Geologic upheaval in Aspen |

Geologic upheaval in Aspen

Willoughby CollectionThe long-ago landslide that created Little Nell stands out in this 19th century photo of Aspen Mountain.

The devastation from the earthquakes in Haiti and Chile is a reminder of ever-present, powerful geologic forces. Aspen is a long way from the present day’s colliding plates of converging continents, but earthquakes and significant geologic events have occurred not only in geologic time, but in historical time as well.

Aspen Mountain, one of the most heavily faulted mountains in the country, features fault movements, as measured inside intersecting mine tunnels, of one to two inches per year. Contemporary studies forecast earthquakes of at least 5.0 on the Richter scale four times a decade, in the Aspen area. Stronger quakes are uncommon for two reasons: There is constant movement along Aspen’s faults that gradually releases energy, and Aspen’s major longitudinal fault, Castle Creek, is stable. Major quakes are associated primarily with longitudinal faults such as the San Andreas in California.

The Aspen area is not completely immune to large-scale earth movements. The most obvious one is the large mound of material that forms the base of Little Nell. The face of Aspen Mountain between its two book-end cliffs (Shadow Mountain and Ute Trail) manifests an inclined syncline: severely bent sedimentary layers that tilt toward town. Movement between sedimentary layers, especially when they are lubricated with water, usually happens slowly, but large sections can dislodge all at once.

Had Aspen existed at the time, the Little Nell slide would have buried part of town.

Those same steep sedimentary layers still exist, along with the possibility of mass movement. The topsoil layers moved 40 years ago. Mudslides inundated buildings along the base of the mountain. The Shadow Mountain condominiums, built on a very steep and questionably stable slope, had to be anchored with cables to prevent them from joining the downhill tendency.

Hikers who have ventured into the valley above the Markley Hut, a few miles from Ashcroft, may have noticed another example of an inclined syncline. That valley has a similar geologic structure to Aspen Mountain, only on a smaller scale, and was also a major silver producer. As you climb the west side of that valley, the slope increases with elevation. You traverse three slope changes where geologic layers slumped, resulting in the small ridges that dip toward Express Creek. From the top of the ridge, it is easy to see how geologic forces shaped the valley and how a significant earthquake could set off a large landslide.

In historic time, the Aspen area experienced the second largest landslide in the lower 48 states. You drive through the base of the slide as you drive Snowmass Creek Road to Campground. The slide happened after the area was saturated with water, and it could have been triggered by a small quake. No one witnessed the event in the late 1950s, but it was a popular destination for locals’ Sunday drives.

The combination of glacial material overlying tilted sedimentary layers, significant groundwater buildup, steep slopes and earthquakes could lead to large land movements in the Aspen area. While such movements are less likely in historic time, they could quite possibly happen in future geologic time. Don’t lose any sleep; building codes in the most vulnerable locations exclude residences.

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