Homestake Pass

The granite exposed in road cuts along the uphill grade From Butte to Homestake Pass is part of the Boulder Batholith. This large mass of granite extends from Butte north to the Helena area and is composed of at least sixteen separate bodies that formed from individual pulses of rising magma between about 81 and 74 million years ago. Slow cooling of this magma formed granite that contains glass quartz crystals and grayish to pinkish feldspar crystals. After some of the magma crystallized to granite, additional magma flowed into new-formed planar cracks and crystallized to form dikes. Dikes that are generally a foot or so thick are composed of a fine-grained and somewhat lighter colored rock called aplite. Thicker dikes are composed of a coarse-grained rock called pegmatite, which is large crystals of quartz and feldspar. Smoky quartz and amethyst have been collected from these pegmatites. Granite taken from Welch’s Quarry about 2½ miles north of this sign was used to construct buildings in Butte from about 1899 until the early 1920s. Traveling down Interstate 90 to Whitehall in the Jefferson Valley you can see spectacular exposures of granite north of the highway. Weathering of the granite along well-developed fractures called joints formed the large, smooth boulders of the Boulder Batholith. Similar fractures provided natural planes that aided in the quarrying of large blocks of granite. The Butte Short Line The railroad that snakes its way over Homestake Pass is the Northern Pacific Railway’s Butte Short Line. Completed in 1890, the line branched off the Northern Pacific’s main line at Logan and terminated in Butte. Although the railroad intended the line to carry coal to Butte and Anaconda from the mines in Red Lodge, it served primarily as the passenger main line after 1895. The Northern Pacific’s famed North Coast Limited passenger train was part of this line. The Burlington Northern –Santa Fe Railroad Company ended passenger service on the line in 1979 and stopped using the tracks in 1983.