Geologic Activity

Wedding Canyon view at the end of Ottos Trail

NPS / N Scarborough

 

Upper Ute Canyon in early spring

NPS Photo / N Scarborough

Rocks are always in a state of change. On the surface, they're usually getting broken down into smaller pieces ("weathering"). Sometimes they're carried to new places due to water, wind, ice, and gravity ("erosion"). Eventually they settle in one spot ("deposition"), where they might eventually turn into other rocks ("lithification").

Colorado National Monument was preserved in 1911 because of our exceptional examples of erosion. You can see proof of the geologic activity just by looking at the canyon bottoms. All sizes of rock lie at the base of cliffs. You can even try matching the shape and color of a boulder below with an area on the cliff face, imagining the exact spot that it fell from.

After a rainstorm, you can find waterfalls at the heads of almost every canyon. Streams run down the middle of the canyons soon after. It may be hard to see the change in action, but every storm changes the Monument. Keep reading for a recent episode of sudden, larger-scale erosion ("mass wasting") that happened in the year 2000.

 

The Rock Fall of 2000

 

Rock fall of 2000.

Ron Young

 

What happened? At about 9:45 in the morning on January 8, 2000, a section of cliff dropped onto Rim Rock Drive near the Liberty Cap trailhead, completely blocking the roadway.

What fell? The rocks that fell are sandstones that were originally deposited about 140 million years ago by ancient streams. Geologists describe the rocks as being from the Salt Wash Member of the Morrison Formation.

Why did they fall? The salt wash sandstone has a distinct layer of soft shale in the middle and a layer of easily eroded shales and sandstones below. Ancient earth movements made two distinct sets of nearly vertical fractures in the salt wash sandstone. These two fracture planes meet at nearly right angles, creating large rectangular blocks that are not firmly connected to the rest of the cliff. This lack of a firm connection to adjacent rocks, plus the weak shale, means salt wash sandstone is easily eroded in this area.

Has this happened before? Certainly. Our earth is a dynamic and ever-changing place. The rock fall of January 8, 2000 is simply a continuation of the erosion that has carved the canyons of Colorado National Monument. Erosion is a natural process, and in fact is the architect of the spectacular landscape. You can see lots of examples of erosion within the Monument. Look for fallen boulders, small rock falls, and stream scouring and widening during flash floods.

While larger rock falls occur frequently in geologic time, it is a rare and exciting opportunity to see and study one during our lifetime. Some of these broken rocks last saw the light of day 140 million years ago when they were laid down in a streambed during the time of the dinosaurs. As geologists study them today, the rocks may reveal more about those ancient times. They might even help us understand how to predict the likelihood of future rock falls.

Geologists, engineers, and historians examined the rock fall and tried to figure out several things:

• Was there any immediate danger of further rock falls?
• Was it safe to work in the area with heavy equipment?
• How much damage was done to Rim Rock Drive?
• How should they clean up and repair Rim Rock Drive with minimal effect, since the road is a listed structure on the National Register of Historic Places?

Considering all of these factors, Rim Rock Drive was cleared and repaired as quickly as possible. There were no injuries, but because the area was considered unstable, it was closed to the public. Rim Rock Drive remained closed until the rock debris was cleared off and the road repaired.