A major rockfall, approximately 1,500 yd3 (1100 m3), caused significant damage to Interstate-70 at mile-marker 125 in Glenwood Canyon, Colorado, Thanksgiving Day, November 2004. Photo credit: Colorado Department of Transportation.


One of the primary missions of the CGS is to help reduce the impact of all geologic hazards on the residents of the state. The CGS responses to Colorado’s rockfall hazard include:

A rockfall is a type of fast-moving landslide that happens when rock or earth falls, bounces, or rolls from a cliff or down a very steep slope. Rockfalls start from high outcrops of hard, erosion-resistant rock that become unstable for a variety of reasons. The size of the falling rock depends on the source area geology (bedding thickness, bedding dip and dip direction, hardness, joint/fracture orientation), weathering, position, and steepness of the slope. Generally, an individual rockfall has one to only a few rocks, with sizes that vary from gravel to boulders (two inches to five feet or larger in diameter). When a large mass of rock fails and the resultant fall spreads out into a debris fan, it is referred to as a rockslide or even a debris avalanche. Rockfalls can be very dangerous depending on where they occur, the size of the rocks involved, and how fast the rocks fall or bounce downslope. When people, buildings, vehicles, or highways are in their path, these rockfall events can cause property loss, personal injury, or even loss of life. Rockfalls can also create unexpected hazards on roads, causing damage to vehicles who unexpectedly drive over or into recent rockfalls.

Because they are often catastrophic and without warning, it is difficult to predict how often rockfalls occur. However they are a common erosional process in mountainous areas near cliffs of broken, faulted, or jointed bedrock; on steep slopes of rocky soils; and where cliffs and bedrock ledges are undercut by erosion or human activity. Young, actively rising mountain ranges like those in Colorado do not weather away slowly, grain by grain, but typically erode through larger events including rockfalls, landslides, and debris flows. Many regions and towns in Colorado’s Front Range and Western Slope are exposed to the rockfall hazards of nearby mountains and cliff-rimmed mesas.

Active rockfall areas are those showing evidence of recent falling and rock movement. Rock displaced or damaged vegetation, fresh “tracks” of rocks rolling down-slope, fresh scars on cliffs, anomalous or disoriented lichen growth on rock blocks, eyewitness accounts, and damage to fences or man-made works are some common criteria for identifying active rockfall areas. The most common difficulty with ‘inactive” rockfall areas is unexpected reactivation due to human activities or exceptional natural conditions. Questionable rockfall areas should be monitored if there is the possibility that reactivation of a rockfall may take place and present a hazard to man.

Most fatalities from rockfall in Colorado happen on roads or major transportation corridors, but some people have been seriously injured or killed in their homes where they are located in the path of rockfalls. Rockfalls are often not covered by homeowners insurance because they are classified as a type of landslide. It is important to be extra alert during three particular times on Colorado’s highways: spring thaw, after heavy rains, and at night. Be particularly alert during these times where you are approaching blind curves.