- In the summer of 1925, an entire mountainside collapsed and slid into the river valley below in Teton County.

It's known as the Gros Ventre Slide, and it's still regarded as one of the biggest landslides ever recorded.

I'll speak today with two experts who know all about it.

I'm Steve Peck of Wyoming PBS.

This is "Wyoming Chronicle."

(bright upbeat music) Welcome to "Wyoming Chronicle."

I'm here today at the Gros Ventre Slide geologic site with Mariah Radue, Todd Stiles of the U.S.

Forest Service.

This is not in Teton National Park, but it's close to it.

It's in the Bridger-Teton National Forest instead.

That's your employer.

Mariah, what's your specialty within the Forest Service?

- I'm a geologist with the Forest Service.

- And Todd, you're a?

- Yeah, I'm the district ranger here on the Jackson Ranger District of the Bridger-Teton National Forest.

- How long you been with the Forest Service?

- With my seasonal time and as a permanent employee, about 28 years.

- 28 years.

How about you, Mariah?

- It's just about six years.

- Six years?

We need a geologist here because we're on site of the Gros Ventre Slide, which I think is familiar to people much in the way it was familiar to me.

I've lived in Wyoming all my life, driven from Moran Junction over to Jackson many times.

My dad, my uncle, always said, "Look!

There's the Gros Ventre Slide!"

And you can see this area in the mountain, we're much closer to it now, and I think in the shot we're in, people can see more of it behind us.

It's one of the biggest landslides that's ever really been recorded in human history.

Is that still accurate to say?

- Yeah, certainly in the United States.

It's one of the most notable, one of the largest, that we have a good record of what happened.

- And we're here in early July of 2025 and we're just almost exactly at the centennial of the event itself, which happened in very late June of 1925.

I wanna give it a little bit of context related to the perspective from a much more recent and very well publicized landslide that occurred on Teton Pass, on the highway there.

That was just last year, in 2024.

The highway comes down, it's almost a spiral road in some places, and a big chunk of the highway just gave way, I think 30 yards long and probably 10 yards wide, and displaced a hundred feet of support material below it.

Closed the road, tons and tons of publicity for it.

A huge mess, a lot of impact, and maybe displaced 10,000, 15,000 cubic yards of material.

Striking thing to see.

Nothing compared to this, which I believe my calculations are, this was 5,000 times bigger than that.

- So it was June 23rd, 1925, and the north face of Sheep Mountain, it fell away catastrophically.

It was almost within the blink of an eye.

We're looking at the Gros Ventre Mountain Range here, right on the northern edge.

And as you said, it's mentioned, it's assumed that 50 million cubic yards.

There were people there at the time who witnessed it.

One of the cowboys was driving cattle on the north side of the valley and he said the winds were so great that it almost knocked him off his horse.

- That's what I wanted to talk a little bit about.

I mean, this is a multi-sensory experience if you happen to see it.

And we know there were people who... There's people who did!

It couldn't have been recorded, of course.

Nobody had a phone to say, "Look at this!"

What must that have been like?

I mean, you thought about that Todd?

The impact and the sort of spectacle of what... I wonder if they could comprehend what they were seeing even.

- Yeah, and you have to kinda think whether or not too the horse understood something was coming too.

Because a lot of times they're pretty perceptive.

- [Steve] Yeah!

- But definitely, he was lucky he had a horse that was fast enough to get out of the way.

'Cause it was a major close call and he was right there.

- Of course, he heard something happening and of course he felt it probably.

Wind I hadn't thought of.

Of course you could see it happening, if you could sort of comprehend what it was.

I've heard people in the distance thought, "Is there a fire over there possibly?"

'Cause of course an enormous cloud of dust flew up.

I've been to a dam construction site once where a ton of earth had been moved and it had been many tons and it had been wet and the smell of it was so prevalent also, the smell of that wet soil.

So other than eating it, I guess.

- [Todd] Yeah!

- It affected every single sense that you could have.

- Had to be terrifying for sure.

I'm assuming feeling pretty lucky though, to make it out unscathed.

- The very top of what we're seeing here at the mountain is what altitude, what elevation?

- It's about 9,000 feet at the top.

- And down at the river where there's a river below us, that's at what elevation roughly?

- 7,000.

So yeah, 2,000.

- It was a vertical drop of 2,000 feet.

- [Mariah] Yep!

- Of 50 million cubic yards of material.

- The entire mountain came down.

So some of the trees and vegetation came down in just entire blocks.

