Dillon in Beaverhead County, Montana — The American West (Mountains)
”…you will take careful observations of …objects distinguished by such natural marks & characters of a durable kind…”
—President Thomas Jefferson
There is a remarkable view from the crest of Clark's Lookout and it is easy to see why Clark chose this rocky outcrop. Look south to see the Blacktail Deer Range and the Tendoy Mountains; the Sweetwater and Ruby Ranges rise to the east; the Tobacco Root and Highland Mountains are to the north; and the Pioneer Mountains dominate to the west. These mountain ranges, and the valleys that lie between them, have been forming for approximately 20 million years as a result of the Earth's crust pulling apart (see cross section). The sinuous and shallow rivers that developed in these broad, flat valleys presented a time-consuming and demanding physical challenge for the Corps of Discovery. Clark noted in his journal that on August 11th they traveled 14 miles by water for 5 miles in a direct line. On August 12th he wrote:
”…men complain verry much of the emence labour they arte obliged to undergo and wish very much to leave the river. I passify them.”
This exposed chunk of limestone originated as shell material that settled to the bottom of a shallow tropical sea, which covered the area about 360 million years ago. Look carefully at the rock and you might find the shells of animals that lived in this ocean. The limestone was heaved upward by movements along faults in the Earth during the process of mountain building. Similar faults have uplifted entire mountain ranges, including the ones you can observe from the top of Clark’s Lookout. The limestone has not been eroded away since it was uplifted and exposed because this rock type resists erosion in the dry climate of southwest Montana. Limestone comparable to that of Clark’s Lookout lies hidden beneath the sand and gravel that fills the Beaverhead Valley (see cross section).
A Geologic Timeline of Clark's Lookout
• Cenozoic Era
Present (Clark stands on the exposed rock in August, 1805)
Quaternary Period (about 2 million years ago)
Tertiary Period (about 65 million years ago) (Uplift of the Clark’s Lookout rock begins)
• Mesozoic Era
Cretaceous Period (about 144 million years ago)
Jurassic Period (about 208 million years ago)
Triassic Period (about 245 million years ago) (the first dinosaurs)
• Paleozoic Era
Permian Period (about
Pennsylvanian Period (about 320 million years ago)
Mississippian Period (about 360 million years ago) (Formation of the limestone that makes up Clark’s Lookout)
Devonian Period (about 408 million years ago)
Silurian Period (about 438 million years ago)
Ordovician Period (about 505 million years ago)
Cambrian Period (about 545 million years ago)
Proterozoic Eon (about 2500 million years ago)
Archean Eon (about 4500 million years ago)
(Earth and other planets formed)
Erected by Montana State Parks and University of Montana Western.
Marker series. This marker is included in the Lewis & Clark Expedition marker series.
Location. 45° 14.115′ N, 112° 37.983′ W. Marker is in Dillon, Montana, in Beaverhead County. Marker is on Clarks Lookout Road west of State Highway 91, on the right when traveling south. Touch for map. Marker is located in Clark's Lookout State Park, at the trailhead on the west side of the parking lot. Marker is at or near this postal address: 25 Clarks Lookout Road, Dillon MT 59725, United States of America. Touch for directions.
Other nearby markers. At least 6 other markers are within 10 miles of this marker, measured as the crow flies. Mapping the Way (here, next to this marker); William Clark (here, next to this Distant Features (here, next to this marker); Clark Pocket Compass Monument (about 300 feet away, measured in a direct line); Old Trail to the Gold Diggin's (approx. 9.4 miles away); Bannack (approx. 9.4 miles away).
More about this marker. Marker is a large, framed composite plaque, mounted horizontally on waist-high metal posts.
Related markers. Click here for a list of markers that are related to this marker. Clark's Lookout State Park
Categories. • Exploration • Landmarks • Notable Places •
Credits. This page was last revised on January 3, 2019. This page originally submitted on January 3, 2019, by Cosmos Mariner of Cape Canaveral, Florida. This page has been viewed 44 times since then. Photos: 1, 2, 3, 4, 5, 6, 7, 8, 9. submitted on January 3, 2019, by Cosmos Mariner of Cape Canaveral, Florida. • Andrew Ruppenstein was the editor who published this page.