Lone Mountain, the prominent peak above Big Sky, was shaped by geologic processes that left igneous dacite within layers of sandstone and shale after magma rose through a vertical crack and spread between sedimentary layers, then crystallized before reaching the surface. The dacite and nearby baked sedimentary rocks resisted erosion better than softer surrounding rock, leaving the mountain standing tall, while glaciers, landslides, and rock fall greatly modified it and formed cirques containing rock glaciers that slowly creep downslope. To the northwest, the Spanish Peaks are composed of metamorphic rock billions of years older than Lone Mountain's igneous and sedimentary rocks, uplifted along a huge fault zone stretching from the Tobacco Root Mountains to Gardiner north of Yellowstone Park. The mountains also became a backdrop for visitors to Gallatin Canyon's dude ranches, where guests rode horses, hiked trails, fished trout streams, and enjoyed mountain solitude; these ranches were immensely popular before World War II, then declined in the 1950s until only a few remained.