Ahhhh, Another Day at the Office: Rock Fall in Zion National Park

A large rock fall smashes a truck near the Oak Creek maintanance facitlity in Zion National Park. NPS photo.

We have been spending quite a bit of time down in Zion the last couple of weeks. The Utah Geological Survey has contracted with the Park Service to produce a series of geologic hazard maps. Much of this mapping can be done with existing data sets and sophisticated GIS software, but, thank goodness, there is some field work involved.

One of the biggest hazards we are concerned with is rock fall. Compared to places such as Yosemite (where people have died due to falling rocks), the hazard at Zion appears to be a bit less but still a very real possibility made evident by the amount of rock-fall boulders near park housing, roadways, and even the Zion Lodge. Buildings have been struck and roads have been damaged but we are not aware of anyone hurt or killed by a rock fall in Zion (other than stories of big-wall climbers).
One only needs to look a few miles down the road to see the devastating potential of a large rockfall. Above is a picture taken by my boss in 2001 in the nearby town of Rockville. The boulder came within inches of the then-sleeping residents' face in the early morning. Just beyond the yellow tape you can see a deep crater where the massive boulder bounced before finally coming to rest in the home.


Some key things that are investigated in the field in order to properly characterize the rock-fall hazard include identifying what formation the boulder came from, the angle from the boulder to the source area, and the size of the boulder. Some boulders are small enough to carry, but others are as big as a house. Vincent and Julien, two very helpful volunteers from France, were more than eager to climb and measure some of the more monstrous boulders in 110-degree heat for the sake of science.

The small and angular shards of rock above are good evidence for a recent rock fall. Other rock-fall boulders are anything but recent. If you click on the panarama above, you will see several huge boulders that are too far away from the cliff face to imagine being recently deposited. Our theory is that these boulders fell when the cliff was much closer and the canyon much narrower; but over the last several thousands of years, the cliff has eroded back to its present location leaving the boulders "stranded" out in the middle of the valley.
Above is further evidence that most if not all of the very large boulders way out in the middle of the valley are very ancient. It would be basically impossible for the small boulder above to roll from several 100 yards away and come to rest on top of the larger one and have the bedding be the same orientation. A more plausible explanation is that these two boulders are the eroded remnants of a once much-larger single boulder that fell when the cliff face was much closer eons ago.
Anyway, it's been fun working in Zion and I look forward to a few more field days.

We came across a few interesting things traipsing around off the beaten path including some petroglyphs, and some peculiar etchings (below) that appear to be some sort of Native American calendar(?) - also, we found a purse amongst the boulders that we turned into the Park, and later found out it was tossed out a car window after a smash-and-grab job from a few weeks prior.