Filed Under Environment

BA&P Trail - Granite Outcrops

Butte Granite is made up of light-colored minerals (quartz and feldspars) and dark-colored minerals (biotite mica and amphibole). The crystals can be distinguished by the naked eye as they range from 1/16” to sometimes 1” in dimension. This outcrop is stained reddish-brown by the decomposition of the iron-bearing dark minerals, producing iron oxides, commonly known as “rust”. Most of the granite building stone in Butte is unweathered and does not have a reddish-brown cast. Some nearby examples of this rock used as building stone are the rock walls around houses at the corner of North Excelsior and Copper or Quartz streets.

Walk east along the trail towards the East Ridge. The large fenced-in area behind the Anselmo mine yard is the site of the Anaconda wood works where timbers were milled for the underground mining operations.

Images

Untitled The rock you see in this outcrop is all one rock type. Fractures oriented in various directions have broken the outcrop into boulders. Don’t be deceived by the fractures! If you look closely at each boulder, the rock type is identical.

Location

Location: Latitude 46.01910 Longitude -112.54566

Metadata

Larry Smith, “BA&P Trail - Granite Outcrops,” Story of Butte, accessed April 14, 2024, https://storyofbutte.org/items/show/3457.