Over a Billion Years of Montana’s Geologic History

From this overlook you can see areas of very old rock and nearer relatively young deposits. The Big Belt Mountains on the skyline east of the Helena Valley in the far distance are primarily composed of Belt Supergroup rocks deposited about 1.4 billion years ago in a large, shallow sea that covered much of western Montana and extended into Canada. Sand and mud deposited in this sea became hard rock such as quartzite as a result of burial under thick layers of younger sedimentary rock. The crags to your left are part of the Boulder Batholith, a huge mass of granite that extends to south of Butte. It formed as molten rock (magma) cooled before reaching the earth’s surface from 80 to 75 million years ago. If you could strip the vegetation and soil from the grassy hills in the foreground toward the Helena Valley you could see beds of sandstone and shale that were hardened (metamorphosed) by the adjacent hot magma that cooled to form the granite. During the Ice Age the mountains near MacDonald Pass were partly covered by glaciers. Round boulders to the north of the highway just above this overlook were left when the glaciers melted about 14,000 years ago. Ten Mile Creek that flows in the gentle valley between this overlook and the Helena Valley was a raging torrent when the mountain glaciers melted. Boulders up to 4 feet across were deposited by this flood near the present well-behaved Ten Mile Creek in this valley.