Wyoming
Wyoming’s Heart Mountain Formed By The Two Largest Landslides In Earth’s History
Cody’s Heart Mountain isn’t the result of one of the largest known landslides in Earth’s history. It’s the result of two of the largest landslides in Earth’s history separated by nearly 50 million years.
There’s a lot that makes Heart Mountain, the iconic peak north of Cody in Park County, unique in the geological context of Wyoming. That’s saying a lot, considering the Cowboy State is one of the world’s most lauded geological wonderlands.
It’s a story of a block of rock the size of Rhode Island moving at 700 mph, an eruption of the Yellowstone supervolcano, and two separate but equally cataclysmic events in Earth’s geologic history.
Thomas Bown and Albert Warner are the authors of “The Heart Mountain Detachment: A Critical Reappraisal,” which was published in February.
It’s a comprehensive look at the geology of Heart Mountain that presents a new interpretation of how it came to be, revealed by the complex geological landscape and evidence scattered across the northwest corner of Wyoming.
Their conclusion is that it took less than 30 minutes to make Heart Mountain, but that 30 minutes was stretched across 47 million years.
“We believe that Heart Mountain constitutes the world’s largest known landslide, but there are two different structures from two different events,” Bown told Cowboy State Daily. “They’re contiguous but separate. It’s complicated.”
The World’s Largest Landslide
Heart Mountain is a geological anomaly for several reasons. For one, the rock at the top of its iconic peaks is significantly older than the rock at its base, violating the geological law of superposition.
Also, the mountain belongs nearly 50 miles northwest of Cooke City, Montana. All the evidence indicates that the geology of Heart Mountain is a massive block of Bighorn Dolomite that was initially attached to the rest of the formation exposed along the Wyoming-Montana state line at the northern extent of the Absaroka Range.
So, how did it get from Cooke City to Cody?
“The debris of Heart Mountain constitutes the world’s largest landslide,” Bown said. “A block of Bighorn Dolomite that was the size of the state of Rhode Island rafted about 40 miles from its source around 49 million years ago.”
Calling this event a “landslide” understates its enormity. The state-sized slab of Paleozoic sedimentary rock moved at 100 meters a second, covering over 40 miles in less than 30 minutes at speeds over 700 mph, leaving behind a field of “mountains” and other debris across an expanse over 40 miles long and 15 miles wide.
“We believe it was caused by a massive subduction zone earthquake, 9.0 or higher,” Warner said. “An earthquake of that intensity would have lasted from 4 to up to 10 minutes.”
An earthquake of that length and intensity would have reduced the surface friction to the point where it barely existed. Bown compared the incredible geological phenomenon to an electric tabletop football game.
“The blocks skidded across the landscape like the football men on the field,” he said. “Paleomagnetic studies have shown that the massive block broke, and those individual blocks rotated as they went. It was like a vibrating conveyor belt.”
The “Heart Mountain Detachment” came to rest at the present-day location of Pat O’Hara Mountain, the prominent peak directly west of Heart Mountain, where 500-million-year-old limestone and dolomite sit on top of 55-million-year-old mudstone and shale. It took less than 30 minutes for the blocks to cover a distance of nearly 40 miles.
It’s hard to visualize the scale of this event, but the evidence of it is spread throughout northwest Wyoming. Many of the “mountains” visible from the pinnacle of Dead Indian Pass are pieces of the Rhode Island-sized block of Bighorn Dolomite that broke off and settled during the landslide.
“One of the more recognizable features along the Chief Joseph Highway is the Cathedral Cliffs,” Warner said. “That’s one of the largest breakaway blocks from this landslide. Republic Mountain right outside Cooke City is another one of these blocks.”
But, according to Bown and Warner, that event didn’t create modern-day Heart Mountain. It stopped just short of reaching that spot and needed another unprecedented geological event to finish the job.
Another One?
This is where the story of the Heart Mountain Detachment usually ends – a single event that occurred 49 million years ago. Bown and Warner believe there’s more to this story.
Their new book focuses on the bigger picture of the world’s largest landslide. They believe the McCullough Peaks, south of Cody, indicate another cataclysmic event that was associated with but not attached to what’s been understood as the Heart Mountain Detachment Fault.
“McCullough Peaks was believed to be part of the Heart Mountain Detachment Fault, but that’s all early Eocene in age,” Bown said. “The deposits around McCullough Peaks are all early Pleistocene in age. They both have older rocks sitting on younger rocks, and they’re contiguous, but they sit on surfaces of very different ages.”
