The Boulder Batholith is a huge granitic magma and chamber that originally cooled 10 miles below the surface during the formation of the Rocky Mountains. Today there are 15 major plutons that are exposed portions of the batholith (approximately 10 miles of material were eroded away to expose them).  The Butte Pluton exposed here was associated with extensive mineralization that formed massive ore deposits.  Gold was discovered in the region in 1864 and the mining boon began.  Copper along with other economic minerals was extensively mined.  In 1955 open-pit mining began in the area to recover low-grade ore.  Although the open-pit mining ceased in 1982, an acidic water filled pit highlights the environmental impact of the mining.