Reliable Landmarks

From the crest of Clark's Lookout, a remarkable view takes in the Blacktail Deer Range and Tendoy Mountains to the south, the Sweetwater and Ruby Ranges to the east, the Tobacco Root and Highland Mountains to the north, and the Pioneer Mountains to the west. These mountain ranges and the valleys between them formed over approximately 20 million years as the Earth's crust pulled apart. The broad, flat valleys developed sinuous, shallow rivers that created a time-consuming and demanding physical challenge for the Corps of Discovery; William Clark recorded that on August 11, 1805, the party traveled 14 miles by water for 5 miles in a direct line, and on August 12, 1805, he wrote that the men complained greatly of the immense labor they were obliged to undergo and very much wished to leave the river. The exposed limestone at Clark's Lookout originated as shell material that settled on the bottom of a shallow tropical sea about 360 million years ago, and shells of animals that lived in that ocean may still be found in the rock. During mountain building, movements along faults in the Earth heaved this limestone upward, and similar faults uplifted entire mountain ranges visible from the lookout. Because limestone resists erosion in the dry climate of southwest Montana, this rock has remained exposed since its uplift, while comparable limestone lies buried beneath the sand and gravel filling the Beaverhead Valley.

Montana · United States