A cliff where two house-size boulders came loose and tumbled onto Colorado Highway 145 on May 24 will be analyzed further for potential rockfall risk, according to the Colorado Department of Transportation.

The 90-foot-long block of Dakota sandstone sheared off and broke into two massive boulders that plunged 1,000 feet onto the roadway, narrowly missing three vehicles, witnesses said.

A Google Earth image of the section of rock before it fell shows a large gap where it was separated from the main cliff band.

“Everyone has been looking at that image. You can see the separation,” said Ty Ortiz, a geohazard program manager for CDOT.

The area is not known to have frequent large rockfalls, he said, and the unstable cliff section far from the road apparently was not on anyone’s radar.

Surveying and mitigation work by a geotechnical crew was done right after the incident. In the immediate location, no visible signs of pending failure were discovered.

A geotechnical crew will be dispatched next week to reinspect the area, Ortiz said.

“They will be walking the ridgeline surveying for other areas with possible rockfall hazards,” he said. Stabilization work will be done if necessary. CDOT also has a plan to inspect the Colorado 145 corridor for rockfall hazards.

The detached section of cliff wore over time from a combination of weathering, erosion, freeze-thaw cycles, gravity and perhaps even a 4.5 magnitude earthquake that occurred 35 miles north of Dove Creek on March 4 and could be felt in Cortez.

A phenomenon called ice-jacking occurring in a gap behind an unstable area also might have contributed, Ortiz said. Ice-jacking occurs when water enters a confined space athen freezes and expands, causing structural fractures. The area had seen frequent rain in the weeks leading to the fall.

Ed Archuleta, a contract engineer with CDOT, hiked to the cliff band the day after the slide and noticed moist and saturated soil.

Where the rock sheared off, “there was visible water seeping from the face in numerous locations,” Archuleta told The Journal. “The winter’s high moisture had to play a role.”

Significant low-elevation snow this year likely intensified freeze-thaw action within the gap as well. When snow melts then freezes again, it forces rock apart.

The soft soil where the boulders detached kept them intact as they tumbled down, Ortiz said.

Typically, when large boulders fall, they break apart on rock surfaces and cliffs before hitting roadways, he said.

That slope has blocks that have separated from the cliff, but none was large enough to gain momentum to reach the valley floor. Historically, at least one other boulder nearly reached Colorado 145, located about 4 miles south of the rock slide.

The large trench of torn-up earth from the boulders will heal naturally and does not appear to have created a potential for future failures or risk to the highway, officials said.

Jim Ellson and his son where fishing at a nearby pond when they saw the boulders fall.

“It sounded like a jet engine,” said Ellson. “I told my son, ‘Look at that!’ You could see massive boulders crashing down and tons of dust and debris being plowed up. We were in awe, kind of shocked. What a sight!”

Ellson said the boulders just missed two vehicles, which continued southbound without stopping.

“I guess they didn’t bother to look in the rearview mirror!” he said.

He threw his fishing pole down and ran to his nearby home on Road 37 to call 911.

Neighbors raced to the scene, one on an ATV. They found another southbound vehicle that had stopped and reversed just in front of where the boulder came down. The truck was pummeled with large rocks, but the occupants were uninjured. A huge plume of dust enveloped the area.

“The house shook long and hard, then shook again with the second rock hit,” said Ellson’s wife, Barb.

Road 37, a private road that parallels Colorado 145, served as an initial detour around the fallen boulders. The Montezuma County road department graded it to improve the route, and Sheriff Steve Nowlin was on scene.

“It was the first traffic jam our road has ever seen,” said Barb Ellson.

The highway temporarily was closed, except for residents, while CDOT put in a temporary lane allowing alternating traffic controlled by a signal.

CDOT said the road repair will cost an estimated $1.3 million. Once the contract is awarded, the repair will take four to six weeks, depending on the weather.

The boulder that came to rest on the road weighed 2.5 million pounds and was blown up with explosives. The other boulder weighed 8.3 million pounds and came to rest just off the roadway after creating a huge trench through it.

CDOT decided to leave the big boulder where it is, and Gov. Jared Polis announced it will be named Memorial Rock because it came down at the beginning of the Memorial Day weekend.

CDOT spokeswoman Lisa Schwantes said CDOT maintenance crews who responded to the incident suggested the name.

“This name not only reflects when the slide occurred, but it can also be a reminder of those who lost their lives protecting our country,” she said.

The highway will not need to be rerouted. It will be widened with additional shoulder, and a guardrail will be installed at the location of the rock for driver safety.

jmimiaga@ the-journal.com

Did you know?

An 8.5 million-pound boulder fell from a cliff band May 24 and came to rest on the side of Colorado Highway 145. That’s about the weight of 1 million gallons of water, or about 3 acre-feet. Another way to think about it: It’s about the weight of 650 African bush elephants.

Update: Crew blasts boulder blocking Colorado 145