On A Drive Around Grand Mesa A Geologist Explains Why It's Prone To Slide
May’s massive landslide on the edge of the Grand Mesa near Collbran claimed the lives of three men.
Geologists I spoke to said landslides in western Colorado and the Rocky Mountain region are a normal occurrence because the earth is dynamic and erosion is happening all around us.
To get a better understanding of why experts told me the nature of the flat-topped mountain is to slide, I took a drive on Grand Mesa with a geologist.
I met Andres Aslan, a Colorado Mesa University professor and geologist, at the visitor center on the mesa.
Before we got into the same vehicle, Aslan opened the truck of his SUV and showed me a satellite map of the area.
"Grand Mesa as you can see [on this map] is this flat feature that is capped by very resistant lava flows," he said. "And these lava flows, formed 10 million years ago and at the time when these lava flows were extruded, they probably filled ancient valleys similar to the present day Grand Valley. When those lava flows hardened, they formed this resistant cap of rock such that subsequent erosion was focused on the areas around the lava flows and areas outside of the lava flows such that the surrounding valley walls were eroded away leaving behind these resistant areas which today form the highest points in the landscape. So in a nutshell, [the] Grand Mesa is an ancient valley bottom that used to be the lowest point in the landscape and today is the highest point in the landscape."
As we drove down Highway 65 towards the Grand Valley, I asked Aslan to explain the different formations we were seeing as we moved past the lakes sprinkled across the mesa.
"The lakes that exist on Grand Mesa are very important because unlike most places in Colorado high elevation lakes in the Grand Mesa are not related to glaciation," he said. "These lakes that populate the south and north flanks of Grand Mesa are related to huge landslides that have occurred in the past not recently but probably on the order of tens of thousands of years to hundreds of thousands of years ago. Huge pieces of that basalt cap rock that marks the surface of Grand Mesa calves off from time to time. And as these huge blocks fall downhill, they basically create closed depressions on their uphill side that then fill in with surface water and groundwater forming the present day lakes we see."
As we continued, Aslan pointed out examples of areas that show mass wasting, the general movement of material downhill largely due to the affects of gravity. Three factors determine if an area is landslide prone: gravity, slope, and types of sediment. Aslan said if there’s clay in the bedrock of an area that has steep slopes and lots of water it makes it more prone to landslides.
The Grand Mesa gets a lot of water. The National Weather Service said the Bonham Reservoir monitoring station on the mesa measures about 18 feet of snowfall and about 33 inches of precipitation each year. With an elevation of more than 11,000 ft the mesa has steep slopes. The mesa also has clay present in its bedrock.
Aslan hasn’t been out to the Collbran site, but said he has studied aerial images of the three-mile-long mudslide. He said you can see clear examples of landslides that happened in the same area before the Collbran slide.
He compared geology to history, and said it repeats itself.
At the bottom of the mesa, Aslan pulled over to explain the geology of the area as we looked up at it from the Grand Valley floor.
"What I want to do is just look uphill and from the road what we’re basically looking at on the horizon is the northern edge of the landslide bench," he said. "The edge would be marked by things like the old ski runs of Powderhorn. And you can see that there’s some creeks that are draining those steeper slopes and that those streams are eventually going to converge downhill so there’s surface water affecting that slope. But, the other thing I wanted to point out is the gentle sort of undulating nature of the slopes at the lower evaluations. You sort of see these mound or ridge like geometries to the slopes [and] to me those are very characteristic of ancient debris flow, slide, [and] mass wasting deposits. I see much greater evidence of ancient landslides [here that are] more similar to what we saw at Collbran."
Throughout the drive Aslan repeatedly stressed there’s evidence of landslides all over the Grand Mesa and they’ll continue to happen until the mesa eventually erodes away.