And at conclusion of the slide, there were still actually trees that were partially upright in the slide, 'cause they'd moved in larger blocks with the soil and everything.

- So it's a mountain.

And so there, I mean, the construction of the mountain was what was coming down, not just top soil or even tree roots, but there's rock underneath.

Wyoming is a really interesting state for geology anyway.

And this part of the state in particular, geologically speaking, we sort of figured out why this happened.

- Yeah!

- [Steve] What was the day like?

What were the conditions?

- Well, there are four reasons why people think all contributed to the Gros Ventre Slide, and the first one is the underlying geology.

So we're in the Gros Ventre Mountain Range.

There are sediments that were deposited 500 to 300 million years ago.

And when they were deposited, they were originally flat, you know, coral reefs where limestones are built up, beaches with sandstones.

And importantly, the Amsden Formation is in there, which is full of clays, which end up being very important for the landslide.

And so they're originally flat, but with the mountain building of the Rocky Mountains about 70 million years ago, these rocks were tilted on their side, and so that created a slope.

We're looking at the north slope and the dip of the rocks.

The tilt is about 30 degrees, which aligns with that slope.

So that set it up for a landslide, which you see all throughout the Gros Ventre.

This is not unique because the geology really sets itself up to being landslide prone.

- You said, "Clays."

I've done some work during my newspaper career on the bentonite mines in the Northern Bighorn Basin.

That's clay.

Slippery!

- Yeah, exactly.

And if you've ever driven a clay road, you notice you wanna be out of there.

- Yeah.

- If it started raining.

And so, water was the next big factor.

- It had been a rainy spring.

- Exactly!

Yep!

And when it's springtime, a lot of snow melt, it was melting quickly, sends a lot of water into the rivers and then also into the groundwater.

- That had happened before.

There's always a lot of snow melt up here.

It's... This was the straw that broke the mountain's back, I guess.

- Yeah, and there was discussion and all that, you know, the Gros Ventre River is a meandering river and so naturally it moved throughout the valley with ox bows and everything and it had cut on that south side of the bank of the river.

And so you had a pretty raw cut slope.

And so when you put together this clay layer that was impermeable, not very porous, this moisture, like you said, that was running against it, this formation above it and then the tilted angle, you really didn't have any integrity at the bottom of the river to even hold it in place either.

So it's just all these factors at once.

- That happened on that day.

How long do we think it took from the moment that you might have said, "Whoa, landslide," to, "Thank God that's over."

Any ideas?

Did it take a couple of minutes or an hour, or?

- That's my guess is minutes.

- Really?

- I mean, it is cool that we have such a good account of this landslide.

It was a hundred years ago, but we did have eyewitness and they were interviewed and so, we have a understanding.

But I've heard a geologist might even use the word, "A debris avalanche" to describe this because it was so fast.

- Thinking back to the recent, fairly recent history in Jackson, there was this slow creep of a landslide that had a commercial building on it as I recall.

And it was moving, what?

A half inch a month or something?

Avalanche compared to landslide as the speed, is that part of the definition of it?

- Geologists can get very technical about how to describe slope movements, and landslide is a general term.

But thinking about how much the debris was flying around, they estimate it was maybe 50 miles an hour.

- Wow!

- It was moving down the downhill and the speed at which it happened, maybe avalanche.

- Well, the galloping horse, I guess we've heard of this.

- Yeah!

And I think some of the difference of what you were referring to there on West Gros Ventre Butte is this kind of slow moving geological event.

- Yeah!

- Versus this catastrophic event that happened within minutes here at the Gros Ventre Slide.

- The French explorers and trappers who explore the country and began naming things, gave things some pretty crude names sometimes.

It means like fat belly or something, doesn't it?

Gros Ventre?

- That's correct.

- And that's what the name of the river was.

And it's down there and a lot of this came right down into it.

But it didn't stop there, did it?

Because there's another ridge on the other side of the river and it boiled right back up to considerable height.

- [Todd] Yeah.

We're standing on the debris here.

This entire site's built on it.

And our geologic trail here that's right here in the geologic site that you can hike to, it's all built up on the talus rock field of the slide debris.

So it's very impressive because the ecology of the area, there's only basically been a hundred years of recovery for vegetation to come back from that really catastrophic event.

- [Steve] It gives you an idea of what can happen in a hundred years, but also what can't happen in a hundred years.

I mean, we think about other events on the landscape, fires, and floods and things, "Well, it'll come back," and we get impatient for that sort of thing.

