East of Enoch

Millions of years ago, three massive landslides rocked the ancient landscape of southwest Utah, creating enough debris to fill the Grand Canyon nearly to its rim.

Geologists have dubbed the ancient slides the Marysvale gravity slide complex. And it could be the largest such complex on land in the world, encompassing approximately 3,000 square miles, an area “roughly the size of Yellowstone National Park,” according to the Utah Geological Survey booklet, “Utah’s Ancient Mega-Landslides,” written by Robert F. Biek, Peter D. Rowley, and David B. Hacker.

One of these events, the Markagunt Slide, was approximately 60 miles long, 23 miles wide — at least 1,150 square miles — and estimated to be over one mile thick in some areas, according to the publication.

Comparatively, the largest terrestrial landslide in recorded history occurred in connection with the eruption of Mount St. Helens in Washington state in 1980, which traveled about 14 miles and had a maximum depth of 600 feet, according to the U.S. Geological Survey.

A gravity slide is “a special class of extremely large and geologically complex landslides several tens to thousands of square miles in extent,” the booklet says.
Ancient ‘mega-landslides’ left marks across Southern Utah

The size of the Marysvale gravity slide complex compared to the Wasatch Front, the San Francisco Bay and the Seattle area to “dramatically show its gigantic scale” | Image courtesy of the Utah Geological Survey, Cedar City News

“They are larger and far more interesting than geologists could ever have imagined,” the authors state. “Though crisscrossed by highways and dotted with a dozen small towns, including Beaver, Parowan, Panguitch, and Cedar City, the mega-landslides remained hidden in the landscape until the mid-2010s.

“These ancient landslides are home to thousands of Utahns and are traversed by millions of tourists each year traveling to southern Utah’s famous red rock country and national and state parks and monuments. You might ask yourself, ‘How can something so large escape our notice for so long?”‘

According to their research, the remains of the slides are “chopped up by subsequent faulting, are deeply eroded and partly buried by younger deposits,” so there isn’t a single vantage point to view them.

The slides are composed of volcanic rocks that erupted from large stratovolcanoes in the volcanic field and ash-flow tuffs, rocks made from volcanic ash that erupted from calderas, similar to the Yellowstone Caldera, both within the field and from as far away as Eastern Nevada.

The Markagunt, Sevier and Black Mountains slides, each named for the plateaus where they are best exposed, were discovered between 2014-19, according to the booklet.

Robert “Bob” Biek, a retired senior scientist with the Utah Geological Survey, told Cedar City News that evidence of the ancient landslides was discovered by happenstance.

Biek, who retired in June 2022 after 25 years with the agency, has continued to research “these gigantic landslides” and complete geologic mapping in the St. George basin, Zion National Park, the Cedar City area and Bryce Canyon National Park in Southwestern Utah.

“Because it’s so much fun,” he said. “Utah just has the most tremendous kind of in-your-face geology of anywhere that I know of.”

The “long and short” of the discovery is that researchers noticed that there was “something weird” about the geology on the Markagunt Plateau while mapping the area in the mid-2000s, Biek said. Before this, researchers had been mapping the area since the 1950s.
Ancient ‘mega-landslides’ left marks across Southern Utah


This photo shows the highly fractured Isom Formation as part of the Markagunt mega-slide, Sydney Peaks, Utah, date not specified | Photo courtesy of the Utah Geological Survey, Cedar City News

In the 1970s, a graduate student from Ohio State University was working on a geologic map for his thesis near Brian Head Peak. He was mapping the volcanic rocks in the area when he discovered “little hills,” now called Sydney Peaks, where the rocks were out of sequence, Biek said.

“Imagine just a section — you’re going up from older rocks to younger rocks and then all of a sudden, you get back into older rocks,” he said. “And so they were really puzzled about that.”

More examples of the phenomenon were found but ultimately, research halted in the 1990s. Biek said when he began to map the area, he “stumbled” upon the old discovery from decades prior.

“And then I found some really exciting examples of it that broke the whole case open,” he said.

The first of the three “gigantic landslides” was discovered in 2014. Biek said once his team realized how “big and important and unique” the slides were, they requested involvement from universities across the country with various expertise to study different facets of the slides.

Additionally, a three-year study is being funded by the National Science Foundation to better understand the events, Biek said.

A Fast-Moving Slide:
The length of the landslide – over 55 miles – also shows that it was as fast moving as it was massive, Hacker said. Evidence showing that the slide was catastrophic – occurring within minutes – included the presence of pseudotachylytes, rocks that were melted into glass by the immense friction. Any animals living in its path would have been quickly overrun.