After studying the existing research and geology of the region, Bown and Warner concluded that there’s a distinct difference between the two deposits. That indicates that another event must have occurred to create the McCullough Peaks.
“It’s completely different and separate from what most people consider the Heart Mountain detachment fault,” Bown said. “The mountain itself is not part of the fault. There’s one structure beginning near the eastern edge of Yellowstone National Park and going all the way east of McCullough Peaks. Now we believe that’s the result of two structures, separated in time by 47 million years.”
In their book, Bown and Warner present their evidence for two different events: the Shoshone/Sunlight/Abiathar Detachment Fault, which created Heart Mountain 47 million years ago, and the Heart Mountain/McCullough Peaks Sturzstrom, a rock avalanche that only occurred around 2 million years ago.
Warner said these were “contiguous but separate events” with similar outcomes. Both involved enormous landslides that led to the creation of mountain ranges, but the 47-million-year difference makes all the difference.
“We believe that an earthquake of great intensity generated the older movement in the Eocene,” he said. “The younger movement, the one that created the actual Heart Mountain and McCullough Peaks deposits, happened during an immense volcanic eruption accompanied by earthquakes that took place about 2 million years ago in the Pleistocene.”
What was the source of “the younger movement?” Look no further than the supervolcano next door.
The Caldera Culprit
The source of the 49-million-year-old landslide earthquake is still disputed. It was possibly caused by “frictional heating” associated with plate tectonics, as magma deep under the Earth’s surface heated and lifted the dolomite, causing the Rhode Island-sized detachment.
Meanwhile, Bown and Warner think the source of the 2-million-year-old landslide earthquake can be directly traced to the volcanic hotspot that moved into northwest Wyoming during the 47-million-year gap between the geologic events.
“That was the result of silicic eruptions in Yellowstone National Park that resulted in the collapse of the caldera,” Bown said. “It was probably the most violent volcanism in the history of that range.”
Around 2.1 million years ago, the Yellowstone caldera underwent one of the largest eruptions in its history. It occurred right when the volcanic hotspot burned its way into the northwest corner of Yellowstone National Park.
Over 590 cubic miles of ash were ejected from the caldera, creating a layer of volcanic ash known as the Huckleberry Ridge ash bed. It stretches as far south as the Mexican border, as far north as the Canadian border, and across the Western U.S. from southern California to the Mississippi River.
An eruption of that intensity would have generated devastating earthquakes in its vicinity. A modern-day analogy would be the landslides and other events prompted by the eruption of Mount St. Helens in 1980, only on a much larger scale.
This incredible event is what Bown and Warner believe created Heart Mountain as it exists today.
Their research concluded that the original detachment ended in the Absaroka Mountains. The massive blocks that became Heart Mountain and McCullough Peaks didn’t descend to their current locations until the Yellowstone eruption 2.1 million years ago.
“The easternmost remnants of the Shoshone/Sunlight/Abiathar Detachment Fault came to rest in the area of the Natural Corral, north of Pat O’Hara Mountain,” Bown said. “They piled up and sat there for 47 million years, and then an immense volcanic eruption caused this mass to break away and sweep down the Shoshone River as a landslide.”
That would mean that Heart Mountain, which is composed of 500-million-year-old rock sitting on 50-million-year-old rock, is a massive breakaway of a much more massive block from a 49-million-year-old landslide but only reached its current location around 2 million years ago.
Two From One
If all these intricacies seem complex and confusing, welcome to the complexity of geology. The “controversial” takeaway from Bown and Warner’s research is that Heart Mountain wasn’t formed as part of the Heart Mountain Detachment Fault.
“The debris from the Heart Mountain/McCullough Peaks Sturzstrom is very different from the Shoshone/Sunlight/Abiathar Detachment Fault, which is an almost completely unique structure,” Bown said. “Both were earthquake-generated but have very different internal structures and mechanisms.”
Their research indicates that Heart Mountain wasn’t the result of a single event, but two separate events of similar scale. Bown and Warner recognize that their interpretation will shake up the accepted explanation for the geologic events that led to modern-day Heart Mountain.
“There’s always more that you can study, so we can learn more about what happened,” Bown said. “With additional study, you can learn more about the speed, internal mechanics, and movement of these landslides, which isn’t completely known today.”
Landslides and sturzstroms are relatively common in the present day and geological history, but not on the scale of what occurred in northwest Wyoming twice in 50 million years. Fortunately, many geologic processes look and behave similarly, even if their scales are immensely different.
