Gravity and rock physics say slopes steeper than about 30°—known as the critical threshold angle, or the angle of repose—shouldn’t really exist, yet they do. These steep slopes and near-vertical cliffs can be seen in the Himalayas and in Yosemite National Park, for example. For some geologists, the question is not only how “oversteepened” slopes exist but also how long they stay that way.

“The rock masses in these oversteepened parts are unstable by nature, and they are prone to failure. These are places where landslides are most likely to occur,” said Gen Li, a geologist at the University of California, Santa Barbara.