Plutons and the Humbug Spires

This rest area lies near the southwest corner of the Boulder Batholith – a very large collection of igneous plutons. “Igneous” refers to rocks solidified from magma. “Plutons” form from great blobs of magma that harden deep in the earth. Plutons surround this rest area. The Big Hole Canyon Pluton makes up the south side of Fleecer Mountain to the west (the one that looks like Mount Fuji, though it isn’t a volcano). The Climax Gulch Pluton rises on both sides of the highway here and to the north. The Butte Pluton, farther north, hosts the copper, gold, silver and molybdenum deposits of the Butte Mining District. The Butte Pluton has been mined almost continuously since 1864. The Moose Creek Pluton, south and east of us, is now a labyrinth of pinnacles and fins called the Humbug Spires. The spires are what remain after millennia of weathering and erosion along fractures and joints. The result is a haven for hikers, rock climbers, and mountain lions.