Mountains can be precarious piles of rocks. In the Rockies, annual freeze-thaw cycles fracture solid rock into loose scree fields and talus slopes. The results are steep mountainsides like the one you see here on the back side of Treasury in western Colorado.

Rocks on these slopes sit nearly at the angle of repose – the largest angle at which rocks will sit stably without spontaneously sliding away in an avalanche. Traverses can be difficult – missteps lead to runaway rockslides, and large deep ravines are common. You can see one here that drains to a river hundreds of feet below.

How are these ravines formed? Over the weekend I witnessed some dynamic geology, though altogether would have preferred to have not. The story starts when I was caught in a heavy thunder and hail storm at the top of a pass on the side of this mountain.

It is a good idea in a thunderstorm to get to cover as quickly as possible. Unfortunately the mile-long trip to below treeline required crossing the same ravine shown in the picture above. And large peaks with little or no soil development have little water storage capacity, so all the precipitation from the storm rapidly became surface runoff. With high flow undermining the stability of the slope, loose rock as well as water began to be transported down-mountain. The result was a very frightening slope failure and debris flow. Foot-long rocks were flying tens of feet in the air as they bounced down the mountain. In the below picture, a person would stand a bit less than one-half the height of the frame.

You can see the power of the flow in this short video I captured. As a warning, some profanity can be heard from our group…

We were trapped by this ravine for fifteen minutes, until a break in the storm that brought a stop to the flying rocks. A running start, a nerve-wracking airborne second, and we were over the gap and heading toward home. Days like these bring new respect for the forces that shape mountains, and new appreciation for safe cozy nights at home!