But a lot of trees where we are now and, as you say, this once was the debris field so to speak.

How high did the debris go back up to the mountainside?

- Heard it was about 300 feet.

- Okay.

So think of a football field stood on end and it went up at least that far again.

So then the river gets blocked up and suddenly you have, if you had a ranch property here, you have a lakeside property here now all of a sudden.

- Yeah, the lake was initially about seven miles long.

And after it was rising, it picked up structures and everything that was down in the valley where there were ranches.

So there were homes, the cabins floating, our Horsetail Ranger Station for service ranger station, that was floating.

Initially picture these structures floating until they no longer floated.

So for that short, and there's some really great photos we have on some of our new interpretive signage.

- The guy sitting up on his roof.

- Yeah, that's Guil Huff.

Yep!

- That's the guy?

- The guy who outran it with his horse.

His ranch got flooded.

He had a bad day.

- Then it seemed okay the following year.

Well, we've got this big dam here now, but that's gonna hold forever.

Just like, well, we learned though that you probably shouldn't think, "Well, this is gonna hold forever" because the mountain didn't.

And sure enough, two years later, was it?

Another big wet spring, a lot of runoff.

What happened then?

- The lake levels rose higher than they thought they would and they overtopped the earth and dam.

So it was May 18th, 1927 that it overtopped the dam and it caused the Kelly Flood.

- The Kelly Flood.

There's a town.

And this was in the earlier days of Teton County.

And there was some question as whether Kelly might actually be the Teton County seat instead of Jackson.

Kelly almost ceased to be when that flood happened.

- It was a tragic event.

Six people lost their lives.

Lots of domestic cattle and all but two buildings were wiped out by the flood.

- Yeah, there's been part of that as discussion that that was part of what ultimately led to Jackson becoming the county seat of Teton County, Wyoming, versus Kelly.

- So there was intense interest at the time.

There immediately began to be evaluated, studied, analyzed, explored.

We wanted to know what was going on and were able to.

When did the sort of historical remembrance of it begin?

- 1962, the Intermountain Regional Forester and the Teton National Forest decided to designate this as a national geologic area.

And it took a couple years to figure out what infrastructure was gonna be needed and everything to accommodate the public.

Parking, signage, a trail, all that so that the public and the visitors could enjoy it and see the interpretation.

And then it was four years later in 1966 when the Forest Service and the YWCA, the Young Women's Christian Association, there was a group of young girls who came out from all over the country and they helped the Forest Service construct the trail here in the geologic slide area and do some of the sign installation and painting as well at the time.

And this I'll mention too, this road was just finally paved in the last couple years up to the Atherton Campground.

So the access is really good for the public.

So right by here, immediately to our east, there's another, a really beautiful overlook of Slide Lake, which was the lake created during the slide here.

And we have another panel of interpretive signs there that speak to history, some of what we just discussed, wildlife.

'Cause this is a very significant wildlife corridor in the Gros Ventre, and also a little bit more interpretation on the geology.

- And there's also a trail if people are better hikers.

- Right from here on this same south side of the road, you can hike through the rock slide and it's a self-guided interpretive trail.

So we have brochures, there's four stops.

And that was... We redid it this spring in conjunction with the celebration and the kids from the Kelly Elementary School helped with that.

- And Mariah showed me a brochure that the young people had taking the lead in doing.

A lot of exclamation marks, you said, but.

(chuckles) - It's true!

Cut out a few of them, not all.

Wanted to keep it as their voice.

(footsteps crunching) - We've come just a little ways down a trail and we're standing now on a huge pile of mostly rocky rubble.

And this is the kind of stuff that came down from there, crossed the river and came back up here.

And it hasn't been reclaimed by nature in the way that the stuff down closer by the river has.

You can see evidence of all of it here still.

- It takes a while for soil to form on the bare rocks like this, so, yeah, plants have a hard time growing.

You'll get lichens growing on the rocks.

Some small, small plants, but the trees haven't quite made it here.

A good comparison would be the glacial deposits in the area.

We had glaciers here 15, 20,000 years ago.

And those marines, you can see small trees growing smaller plants, but there's a definite difference between those and the valley bottoms, which are hundreds of thousands, if more, years old.

So there's some context for this that'll take many, many years.

- As you were walking with me off camera, you were talking about the variety of plant life that has recovered, has originated here essentially, in the past hundred years, and it's diverse.

- [Todd] It is!