Warner is examining possible analogies for the events in the Philippines. Insight into similar events from the past and any possible events in the future could reveal more information on what happened in northwest Wyoming 49 million years ago, and again 2 million years ago.
“The Philippine Area is an incredibly active tectonic region, so earthquakes there are very, very common,” he said. “There is a study of a particular block of Miocene rock that broke away and created a strip surface very similar to the surface of tectonic denudation originally described around Heart Mountain. A very similar situation, but on a much smaller scale.”
Natural Monuments
A lot has changed in the 50 million years since the initial detachment that led to the formation of the modern-day landscape in northwest Wyoming. Evidence that could reveal more about these incredible events has been lost to the inevitable passage of time.
“Part of the problem of studying the Shoshone/Sunlight/Abiathar Detachment Fault is that it immediately preceded a period of enormous deposition,” Bown said. “It happened right before the beginning of volcanicity and tremendous buildup of volcaniclastic sediments in the Absaroka Range, so much of that evidence has been buried.”
Meanwhile, the evidence of the Heart Mountain/McCullough Peaks Sturzstrom is eroding and readily available for scientific scrutiny. Bown and Warner hope their book will increase interest in the area and challenge more geologists to scrutinize their work and the geology that led them to their conclusions.
The geologists also hope that more people become aware of these incredibly dramatic stories of unprecedented catastrophes and mountain-building that created the striking landscapes in and around Cody, Cooke City, and Yellowstone National Park.
When someone looks upon the iconic peak of Heart Mountain, they aren’t looking at a traditional mountain. They’re glimpsing the result of the largest known landslides in Earth’s history — a massive mountain that’s just a small part of a much grander story scattered across the region.
“One of the things Tom and I have discussed is that there’s nothing about the geology posted at the summit of Dead Indian Pass,” Warner said. “We would love to see some geological recognition at that particular site, because there are a lot of natural monuments to these events. When you recognize what occurred there, it’s truly spectacular.”
Andrew Rossi can be reached at arossi@cowboystatedaily.com.
Wyoming
American Rare Earths strengthens board with veteran Wyoming mine builder ahead of planned Nasdaq listing
Veteran mine builder Matthew Gili will join American Rare Earths Ltd (ASX:ARR, OTCQX:ARRNF)’s board as a non-executive director as the company advances the Halleck Creek Rare Earths Project in Wyoming and prepares for a planned Nasdaq compliance listing in H2 2026.
Gili is currently president and CEO of Ur-Energy Inc, a NYSE American and TSX-listed Wyoming uranium producer, and brings more than 25 years of mine development and operational experience across major global mining groups including Rio Tinto and Barrick.
His appointment remains subject to completion of Australian regulatory formalities, which American Rare Earths expects to be completed shortly.
Once formally appointed, Gili will join the company’s Technical Committee and contribute to the Definitive Feasibility Study workstream at Halleck Creek, which American Rare Earths describes as the largest known rare earth deposit in the United States on a total rare earth oxide basis.
Board renewal ahead of US listing plans
The appointment forms part of a broader board renewal process as ARR works toward a Nasdaq compliance dual-listing in H2 2026, while retaining the ASX as its primary listing.
The company is also considering a full US domicile in 2027, subject to a prospective shareholder vote.
CEO Mark Wall said Gili’s operational experience and Wyoming background would strengthen the board as Halleck Creek moves toward construction and production.
“The intended addition of Matt to our Board of Directors further demonstrates our commitment to advancing the largest rare earth element deposit on a total contained rare earths basis in the United States toward construction and operations. Matt brings a tremendous blend of mining technical expertise and Wyoming-specific experience to both the Board and the Technical Committee. His depth of operational knowledge, his relationships in Wyoming, and his proven track record of delivering world-class mining projects, including building the first new copper mine in the United States in a decade, make him exactly the right person to help us get Halleck Creek built.
“As we progress toward our NASDAQ listing later this year, appointments of this calibre send a clear message to U.S. investors about the quality of the team and the seriousness of our intent. Matt’s experience managing ISR uranium operations in Wyoming gives him first-hand knowledge of the hydrometallurgical processing chemistry that will be central to bringing Halleck Creek into production. The parallels between uranium and rare earth processing are substantial and practically meaningful. This is not simply a credential; it is operational expertise that will directly benefit our Technical Committee and Feasibility Study.”
by Dustin Bleizeffer, WyoFile
The Seminoe pumped-water storage hydroelectric project in Carbon County advanced toward final approval this month, when the Federal Energy Regulatory Commission issued its environmental impact statement, leaving critics warning of potential fish kills and other risks to wildlife.
