Original Crater
The original crater is no longer preserved as an obvious landform. Erosion, burial, tectonic deformation, and later geological events have complicated its reconstruction.
AN ANCIENT GEOLOGICAL MYSTERY
Beaverhead
Impact Structure
Discover the evidence of an immense ancient impact preserved in the rocks of southwest Montana and Idaho.
A WINDOW INTO DEEP TIME
An Impact Hidden by Time
The Beaverhead Impact Structure is evidence of an ancient extraterrestrial impact event whose geological traces survive in parts of southwest Montana and eastern Idaho.
Unlike a young, easily recognizable impact crater, the original Beaverhead structure has been profoundly altered by geological processes over hundreds of millions of years.
Scientists study unusual rock structures and their geological setting to reconstruct what may have happened in the distant past.
THE GEOLOGICAL EVIDENCE
What Are Shatter Cones?
One of the most important clues to the Beaverhead impact is the presence of shatter cones.
Shatter cones are distinctive, often conical fracture patterns that can form in rocks subjected to extremely powerful shock waves. Their surfaces commonly display radiating or branching grooves.
Because shatter cones are strongly associated with impact-related shock processes, geologists use them as important evidence when investigating possible ancient impact structures.
Why Shatter Cones Matter
A landscape may lose its recognizable crater shape, but diagnostic shock features preserved in its rocks can provide evidence of the impact long afterward.
How Do They Form?
During a large impact, shock waves travel through surrounding rocks at tremendous pressures. Under suitable conditions, these waves can create the distinctive fracture surfaces recognized as shatter cones.
Their occurrence and orientation can help researchers understand the history of shocked rock, although later deformation may complicate interpretation.
RECONSTRUCTING AN ANCIENT EVENT
How Large Was the Impact?
Published research has proposed that the original Beaverhead impact structure was at least approximately 75 kilometers (47 miles) in diameter.
That estimate reflects geological interpretation, not a clearly visible circular crater that visitors can identify on the modern landscape.
The impact's age is also uncertain. Some research has suggested that the event occurred at least about 900 million years ago.
Scientific Uncertainty
Researchers continue to interpret the extent, timing, and geological relationships of impact-related rocks. Published estimates should not be treated as exact measurements.
GEOLOGY AFTER THE IMPACT
Why Can't We See the Crater Today?
Southwest Montana and eastern Idaho have experienced an exceptionally long and complicated geological history.
Since the proposed impact event, rocks in the region have been affected by sedimentation, mountain building, faulting, uplift, and erosion.
These processes can move, deform, bury, and expose rocks that were originally part of an impact structure. The result is a geological puzzle rather than a simple circular depression.
A Geological Detective Story
Geologists examine rock textures, shock features, geological maps, and relationships between rock units to piece together the history of the structure.
The Beaverhead Impact Structure demonstrates how evidence of an extraordinary event can survive even when the original landscape has disappeared.
EXPLORING THE SCIENCE
Can You Visit the Impact Structure?
The Beaverhead Impact Structure is not a developed recreation destination with a single visitor center, marked crater rim, or established crater-viewing trail.
Geological evidence occurs across a complex region of southwest Montana and eastern Idaho. Some scientifically important locations may be remote or difficult to access.
Respect Geological Sites
Do not assume that a research locality is open for public collecting or exploration. Respect private property, land-management rules, and scientifically significant rock exposures.
For most visitors, geological maps, scientific publications, and interpretive materials are the best starting points for understanding the impact structure.
SCIENTIFIC RESOURCES
Explore the Research
Scientific studies of the Beaverhead Impact Structure help explain the evidence for the impact and the challenges of reconstructing its history.
Montana Bureau of Mines and Geology ↗
We will add specific research papers, geological maps, and photographs as this resource develops.