Science column: It’s raining rocks

James W. Hagadorn
Just the Facts
James Hagadorn
Chris Schneider |

Ever been driving on a winding road late at night, only to round a bend and have to slam on the brakes to avoid rocks in the road? If so, you’re not alone. Rockfalls are regular occurrences in Colorado, and the debris they jettison onto roadways and other structures is hazardous.

Fortunately, Colorado’s got some of the top gurus in the nation to track these geohazards, including teams at the Colorado Department of Transportation, at the Colorado Geological Survey and their partners. Using laser mapping, aerial photography, remote sensing and a bevy of crack, til-, and movement meters, they regularly assess and monitor our known rockfall regions. The data are plugged into modeling software to assess the nature of potential future slope failures. In fact, the most widely used modeling software for characterizing rockfalls was developed right here in Colorado. It arose from the need to minimize risk from snaking an interstate through Glenwood Canyon. To head off future falls, whether in Glenwood or in the seemingly innocuous hillsides of South Park, agencies build preventative berms, moisture-diverting sluices, rock-catching troughs, fences and walls. Known problems get remediated with an arsenal of bolts, buttresses, cables, anchors, netting and related gear. Sometimes crack-filled exposures are even injected with vast quantities of resin — akin to mountain superglue.

But what is it about our landscape that makes all of this prevention necessary? After all, New Mexico, Utah and California have huge mountains but we don’t hear about many of their catastrophic rockfalls. Short of blaming population penetration of the mountains, Colorado’s rockfall abundance stems from our distinctive climate, topography and diverse blend of rock types.

In Colorado, falling rocks mainly come from three sources, all of which are plentiful on hillsides and mountains. The first source is fractured bedrock, like the stuff exposed in Black Canyon of the Gunnison, Golden’s Clear Creek Canyon and Pikes Peak. These rocks, cooled from magma or cemented together during burial and mountain-building, are full of fissures and cracks. As water seeps into the cracks, sometimes it lubricates them or degrades crack-filling crystals into clays that help the rocks slip apart. The rocks eventually calve off like glaciers. An even more destructive process, called ice-jacking, occurs when this infiltrating water freezes and expands, gradually wedging the rock’s cracks open more and more during the daily and seasonal freeze-thaw cycles.

Alternating layers of sedimentary rock are the second major contributor to rockfalls. In exposures such as those that rim Castle Rock, Durango, Grand Junction, and Manitou Springs, massive cliff-forming beds of these rocks regularly spall off from mesas, buttes and bluffs. The cause? Underlying the durable well-cemented cliff rocks are soft, mud-rich layers. These layers erode from rainfall and seepage, undercutting and allowing the overlying rocks to topple downhill. Ice-jacking of cracks in cliff rocks also contributes.

Relict soils and sediments left behind by ancient landslides and the bulldozering of Snowmastodon-era glaciers are the last source of falling rocks. These crazy-looking piles of dirt and rock are a geological mishmash of giant boulders suspended in sand and gravel. They line most of Colorado’s valleys and canyons. As water from snowmelt or rainfall saturates sediment that holds boulders in place, rocks can slip or fall out of these piles, tumbling down to burgs like Canon City or the geological bullseye of East Vail.

Falling rocks from these sources directly impact homes, vehicles, infrastructure and people. They have indirect impacts as traffic hazards and because they catalyze landslides. To add to natural triggers, anytime we undercut, oversteepen or disturb hillsides and mountainsides, rockfalls will be more likely. Ditto when we build roads and homes at the foot of or on top of ancient rockfalls and landslides.

Given that Colorado’s freeze-thaw cycles and rainfall are each three-season phenomena, it isn’t surprising that crack-related rockfalls are a year-round phenomenon. Witness the epic traffic-clogging examples on the I-70 corridor. In contrast, erosion- and slippage-related rockfalls tend to occur after prolonged or massive rainy intervals, especially when it’s windy. Highway 133 between Paonia and Carbondale is a classic example.

Rockfalls have been shaping the surface of our state for a billion years and will continue to do so. They’re an indication that our Earth is always in action, and they signal the risks of living and playing in mountainous and hilly terrain. Understanding what triggers them and how they behave helps us predict and minimize our risk. To learn more, check out colorado hazards/ or

James Hagadorn, Ph.D., is a scientist at the Denver Museum of Nature & Science. Suggestions and comments welcome at