Wednesday, September 27, 2017

A GRANITE BOULDER TAKES A RIDE IN UTE PASS

 By Steven Wade Veatch

With the suddenness of a rattlesnake’s strike, an enormous boulder of Pikes Peak Granite moved down one of the steep slopes of the lower part of Ute Pass, Colorado. As this rock—larger than a yellow school bus—traveled down the hill, it flattened the bushes growing in front of it, and left a trail of scraped ground behind it.

Figure 1. Gravity’s relentless force pulled this huge boulder
down the hill to its resting place near US Highway 24
between mile marker 295 and 296. This is a geohazard.
Photo © S. Veatch.
This giant rock, perched on a slope in Ute Pass along US Highway 24— between Manitou Springs and Green Mountain Falls—moved downslope from the pull of gravity in a type of erosion called mass wasting. When combined with the water of winter snow melt or rain that alters ground conditions, gravity can move rocks downhill—the steeper the slope, the faster the rocks and boulders move (McGeary, Brown, & Plummer, 1992).

During a recent summer, thunderstorms poured rain on the pass.  The slope where this boulder rested was saturated with water, making the ground a muddy, slippery mess. As the rain soaked into the soil, it filled pore spaces, which pushed apart individual grains in the soil—decreasing the resistance of the boulder to movement (Murck, Skinner, & Porter, 1997). Also, some of the grass was washed away by rivulets and rills running downslope, also adding to the conditions that mobilized the boulder.

One night when it was quiet, except for the rasp of a cricket and the passing of an occasional car on the highway, the force of gravity became greater than the resistance of the ground holding the immense boulder in place. Catching the sleeping birds in the pine trees off guard, the giant rock yielded to the endless pull of gravity and slid down the slope—a geological event that starts within the blink of an eye.

Figure 2. A once moving boulder left behind a trail and pushed up
loose gravel in front of it as it slid down the slope of Ute Pass.
Photo © S. Veatch.
This rapid movement of rocks is a geohazard that develops over time and locally impacts Ute Pass and Manitou Springs. Ute Pass and Manitou Springs are in the path of sliding and falling rocks. Work is ongoing to mitigate some of these hazards. Travelers going through Ute Pass not only have to watch other drivers, but must also look out for moving boulders.

References
McGeary, D., Brown, W. C., & Plummer, C. C. (1992). Physical Science: Earth Revealed. Dubuque: William C. Brown.

Murck, B. W., Skinner, B. J., & Porter, S. C. (1997). Dangerous Earth: An Introduction to Geologic Hazards. New York: John Wiley & Sons, Inc.





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