It's really impressive.

So you've got this mixed conifer forest of Douglas fir, some lodgepole pine, and then you've got deciduous trees, you've got aspen, you've got narrow-leaf cottonwood in here, and a really impressive shrub understory too.

- [Steve] We saw sagebrush, we saw some Indian paintbrush here.

Mariah, you're the geologist.

And just to my untrained eye, this is really, this is geology as I think of it.

A bunch of rocks lying around that aren't covered by vegetation.

What are we seeing here?

- Yeah, so we're looking, this is the Tensleep Sandstone that is the strong, competent sandstone that was on top of the shale layers.

So we're looking at rocks that were originally all the way up there.

And what you also see is the reds and the pinks of the shale from the Amsden Formation.

So those two rock types, those are the two ones that were responsible for the landslide.

Amsden Formation up here, Tensleep Sandstone all the way down here.

- So pick up something and show us what you're talking about.

- Oh, sure!

So sandstone, that rock is a pretty descriptive name.

You know, it was once a beach hundreds of millions of years ago.

And you can still feel the... There's fine sand grains in the rock.

This is also a cool place to look for calcite crystals.

There's a lot of limestone in the area and the water has picked up calcium carbonate from those and deposited some very spectacular crystals.

So it's a fun place to explore and pick up rocks.

And here's some of that calcite.

And so one of the tools in a geologist's field kit is usually hydrochloric acid.

And if you put it on the calcite, it'll fizz.

You can tell that.

And then the sandstone is made up of silica.

And so, that won't fizz.

- [Steve] It won't fizz?

- [Mariah] Yeah!

- So even if you can't tell as visually clearly as we can here, that's the surefire way.

- [Mariah] Yep!

Yep.

- We moved up the road a little bit to a newer interpretive area, and I know that one of the things that's important to your agency is recognizing the historic significance of the land.

Who was here before the Forest Service was, before this development was, before the ranchers were.

There's another interpretive sign about the wildlife, another one more specifically about the slide.

But what it's demonstrating is this is a great area to be in, even if the Gros Ventre Slide had never happened.

- [Todd] The Gros Ventre's just really one of the more beautiful valleys in the West, arguably.

You know, you've got wild and scenic river here.

- [Steve] What is that exactly?

What does that mean?

- So it's protected by the Wild and Scenic Rivers Act.

And this was 2008 era, the Craig Thomas Headwaters Act, and it's many of the headwater significant rivers and streams of the Snake River that are designated in that.

And it's really to make sure that we maintain the wild character and the free flow of the river.

- And Wyoming is one of the great, maybe the greatest headwater state really in North America.

So many start here!

- Yeah, absolutely.

So the Bridger-Teton National Forest has a headwaters of the Snake, the Green and the Yellowstone.

So some of America's great rivers and many other beautiful high-volume streams and rivers.

- What we're in and we're seeing is wilderness area as well.

And also, a protected migratory corridor.

- South of the river where the slide is, that's within the Gros Ventre Wilderness Area.

And so that was designated by Congress in 1984 in the Wyoming Wilderness Act.

And so wilderness areas are forever protected to remain in the state that they were, when really we made contact with them.

And in terms of the pronghorn migration, so on the north side up here in the Red Hills, the Sublette pronghorn herd, they move all the way from Grand Teton National Park, which is a little bit to the northwest of us, they go all the way through the Gros Ventre and up into the Upper Green Country, by the Wind River Mountains, and then down into the Sagebrush Flats more in the desert.

And that's one of the longest migrations left of an ungulate in the lower 48 states.

So in the Bridger-Teton National Forest, our forest plan, we've recognized that that's a really important thing to ensure that those pronghorn are be able to continue to migrate through there.

So we have protections in our plan for that.

- [Steve] For viewers who don't remember what wilderness means, you said you can get out here in the lake and do water sports.

What can and can't you do on the other side up there near the slide?

- So in a wilderness area protected by Congress, by the Wilderness Act, it's the traditional uses that are still allowed.

So some examples: Hiking, horseback riding, horse packing, canoes and non-motorized boats on the lakes, cross-country skiing, snowshoeing.

But there isn't motorized use allowed.

And when we actually maintain the trails in wilderness areas to keep them open for the public, we still use traditional tools.

- [Steve] You do?

- So we use crosscut saws, two person crosscut saws, double bit axes, all the tools that the Forest Service would've been using in the early 1900s when we first started managing the forest reserves.

