Bown’s New Book Offers Fresh View on Wyoming Landslide

February 2025
Josh Zaffos

Anthropology and Geography faculty Thomas Bown, Ph.D, is the coauthor of a new book, Heart Mountain Detachment Fault: A Critical Reappraisal, published by Archway Publishing. Bown and Associate Teaching Professor Kim Nichols co-direct the CSU Paleontology Field School — which runs in Wyoming’s Bighorn Basin each summer — and the CSU Primate Origins Lab.

Bown and coauthor Albert Warner’s research presented in the monograph suggests that Heart Mountain and its rocky surroundings, located outside of Yellowstone National Park in northwestern Wyoming, was formed through two separate massive geologic episodes 47 million years apart. The more recent event — called a sturzstrom — was an immense, horizontal landslide that occurred roughly 2 million years ago, which the authors believe was caused by a gigantic super eruption of a volcano in present-day Yellowstone. The eruption may have triggered huge 5-mile-wide blocks of rock to break and horizontally slide almost 75 miles, accounting for the jagged modern landscape. The findings overturn past conclusions that a single event caused the fault and resulting outcrop.

Bown and Warner have been studying geology in northwestern Wyoming since the 1960s.

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NEWS RELEASE: Bighorn Basin Dinosaur and Geoscience Museum, Cody, Wyoming

Heart Mountain is a giant block of limestone and dolomite that was formed in shallow seas 320-485 million years old. It sits on rocks that are hundreds of millions of years younger. This enigmatic structure has puzzled geologists for more than 100 years. The scientific consensus is that a massive slab of limestones and dolomites broke away from an outcrop near Silver Gate, Montana, just outside of the northeast entrance to Yellowstone National Park about 49 million years ago and slid along the Heart Mountain detachment surface nearly 75 miles to the southeast at speeds up to 700 miles per hour and broke up into several giant blocks up to five miles in diameter. Heart Mountain, the namesake for this displacement, is where some of its geologic relationships were first recognized over 125 years ago. Dr. Thomas Bown, a former U.S. Geological Survey explorer, adventurer, and geologist, Dr. Albert Warner, a highly successful retired petroleum geologist and Mark Mathison, a geologist and skilled commercial drone pilot, now believe that Heart Mountain and several other similar geologic features found in the Cody area are instead the result of two geologic events separated in time by more than 47 million years.

The first was a massive slide that occurred approximately 49.5 million years ago when the Absaroka Mountain Range east of Cody was formed. This slide traveled as much as 36 miles from the Silver Gate area covering that distance within minutes. The second phase occurred approximately two million years ago when the earlier landslide deposit broke up in a catastrophic earthquake and formed into the world’s largest known landslide that traveled another 40 miles down slope to the southeast covering a vast area that now includes Cody, Heart Mountain, and areas far to the east and south. Both the more ancient and more recent landslides were initiated by huge earthquakes with magnitudes as great as, or larger than, those that created the 2004 and 2011 tsunamis in Indonesia and Japan, respectively. The younger earthquake-induced landslide can be linked to an extremely violent volcanic eruption that produced the Huckleberry Ridge Tuff in what is present-day Yellowstone Park.  Their research has huge implications for the impact of very large magnitude earthquakes and volcanic eruptions on the Earth’s surface and sheds more light into the violent geologic past of the greater Yellowstone region.

The two senior authors’ interest in Heart Mountain date back to the mid 1960s. Since then, they have been scouring the landscape looking for clues as to its origin and have managed to access lands no longer accessible to most researchers. They have recently summarized their findings in a new book titled The Heart Mountain Detachment Fault: A Critical Reappraisal, now available through Simon and Schuster’s Archway Publishing.