Building Stone Sources- A walking tour of examples along the Butte, Anaconda and Pacific Railway Trail and west Butte.

Everyone walking around Butte will notice the use of stone for building foundations, retaining walls, lintels, and sills (above and below windows), steps, and even some entire structures. Close inspection will show that only a few varieties of stone are widely used. Most of the building stones are granite and related rocks, such as aplite (light-colored non-mineralized dikes or veins in the granite), and oxidized rock from ore veins, the most distinctive type. The oxidized ore vein rock makes up the black rubblestone common in walls and some foundations. The oxidized ore vein rocks were derived from near-surface outcrops of ore veins, which were important targets for underground mining in the history of Butte.

A few other rock types, such as quartzite and sandstone, were quarried locally and outside of city limits. A few buildings incorporated rock that came from greater distances in Montana and beyond the nation’s borders.

Although specific quarries in the city have not been found, this walking tour highlights natural occurrences (outcrops) of rocks that were quarried locally.

The tour includes outcrops (where rocks are exposed in their natural occurrence) along the Butte, Anaconda, and Pacific (BA&P) walking trail from the Anselmo Headframe parking lot and outcrops near St. James Hospital to near the intersection of Platinum Street and locations on Excelsior Avenue.

Main Hall's granite steps with dark (mafic) inclusions. This texture of dark blobs in granite is typical of many parts of the Boulder Batholith, including the Welch Quarry near Homestake Pass. Also pictured are examples from the Science & Engineering Building as well as the granite arch…
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This wall, and others in the neighborhood, contains large boulders of the oxidized ore vein rock, especially low in the wall. As the boulders were likely not transported far, these boulders were obtained locally during excavation of the bedrock in the area.
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The Buffalo walking trail spur routes back to the west from the intersection of the BA&P trail with Wyoming Street. Near the bridge where Main St. goes over the trail, good exposures of unweathered and iron oxide-stained granite form the footings for the bridge.
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The south side of the rock-cut displays Oxidized ore veins and veins that were not ore-bearing cutting through the Butte granite. Veins, or dikes, are sheet-like narrow igneous bodies (once molten rock) that intrude into the granite along upright fractures. All three rock types were used as…
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As you approach the bridge below Montana Street, you can see rock outcrops mostly on the south side of the trail. You should notice fractures in the granite that trend about east-west, nearly parallel to the trail. The major ore-bearing veins in the area (Butte’s “main-stage” veins) intruded the…
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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…
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