from marine animals which have been compacted and cemented together), some dolomite (similar to limestone but with a different mineral composition (more magnesium) as well as sandstone (made up of quartz and feldspar).  These rocks are generally from the Mississippian (354 - 323 million years ago) Pennsylvanian (323 - 290 million years ago) and Permian (290 - 248 million years ago) periods. The rock structures are part of the Madison limestone, big snowy dolomite and Quadrant Sandstone.

Northwest of Sheridan are additional sedimentary rocks including shale (formed by compacting and cementing fine grained mud, silts and clays). The shale is part of the Three Forks Formation.  Other structures include Jefferson Limestone, Pilgrim and Meagher limestone, Park and Wolsey shale and flathead sandstone. These rocks date from the Devonian (417 - 354 million years ago) and Cambrian (543 - 490 million years ago) periods.

Some of the oldest rocks in North America are found in this area and date from the Archean (2.7 billion years ago) Eon. These lie primarily in the Rubies and Tobacco Root Ranges.  These rocks include metamorphic rocks. Metamorphic rocks are highly compressed, almost re-melted sedimentary or igneous (previously melted rocks) rocks. Their specific make up includes quartz (a glass like mineral comprised mostly of silica) and feldspars gneiss (a highly banded rock) as well as amphibolite (a dark colored and heavy rock) and marble (metamorphic limestone).

Finally the core of the Tobacco roots includes younger cretaceous (144 - 65 million years ago) granitic rocks. Granites are igneous rocks and are made up generally of quartz, hornblende and feldspars. The Tobacco Root Batholith was formed by a large magma intrusion or chamber which occurred beneath the earths crust. In addition many other minerals including gold, silver and other semi precious minerals and stones are commonly associated with batholiths. In addition to granite, there is quartz monzonite.

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For a more detailed map please you can view one at http://geology.about.com/library/bl/maps/n_statemap_MT3200.htm

Sources: Sources

Geological Map of Montana Montana State University. Map by Robert L. Taylor, Joseph M. Ashley, R. A. Chadwick, S. G. Custer, D. R. Lageson, W. W. Locke, D. W. Mogk, and J. G. Schmitt.

Schumann, Walter Handbook of Rocks Minerals & Gemstones. Boston New York: Houghton Mifflin Field Guides 1993

USGS The Geologic Time Scale from

http://vulcan.wr.usgs.gov/Glossary/geo_time_scale.html

Wikipedia, The Free Encyclopedia. 9 Nov 2006, 00:38 UTC. Wikimedia Foundation, Inc.