A Tumultuous Beginning
The Bitterroot Valley evolved under dramatic geologic influences. Between 70 and 90 million years ago, tremendous pressure deep beneath the earth's surface forced molten granite to rise upward through layers of bedrock forming what we now call the Bitterroot Mountains. The unstable upper portion of the developing Bitterroots separated from the rest of the range and slid eastward. The eastern front of the ancient Bitterroots became the Sapphire Mountain Range. Glaciers followed, carving out rugged drainages in the Bitterroot Range.
As the glaciers receded, streams flowed down the mountain drainages into the valley below, joining to form the Bitterroot River. Over time, waters draining from the Sapphire and Bitterroot Mountains deposited sediment onto the valley floor, creating a rich foundation for plant and animal life.