- Has there been any sort of stabilization done of this naturally forming dam from a hundred years ago?

Or is it still largely the way it was?

We haven't gone and put a spillway under it or anything like that?

- No, it's how it was.

The Kelly Flood did a good job of turning that more into a stable land form.

But it's interesting.

The decision to build a spillway at another landslide dam up in Montana, they thought about the tragedy that happened here.

So it was the Hebgen Lake Landslide, after that.

Similar landslide on National Forest land, they decided to cut a spillway in that to prevent what happened here.

- We're talking about a famous incident from, the what?

The 1950s?

- 1959!

They had the earthquake, yeah.

- Earthquake that caused that.

Do we think, did an earthquake have anything to do with this or do we know for sure?

- It might have!

So I said four different reasons that built into causing a landslide, and that last one is possibly an earthquake.

This is a seismically active area.

We've got the Teton Fault, Yellowstone, and there are reports documented that people felt shaking in the weeks up to.

- [Steve] There was one person in particular who claimed at least that he saw all this coming and sort of like a little bit of insider trading in its way.

What did he do ahead of time?

- Yeah!

So it was a man named William Bierer and he had the private land in holding right here on Sheep Mountain, and he noticed the signs of a landslide.

We don't know exactly what they were, but I suspect maybe he saw some cracking, some pooling of water.

Sure signs that the slope wasn't totally stable.

So what he did is in 1922, he sold the land to Guil Huff, the man we've been talking about, the local landowner for a ranch.

- [Steve] And he may or may not have told Mr.

Huff of his worries, right?

- The story is unsuspecting landowner was Guil Huff.

And, yeah, Uncle Billy, William Bierer, he never saw the outcome of the landslide.

He passed away in 1923 before the landslide happened, but his prediction came true.

- [Steve] Boy!

- Pays to be a keen observer of your environment.

- There's a comparison to the Big Fill Slide up on Teton Pass.

It's great to see that here.

And you've involved in making this presentation.

It's a useful side-by-side, isn't it?

Just to make people appreciate the difference between the two.

- It is, yeah.

I think people's memories are short, you know?

People were surprised that the Teton Pass, that Big Fill Landslide happened, and there'd been a landslide there 15 years before.

- So now this is done, this is finished happening, right?

This will never happen again, correct?

Mariah?

- Oh, no!

I mean, in geologic time, the Gros Ventre, the Teton Range, they'll keep on moving.

I mean, the tricky thing about landslides is we don't get to see them quite as often as other national... Natural phenomena.

But they'll keep on happening.

It could be 10 years, could be a hundred years.

We see it a lot in the transportation corridors.

The landslides definitely keep people busy because that's where landslides and expensive infrastructure come together.

- Yep!

And also where people are and can be eyewitnesses to them as well.

- Yeah.

- But a slide like this, in other words, has happened many, many times before, obviously, and throughout geologic time.

- And there's more examples in the Gros Ventre.

The Upper Slide Lake, which prior to this was Slide Lake.

And so, when this happened in 1925, this became Slide Lake and the lake further up the valley became Upper Slide Lake.

- You can look over and see small landslides all over the place.

- [Mariah] Yeah!

- Especially where there's a stream bank, for example, that's, as you mentioned earlier, undercut something and something falls down in a much, much smaller scale.

As a geologist, is this the super coolest thing there is?

Or how do you rate it?

- Oh, I think it's super cool.

When I, yeah, when I found out that this was a geologic area and a place with interpretation, I thought that this was just a wonderful place to describe how earth, you know, earth can move very slowly is how it transforms.

But there are places where the earth moves in a blink of a eye.

I grew up in Maryland, so people say, "Oh, it must be nice to have geology out where you are."

And I'm like, "Geology's everywhere!"

- Planet Earth, geology.

- Yes!

It's much easier to see here in Wyoming where there aren't quite as many plants than there are in the East Coast where plants hide a lot of what's going on.

- Wyoming person naturally, or?

- Originally as a kid, I lived in Nebraska.

But, yeah, I've been in the West since I was 18, 19 years old.

- In terms of just the proximity to the national forest, the time that we're living in, the expertise that we have, it's just hard to beat it as a forest site, a geology site and education site.

- I'd say this is what public lands are all about, you know?

This site, it's kind of this bringing together people, culture, and the landscape and how those things interact.

And those historical stories really make it rich, being the land manager and have a better understanding of that and how that's affected the county at the time, and it's continuing to.

(bright upbeat music)