Though the newest plan to minimize myriad impacts to fisheries, wildlife and local recreation economies makes concessions “around the margins,” project skeptics say the FERC ignored calls — including from local and state elected officials — to make more meaningful changes regarding threats, including to a “blue ribbon” trout fishery and a vital bighorn sheep herd.
“I’m very disheartened by the final EIS,” Trout Unlimited’s Wyoming Government Relations Director Patrick Harrington told WyoFile.
The plan still doesn’t mandate operational responses that would effectively prevent a trout kill in the prized Miracle Mile of the North Platte River immediately downstream of Seminoe Reservoir due to the threat of rising water temperatures, Harrington said. Trout are a cold-water species and particularly sensitive to warmer temperatures. Groups like Trout Unlimited and Friends of the North Platte have warned that even one day of higher-than-tolerable water temperatures could result in a devastating fish kill.
The potential for a Miracle Mile fish kill still exists, Harrington said, because FERC declined to update its water forecast modeling to include more recent climate-change analysis that shows higher temperatures and lower annual snowpack for cold water runoff. That leaves the protocol to respond to rising water temperatures woefully inadequate.
“It still leaves serious risk to fisheries — and those go back to our concerns over the data that informs the [water quality] model,” Harrington said.
The revised plan also retains multiple waivers to bypass seasonal construction limitations designed to protect wildlife, including the Ferris-Seminoe bighorn sheep herd. Developer rPlus Hydro says the waivers are vital to the economic feasibility for what it hopes will be a five-year construction period. Complying with the slate of seasonal wildlife restrictions will add major cost, the company has testified.
“These [wildlife timing restrictions] did not come as a surprise to them,” Wyoming Wild Sheep Foundation Executive Director Katie Cheesbrough said, adding that granting waivers of science-backed protections would set a dangerous precedent for other industrial projects in the state. “Those wildlife restrictions were publicly available, and they knew that going into it. If it was going to make the project cost-prohibitive, then they shouldn’t do the project. It’s not on Wyoming to ensure that [wildlife protections] are within their cost range.”
rPlus Hydro responds
The Utah-based company proposes building a 13,400-acre-foot reservoir in the Bennett Mountains overlooking Seminoe Reservoir near the dam — one of several reservoirs on the North Platte River. The $4 billion facility would pump water uphill during daytime “off-peak demand” hours for electricity when wind and solar power are plentiful and wholesale electricity is cheapest, according to rPlus Hydro.
“Think of it as a ‘water battery’ that stores energy generated when demand is low,” the company told WyoFile. “When demand increases, water is released from the upper reservoir back into Seminoe, driving hydroelectric turbines to produce electricity.”
Skeptics in Wyoming have cast doubt on the necessity and consumer benefit of the electrical generation daily balance strategy.
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This map depicts the Seminoe pumped water storage hydroelectric project area. (Wyoming Department of Environmental Quality)
For its part, the company contends that the Seminoe pumped-water storage project represents a $200 million annual savings to ratepayers. A company representative also told WyoFile the FERC’s final EIS “confirms the project is needed for future energy growth and reliability while also safeguarding both the North Platte River and bighorn sheep.”
rPlus Hydro Deputy General Counsel Kevin Baker pointed to the fact that the Wyoming Department of Quality granted a “section 401” water quality certificate for the project earlier this year. The state certificate is proof that “the project will not harm downstream waters, including the Miracle Mile, so drinking water, fishing and recreation remain protected,” Baker wrote.
“The state’s conclusion is backed by a robust, state-led Water Quality Adaptive Management Plan which provides real-time monitoring and strong enforcement measures designed to identify and correct any potential issues before they develop.”
The Environmental Protection Agency agreed with Wyoming DEQ’s findings and stipulations, Baker added.
But there remain huge holes in the modeling — rooted in the failure to consider a changing climate — that FERC, DEQ and the EPA have based their analysis on, Harrington contends. “It’s a castle made of sand.”
Regarding wildlife, and the Ferris-Seminoe bighorn sheep herd in particular, rPlus Hydro contends it is committed to “strict construction practices to minimize disturbance and significant investment in habitat and herd management to ensure its continued health and viability.”
But those promises are not enshrined in FERC’s stipulations for the project, said Cheesbrough of the Wyoming Wild Sheep Foundation.
