“The Crazy Mountains”

The scenic Crazy Mountains form an “island” mountain range near the center of the Crazy Mountain Basin. Unlike many mountain ranges that are blocks of crust uplifted during faulting, the Crazy Mountains were formed by igneous activity. About 50 million years ago, magma (molten rock) traveled through thick layers of sedimentary rock as it pushed up to the surface. The molten rock cooked (metamorphosed) the surrounding shale to form a hard, resistant rock known as hornfels. Time and erosion stripped the softer sedimentary rock surrounding the intrusion, leaving the resistant core towering over 6,000 feet above the adjacent plains. The Big Timber Stock is the largest intrusion within the Crazy Mountains. It is exposed in the higher, southern end of the range whereas smaller stocks occur to the north. The igneous activity associated with the stocks was also responsible for emplacement of impressive dike swarms—hundreds of dikes (magma-filled cracks in the older sedimentary rocks) that form a roughly radial pattern around the intrusions. These dikes often form narrow linear ridges that extend up the mountainsides. They persist because they are more resistant to erosion than the surrounding shales and sandstones of the Fort Union Formation. The Crazy Mountains are the only mountains in central Montana that were high enough to support glaciers during the last ice age. The jagged peaks, cirque basins, and deep valleys that characterize the present landscape were carved by glacial action. Today, the Crazy Mountains contain about 50 glaciers and perennial snowfields and numerous alpine lakes. The largest glacier is Grasshopper Glacier near Cottonwood Lake.