Normal Faults
Normal faults commonly develop where Earth's crust is being stretched. Movement along these faults can contribute to mountain-and-valley landscapes.
THE FORCES THAT SHAPED SOUTHWEST MONTANA
Mountains, Faults
and Valleys
Discover how geological forces created the mountain ranges and broad valleys of Beaverhead County.
A LANDSCAPE IN MOTION
How the Land Took Shape
Beaverhead County is characterized by mountain ranges separated by broad valleys, a landscape shaped by tectonic activity, erosion, and sediment deposition.
The mountains and valleys we see today developed over millions of years through several episodes of geological change.
Understanding these processes helps explain why the county's terrain varies so dramatically from one location to another.
TECTONIC FORCES
What Is a Geological Fault?
A fault is a fracture or zone of fractures in Earth's crust along which rocks have moved relative to one another.
Faults form as rocks respond to tectonic forces. Depending on the direction of movement, faulting can shorten, stretch, or shift sections of Earth's crust.
Reverse and Thrust Faults
These faults form in settings where rocks are compressed. They can move older rocks over younger formations and contribute to mountain building.
Strike-Slip Faults
Strike-slip faults involve mainly horizontal movement, with rock masses sliding past one another.
Faults and Earthquakes
Some faults remain active. Sudden movement along a fault can release energy as an earthquake. Not every mapped fault is active today.
THE VALLEYS OF BEAVERHEAD COUNTY
Why Are the Valleys So Broad?
Parts of southwest Montana have experienced crustal extension and fault movement that helped create basins between mountain ranges.
Over time, streams transported sediment from surrounding mountains into these lower areas, gradually filling portions of the basins.
The Beaverhead Valley
The Beaverhead Valley around Dillon is surrounded by mountain ranges and contains sediment deposited by rivers and other geological processes.
Its broad landscape reflects a combination of tectonic history, erosion, and sediment accumulation.
The Big Hole Valley
The Big Hole Valley is another prominent mountain-bordered basin in southwest Montana. Its landscape preserves evidence of geological processes involving faulting, sediment deposition, and erosion.
Geology Connection
Mountain ranges and valleys often develop together. As tectonic forces uplift or expose higher terrain, erosion transports material into nearby lower areas.
THE SURROUNDING RANGES
Mountains Around Beaverhead County
Beaverhead County and neighboring areas contain several mountain ranges with distinct geological histories.
Pioneer Mountains
A varied mountain range containing sedimentary and igneous rocks, evidence of tectonic deformation, and important mineral resources.
Beaverhead Mountains
This mountain range along the Montana–Idaho region preserves evidence of a complex history involving sedimentation, deformation, uplift, and erosion.
Tendoy Mountains
The Tendoy Mountains contain sedimentary formations and geological structures that reflect major episodes of deformation in southwest Montana.
Other Mountain Ranges
The county's surrounding ranges and uplands reveal additional chapters of the region's geological history, including faulting, mountain building, and erosion.
SCULPTING THE LANDSCAPE
The Work of Water, Ice and Time
Tectonic forces help create changes in elevation, but erosion plays a major role in shaping the details of mountain landscapes.
Rivers and Streams
Flowing water cuts valleys, transports sediment, and deposits gravel, sand, and finer material across floodplains and valley bottoms.
Glaciers
During colder periods, glaciers occupied portions of the higher mountains. Moving ice carved valleys and transported rock debris, leaving distinctive landforms and deposits.
Weathering
Freezing and thawing, chemical reactions, and other processes gradually break down exposed rock, making material available for erosion and transport.
SEEING GEOLOGY IN THE LANDSCAPE
Explore the Mountains and Valleys
The geological history of Beaverhead County is visible in mountain ridges, broad valleys, river corridors, and exposed rock formations.
Scenic roads and public viewpoints can provide opportunities to observe these features. We will add specific locations and original photographs as this section develops.
Exploring Responsibly
Respect private property, posted restrictions, and seasonal road closures. Roadcuts and steep rock exposures can be hazardous, especially where loose rock is present.
MAPS AND RESEARCH
Learn More About Southwest Montana's Landscape
Geological maps and research help explain the faults, rock formations, and geological history of the region.
Montana Bureau of Mines and Geology ↗
Detailed maps, locally documented geological features, and original photographs will be added as this resource develops.