There’s no way, she said, to ensure the bighorn sheep herd, and other wildlife, will be protected due to the multiple waivers FERC wants to allow for seasonal restrictions. Understandably, Cheesbrough noted, the restrictions for bighorn sheep, sage grouse, raptors and other wildlife would black out much of the calendar, limiting when construction could take place.
Protecting wildlife, Cheesbrough said, would likely add several years and dramatically increase the project’s cost. But, she added, “For them to be like, ‘Well, we just can’t afford to do it here if we have to abide by all of this,’ and then asking for waivers, it seems like a very dangerous precedent to set.”
Public and government pushback
The FERC is the primary permitting agency for the project because of its reliance on federally managed water-storage reservoirs, hydroelectric and electrical transmission systems. It’s a source of heartburn for locals, Harrington said, because the agency seems less beholden to public and local government input compared to other federal agencies.
“It’s frustrating,” Harrington said. “I think this project is headed toward licensing in September because the adjustments FERC has made have sort of just indicated that there’s not going to be a lot of changes to the plan as proposed.”
“For them to be like, ‘Well, we just can’t afford to do it here if we have to abide by all of this,’ and then asking for waivers, it seems like a very dangerous precedent to set.”
Katie Cheesbrough, Wyoming Wild Sheep Foundation
In May, the Legislature’s Travel, Recreation, Wildlife and Cultural Resources Committee heard a large outcry from wildlife and recreation enthusiasts opposing the project, as well as from local officials from Carbon and Natrona counties.
“These concerns are not theoretical for us,” Casper Mayor Ray Pacheco told the legislative panel. “Casper relies directly on the North Platte River for drinking water, wastewater treatment, recreation, tourism and the quality of life.”
Committee members bristled at what they saw as a severe lack of engagement by rPlus Hydro and FERC with the public and local officials. Committee leaders agreed to send a letter to Wyoming’s congressional delegation, as well as to FERC, imploring officials to insist on meaningful protections.
What’s next?
The FERC has indicated that the publication of the final EIS this month does not trigger a public comment period before giving its final approval later this year. Some governmental agencies, however, still have the power to persuade the FERC, according to WyoFile sources.
So what powers can be exerted on the FERC to change course on the project?
For example, the wildlife waivers and other accommodations in the FERC’s plan do not align with the U.S. Bureau of Land Management’s resource management plan for the region, administered by the BLM’s Rawlins Field Office. If the BLM chooses to accommodate FERC’s plan for the project, it would likely have to amend its resource management plan — a process that is more inclusive of public and local government agencies.
Harrington and Cheesbrough both noted that the Wyoming Game and Fish Department, for example, has refused to endorse a carte blanche waiver of seasonal wildlife restrictions. That could be a major factor if the BLM initiates the process to align its management plan with FERC’s proposed certification of the project.
“To me, that’s a massive hurdle,” Harrington said.
This article was originally published by WyoFile and is republished here with permission. WyoFile is an independent nonprofit news organization focused on Wyoming people, places and policy.
Related
CHEYENNE, Wyo. — The Wyoming Department of Family Services recently announced that it will be launching a federal program this week to provide grocery assistance to more than 37,000 school-aged children across the state.
Known as SUN Bucks, the initiative provides a one-time $120 benefit per eligible child to help families cover food costs during the summer months, the department announced in a release. Gov. Mark Gordon previously authorized the program’s implementation through an executive order on April 15.
Gordon described the initiative as an essential tool to support children who may otherwise lack access to healthy food while school is out of session.
“We want our children to thrive, because when our children are successful, so too are our communities,” he stated in the release.
According to DFS, most qualifying children will be automatically enrolled in the program. The department reports that it began sending eligibility notifications this week via mail and email.
Eligible families can expect to receive SUN Bucks electronic benefit transfer cards in the mail starting in early July.
DFS Director Korin Schmidt said in a statement that the program is specifically designed to assist rural children who lose access to school-provided breakfast and lunch during the summer months, adding that the benefits will allow families to purchase groceries as needed to ensure food is available in the home for those missed meals.
The SUN Bucks cards will function similarly to other benefit programs and be accepted at any retailer participating in the Supplemental Nutrition Assistance Program.
While tens of thousands of children are enrolled automatically, some eligible families may still need to apply, according to the press release. Residents can check their child’s enrollment status or submit an application through the DFS SUN Bucks website starting June 22.
For more information, people can visit the DFS website, email ask-sunbucks@wyo.gov or call 307-777-8786 between 8:15 a.m. and 4:45 p.m. Monday through Friday.
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